Skip to main content
PMC home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2009 Dec 1.
Published in final edited form as: J Fam Psychol. 2008 Dec;22(6):905–914. doi: 10.1037/a0012603

Siblings of Adults with Mild Intellectual Deficits or Mental Illness: Differential Life Course Outcomes

Julie Lounds Taylor 1, Jan S Greenberg 2, Marsha Mailick Seltzer 3, Frank J Floyd 4
PMCID: PMC2610343  NIHMSID: NIHMS59671  PMID: 19102611

Abstract

The present study contrasted the later life sibling relationships, patterns of family formation, and psychological distress and well-being of siblings of adults with disabilities to a non-disabled normative group. We identified 268 siblings of adults with mild intellectual deficits and 83 siblings of adults with mental illness from the Wisconsin Longitudinal Study, a prospective longitudinal study that followed participants from age 18 to age 64. Compared to the norm (n = 791), siblings of adults with mild intellectual deficits had more contact with family members and were more likely to live in the same state as the sibling with the disability, but reported less affective closeness. Siblings of adults with mental illness reported more psychological distress, less psychological well-being and less adaptive personality characteristics compared to the norm, particularly for siblings of men with mental illness. There were no differences between groups in the patterns of marriage and childbearing.

Keywords: sibling, adult development, intellectual disability, mental illness

Intellectual deficits and mental illness are prevalent in our society, with broad implications for stress and disruption for the many families affected by these disabilities (Marsh, 1992a; 1992b). Over 15% of the population has IQ scores indicating mild intellectual deficits (Neisser et al., 1996). Although their disability is often “hidden” in adulthood (Edgerton, 1967), these individuals remain at high risk for unemployment, unstable marital relationships, and psychological distress (Fujiura, 2003; Maughan, Collishaw, & Pickles, 1999; Richardson & Koller, 1996). Similarly, serious mental illness affects 5% of the adult population in the United States every year (Kessler et al., 1996). Despite the prevalence of these disorders, little is known about the impact of having a brother or sister with mild intellectual deficits or mental illness on the development of their siblings, especially beyond childhood and during later adulthood.

The present study examined how having a brother or sister with a disability influenced the sibling relationship, psychological development, family formation, and participation with the family of origin by the non-disabled sibling during later adulthood relative to a normative sample of non-disabled sibling pairs. The study uses the Wisconsin Longitudinal Study (WLS; Hauser & Sewell, 1985; Hauser, Sheridan, & Warren, 1998), a population-based sample from Wisconsin that includes approximately 5,800 sibling pairs. The WLS provides a unique opportunity to identify sibling pairs in which one has a disability and examine outcomes over the entire adult lifespan, with the data extending from high school into early old age. In a critical review of sibling research, Hodapp, Glidden, and Kaiser (2005) discussed the importance of adopting a lifespan perspective and considering the development of siblings and sibling relationships into adulthood, as the reactions to the brother or sister with the disability may differ based on the stage of the life course.

Hodapp and colleagues (2005) also advocated for the use of multiple contrast groups to determine the effects of different types of disabilities on sibling development. We were able to adopt this strategy with the WLS, by identifying both siblings of adults with mild intellectual deficits and siblings of adults with mental illness. By comparing both of these groups to a control group of siblings of adults with no known disabilities, we can examine the life course alterations that are common to both mental illness and mild intellectual deficits, as well as the unique correlates of each of these disabilities.

There are a number of reasons why one would expect the life course of siblings of adults with mild intellectual deficits or mental illness to differ from the norm. First, research with normative samples suggests that as adults move from middle into older age, the sibling relationship plays an increasingly significant role. Siblings tend to report feeling closer to each other as they age and sibling instrumental and psychological support becomes more important (Cicirelli, 1995, 1996). Although scant research has examined the sibling relationship in later adulthood when one member has a lifelong disability, such relationships are likely different from the norm; these relationships tend to be less egalitarian with the non-disabled sibling providing instrumental and emotional support that may not be fully reciprocated (Seltzer, Begun, Seltzer, & Krauss, 1991; Zetlin, 1986). A relationship with a brother or sister with a disability is more likely to be based on caregiving rather than companionship and mutual sharing, which could affect the intimacy and emotional closeness of the relationship. Indeed, studies of the normative sibling relationship suggest that these circumstances can reduce the quality of the sibling relationship and the mental health of the help giver (Cicerelli, 1995; 1996). The only study to look at the relationship between siblings beyond childhood when one had a disability that included a non-affected comparison group confirms this premise. Begun (1989) found that there was less intimacy and perception of similarity in the sibling relationship when one had a disability, even though siblings reported equal levels of affective closeness as in non-disabled dyads.

Having a brother or sister with a disability may also affect the life course of the siblings by limiting the opportunities to establish and maintain their own families. Siblings of adults with developmental disabilities report that having a brother or sister with a disability impacts their decisions regarding life partners and having children of their own (Seltzer, Greenberg, Krauss, Gordon, & Judge, 1997). Hodapp & Urbano (2007) found that women under the age of 45 who have a brother or sister with developmental disabilities tend to marry and have children later than the norm, but are just as likely to be married by mid and later adulthood. There may be long-term effects of delaying the acquisition of family roles, particularly on the number of children born to such siblings. Furthermore, Seltzer et al. (1997) found that siblings of adults with mental illness were even less likely than siblings of individuals with intellectual disabilities to be married in young adulthood. The increased support required by individuals with disabilities may make it more difficult for their siblings to sustain positive marital relationships.

Third, the life course of siblings of adults with disabilities may be affected by having higher levels of solidarity with their families of origin, particularly among siblings of adults with mild intellectual deficits. Bengtson and Roberts (1991), in their theory of family solidarity, suggested that having higher expectations about the importance of family roles and obligations is related to more frequent contact with family. The identification of an intellectual deficit generally occurs during early childhood, and past research has shown that siblings of individuals with such deficits are socialized early in their lives that the care of their brother or sister with disabilities is a shared family responsibility (Stoneman, 2005). Therefore, these siblings likely had higher expectations regarding their family obligations from a young age, which may be related to more family of origin involvement throughout the life course. On the other hand, mental illness is typically diagnosed during later adolescence or early adulthood, and therefore siblings of adults with mental illness would have had more typical socialization patterns regarding family obligations during childhood. Consequently, they may not experience higher levels of family solidarity.

Finally, differences in the nature of the behaviors associated with mild intellectual deficits and mental illness, along with differential genetic risk for mental health problems, may have disparate effects on the mental health and personality characteristics of non-disabled siblings. Whereas intellectual deficits are characterized by relative stability in the day-to-day functioning of the individual with the disability (Esbensen, Seltzer, & Krauss, in press), the symptoms of mental illness are more episodic and have a less predictable course (Marsh, 1992a). Some siblings of adults with mental illness also have an increased genetic risk of mental health problems themselves (Farmer, Harris, Redman, Sadler, Mahmood, & McGuffin, 2000; Kendler & Prescott, 1999) which, coupled with the stress of the brother or sister's symptoms and behavior, may lead to increased rates of psychological distress, poorer psychological well-being, and less advantageous personality characteristics relative to siblings of adults with no known disabilities. Indeed, past research has shown that siblings of adolescents and adults with mental illness - specifically siblings of individuals with a diagnosis of major depression - tend to have higher levels of depression and neuroticism and lower levels of cooperativeness and extraversion than siblings of non-disabled brothers and sisters (Farmer et al., 2002; Farmer, Mahmood, Redman, Harris, Sadler, & McGuffin, 2003; Masi, Mucci, Favilla, Brovedani, Millepiedi, & Perugi, 2003), and that these less advantageous personality characteristics are due to both genetic and non-genetic causes. In contrast, siblings of adults with intellectual disabilities are not generally vulnerable to mental health problems (Burton & Parks, 1994; Cleveland & Miller, 1977), perhaps because the symptoms of intellectual disabilities are more predictable and consequently easier to manage. These siblings may, in fact, develop more positive personality characteristics than the norm because of being socialized toward caregiving at an early age. Studies often report more conscientiousness and helpful behaviors for children who have a brother or sister with intellectual disabilities than siblings with typically developing brothers and sisters (Cuskelly & Gunn, 2003; Stoneman, Brody, Davis, & Crapps, 1989), which could manifest itself in higher levels of conscientiousness and agreeableness as they age.

In using the Wisconsin Longitudinal Study (WLS) to examine life course outcomes for siblings, we are able to address and rectify some of the methodological limitations that have plagued past research focused on siblings of individuals with disabilities. First, as noted by Hodapp et al. (2005), much of the extant research on siblings of individuals with disabilities used small samples of convenience, which limits the statistical power available to examine interacting variables and also places substantial restrictions on the generalizability of findings. Because the WLS is a large unselected sample, concerns related to insufficient statistical power and selection biases are minimized. Second, Hodapp et al. (2005) also noted that the majority of sibling research focuses on a selected sibling who is the “most involved” or who has the closest emotional relationship with the person with the disability. It is unclear whether the experiences of this most involved sibling generalize to other siblings in the family. The WLS addresses this concern because the sibling chosen for study was randomly selected from all available siblings in the family. Third, previous research examined siblings of individuals with severe disabilities that are relatively rare, including developmental disabilities such as mental retardation, autism, and Down syndrome, and mental illnesses such as schizophrenia and bipolar disorder (Greenberg, Seltzer, Orsmond, and Krauss, 1999; Orsmond & Seltzer, 2000; Seltzer et al., 1997; Seltzer et al., 1991). In the present paper, we focus on more prevalent nonnormative sibling experiences, namely having a sibling with mild intellectual deficits or mental illness such as depression and anxiety.

The present study

Based on theory and previous research, we developed four hypotheses. First, we hypothesized that siblings of adults with mild intellectual deficits would have a relationship with their brother or sister with the disability that is similar to the norm; in contrast, siblings of adults with mental illness will have a relationship with their brother or sister that is characterized by less closeness and less contact. We expected that the detrimental effects of less egalitarian sibling relationships would be offset by early socialization toward caregiving and more family solidarity among siblings of adults with mild intellectual deficits, resulting in a sibling relationship similar to the norm in contact and closeness. For siblings of adults with mental illness, we did not expect the detrimental effects of unreciprocated support to be offset by family solidarity (as the diagnoses of mental illness occurred later in the lifespan). Thus, we hypothesized that emotional distance in the sibling relationship would persist into older age.

Second, we anticipated that siblings of adults with mild intellectual deficits would have similar marital patterns to the norm in later life, but would have fewer children due to their likelihood of being married at an older age. They also would be more involved with their family of origin because of higher family solidarity. We expected that siblings of adults with mental illness would be less likely to be married in later life due to the stress of providing support to their brother or sister with difficult, unpredictable symptoms and behaviors. Because we did not expect higher levels of family solidarity when one member has a mental illness, we hypothesized that siblings' patterns of family of origin involvement would not differ from the norm.

Third, we expected that siblings of adults with mild intellectual deficits would experience similar levels of psychological distress and well-being compared to the norm, but that siblings of adults with mental illness would have more distress and lower levels of well-being. In terms of personality, we hypothesized that siblings of adults with mild intellectual deficits would have higher levels of conscientiousness and agreeableness due to early socialization toward caregiving, and siblings of adults with mental illness would have more neuroticism, less extraversion, and less agreeableness due to both genetic vulnerability and as a reaction to providing support to their brother or sister with difficult symptoms.

Fourth, we hypothesized that gender would play an important role in the sibling relationship. Earlier research on siblings of adults with mental illness (Greenberg et al., 1999) found that they were less likely to expect to provide future care for brothers with a mental illness as compared to sisters. We expected that these gender patterns would generalize to other aspects of the sibling relationship, and predicted that siblings of brothers with mental illness would have less frequent contact and rate the relationship as less close than siblings of brothers with no disability. However, we did not expect such gender differences for siblings of individuals with intellectual disabilities, as previous research has generally not found the gender of the sibling with intellectual disabilities to impact the sibling relationship (e.g., Greenberg et al., 1999; Orsmond & Seltzer, 2000).

Methods

Wisconsin Longitudinal Study

The WLS is based on a random sample of 10,317 women and men who graduated from Wisconsin high schools in 1957. Follow-up surveys were conducted at the following three time points: in 1975 with 9,138 (90.1%) surviving members of the original sample when they were, on average, 36 years old; in 1992 with 8,493 (87.2%) of the surviving original respondents when they were in their early 50s; and again in 2004 with 7,265 (80.0%) of the surviving respondents when they were in their mid 60s. In addition, parallel data collection procedures were conducted with one randomly selected sibling of the respondents in 1977, 1994, and 2005, with 5823 siblings participating in one or more of these data collection points. At this later time, the siblings averaged 64 years of age, similar to the mean age of the original respondents. Family background data in 1957 and high school IQ scores were available for both the respondents and their randomly selected siblings. Data from the 2004/5 wave, the primary data used in the present analyses, were collected via a telephone interview and a self-administered mail-back questionnaire.

Participants

We use the term “target individual” to refer to the sibling with the disability and “sibling of interest” to refer to the non-disabled member of the sibling pair. We identified three groups of sibling pairs for the current study from the combined sample of original respondents and their siblings.

We identified target individuals with mild intellectual deficits by using IQ scores generated from the Henmon-Nelson Test of Mental Ability (Henmon & Nelson, 1937), administered in high school to all participants. Using similar cut-off scores as in previous research (Seltzer, Floyd, Greenberg, Lounds, Lindstrom, & Hong, 2005; Valliant & Davis, 2000), we classified individuals as having mild intellectual deficits if they had IQs of 85 or below, with 61 being the lowest possible score on the Henmon-Nelson intelligence test. Because IQ tends to be quite stable across the life course (Deary, Whalley, Lemmon, Crawford, & Starr, 2000), this earlier measure of IQ is also indicative of intellectual functioning during middle and later adulthood.

We identified target individuals with mental illness by ascertaining all of the participants who self-reported ever being diagnosed with a mental illness, which was assessed in the 2004/5 telephone survey. Fully 85.5% of individuals in this sample who answered affirmatively that they had been diagnosed with a mental illness had been diagnosed with either depressive disorder or anxiety disorder (or both). The remaining individuals were diagnosed with episodic mood disorders (n = 7 or 8.4%), schizophrenic disorders (n = 3 or 3.6%), or alcohol dependence syndrome (n = 2 or 2.4%). Excluded from this analysis were respondents with mild intellectual deficits who also reported a mental illness or a major depressive episode in order to reduce overlap across the groups.

Siblings were eligible for the present study if they reported never having been diagnosed with a mental illness, had an IQ score of 100 or above – one standard deviation higher than the cut-off criterion for mild intellectual deficits – and had completed both the telephone interview and mail-back survey in 2004/5 (full inclusion criteria and detailed flow chart available by request from the first author). Although eliminating siblings who themselves have mild intellectual deficits or mental illness places some limitations on the ability to generalize our findings to the full population, this selection criterion was necessary to ensure that any deviations from the norm in later life development and sibling relationships could not be attributed to the sibling's own disability. This selection process resulted in 268 non-disabled siblings who had a brother or sister with mild intellectual deficits and 83 non-disabled siblings who had a brother or sister with mental illness.

The comparison group was constructed by ascertaining all of the sibling pairs in which neither member had mild intellectual deficits and neither reported ever having been diagnosed with a mental illness. One person from each comparison group pair was randomly designated the “target individual” and the other was the “sibling of interest”. In order to ensure comparable IQ scores across all groups, only cases in which the sibling of interest had an IQ score of 100 or above were included in the analyses. Finally, the sibling of interest must have completed both the telephone interview and mail-back survey in 2004/5. This resulted in 791 siblings in the comparison group. In all groups, cases were excluded if either member of the selected sibling pair had died prior to the 2004/5 interview.

Table 1 presents the demographic characteristics for each of the sibling groups, as well as analysis of variance and chi-square tests to determine if there were significant differences between the groups in demographic and background characteristics. Follow-up tests for significant ANOVA effects are also presented in Table 1. When considering the characteristics of the non-disabled sibling, few significant background differences emerged. Siblings in all three groups were slightly more likely to be female than male, which is consistent with the gender composition of the WLS at this point of data collection. They averaged about 64 years of age, came from sibships averaging between 3 and 4 members, and just over 50% were older than the target brother or sister. Approximately half were employed in 2004/5. Siblings of adults with mild intellectual deficits had significantly lower IQ scores than comparison siblings (by about 4 points) and had completed about one year less of education, on average.

Table 1. Descriptive Statistics and F-tests/Chi-squares for Family of Origin and Background Variables.

Group means and percentages Test statistics
ID MI Comparison Overall Contrasts with Comparison Group
ID vs. Comparison MI vs. Comparison
Characteristics of non-disabled sibling and sibship
Female 54.5% 55.4% 52.5% .51
Age in 2004/5 63.93
(4.76)		 63.87
(4.42)		 64.03
(4.42)		 .09
Size of sibship 3.92
(1.96)		 3.61
(1.58)		 3.67
(1.88)		 1.98
Older than disabled sibling 50.8% 56.6% 52.2% .85
Currently employed 47.4% 51.8% 48.8% .51
IQ score in 1957 109.84
(7.84)		 114.16
(9.74)		 113.53
(9.90)		 16.34** 30.59** .33
Years of education 13.75
(2.43)		 14.32
(2.32)		 14.56
(2.59)		 10.06** 20.11** .64
Characteristics of disabled (target) sibling
Female 44.8% 66.3% 47.8% 12.13**
Currently married 80.6% 56.6% 82.0% 30.26**
Age in 2004/5 64.78
(4.39)		 62.18
(4.95)		 64.01
(5.03)		 7.83** 3.45 10.39**
Currently employed 47.7% 42.0% 48.8% 1.39
Family of origin characteristics measured in 1957
Mother's education 10.17
(2.84)		 11.42
(2.41)		 11.03
(2.69)		 12.03** 20.14** 1.59
Father's education 9.65
(2.84)		 11.42
(3.47)		 10.87
(3.27)		 16.18** 27.21** 2.10
Father's SEI 266.84
(168.33)		 357.00
(208.87)		 370.96
(230.04)		 23.30** 46.34** .31
Family income in hundreds 55.85
(54.76)		 72.52
(47.78)		 68.69
(49.75)		 6.84** 12.23** .43
Population of town 28448.69
(55888.05)		 48207.83
(68288.72)		 44543.93
(67460.66)		 6.68** 14.88** .22
Open in a new tab
*

p < .05

**

p < .01

Note. ID = Mild Intellectual Deficits; MI = Mental Illness. The test statistic for percentages is a chi-square. The test statistic for means is an F-test. Follow-up tests were only conducted for F-tests when there was a significant difference in the overall test.

There were differences between the mental illness and comparison groups when examining the characteristics of the disabled brother or sister. Individuals with mental illness were more likely to be female, were less likely to be currently married, and were significantly younger (by 2 years) than the comparison group; individuals with mild intellectual deficits did not differ from the comparison group on these variables. There were no differences between the groups in the percentages who were employed in 2004/5.

In terms of the family of origin variables (i.e., the variables measured in 1957, when the sibling was in high school), the mild intellectual deficits group significantly differed from the comparison group on every measure. Specifically, mothers and fathers of individuals with mild intellectual deficits completed fewer years of education, and fathers had less prestigious occupations as measured by Duncan's Socio-economic index (Duncan, 1961; Hauser & Warren, 1997). The families of individuals with mild intellectual deficits had a lower total household income and were living in towns and cities that were significantly smaller in population than the comparison group. There were no differences between the mental illness and comparison groups on any family of origin or background variable. All members of the present sample were Caucasian, reflective of Wisconsin's population at mid-century.

Measures

Measurement of IQ score

IQ was assessed using the Henmon-Nelson Test of Mental Ability: Grades 7-12 (Henmon & Nelson, 1937), a group-administered IQ test that was given to all Wisconsin high school students during their junior year of high school. A detailed description of the measure can be found in Seltzer et al. (2005). The original scale and subsequent revisions have been widely used in educational testing and research on mental ability, and psychometric data on the Henmon-Nelson indicate that it provides reliable and valid scores that reflect general intellectual functioning (Gregory, 2004).

Measures of the sibling relationship in 2004/5

Information was obtained about the relationship between the sibling of interest (i.e., the non-disabled sibling) and the target brother or sister (i.e., the individual with mild intellectual deficits or mental illness, or the target individual in the comparison group) through four questions. Non-disabled siblings were asked whether they currently live in the same state as the target brother or sister (0 = did not live in the same state, 1 = lived in the same state), how close they feel to the target individual (1 = not at all close to 4 = very close), the number of times they had contact (including both in-person and telephone) with the brother or sister in the past year, and how similar they are to the target brother or sister in terms of general outlook on life (1 = not at all similar to 4 = very similar).

Family formation and participation in 2004/5

The following information was obtained about family formation in 2004/5: the non-disabled sibling's current marital status (1 = married, 0 = not married), whether the sibling had been married more than one time (1 = married more than once; 0 = not married more than once), and the number of children born to the sibling. Involvement with the family of origin was measured by the number of times the sibling had visited with relatives during the 4 weeks prior to the interview, and whether the sibling felt they could talk to any of their brothers or sisters (not specifically the sibling with the disability) about their personal problems (1 = yes, 0 = no).

Measures of psychological distress, well-being, and personality in 2004/5

Depression was measured by the Center for Epidemiological Studies – Depression Scale (CES-D, Radloff, 1977), which has been used extensively in samples of mid-life and older adults (Gatz & Hurowicz, 1990). For each of 20 depressive symptoms, the non-disabled sibling was asked to indicate how many days in the past week the symptom was experienced. The data were recoded into four categories (0 = never to 3 = 5 to 7 days), consistent with the conventional scoring of the CES-D. The total score is the sum of the category ratings for the 20 items (α = .82), with higher scores indicating more depressive symptoms. In addition, siblings were asked in the 2004/5 interview whether they ever had a depressive episode lasting 2 weeks or more (1 = yes, 0 = no). Siblings were counted as having a depressive episode if they responded affirmatively to that question, regardless of the cause of the episode.

The 2004/5 survey also included a modified version of Ryff's Psychological Well-Being measure (Ryff, 1989), which consists of a series of 31 statements, such as “I have the sense that I have developed a lot as a person over time” and “I judge myself by what I think is important, not by what others think is important,” to which the siblings respond on a scale of 1 to 6 (1 = agree strongly to 6 = disagree strongly). Items are summed to form 6 dimensions of well-being: Self-acceptance, Positive Relations with Others, Autonomy, Environmental Mastery, Purpose in Life, and Personal Growth. All subscales consisted of 5 items except the Purpose in Life scale, which consisted of 6 items. For the present analyses, all of the items were averaged to obtain an overall well-being score with higher scores indicating more positive well-being (α = .92).

Personality was measured in the 2004/5 survey by a modified version of the “Big Five” Inventory (John, Donahue, & Kentle, 1991). This inventory was designed to allow assessment of the Big Five personality dimensions in a self-report format. The Extraversion (versus Social Inhibition) scale captures gregarious, energetic, and expressive features of behavior (α = .79). The Agreeableness (versus Antagonism) scale reflects prosocial characteristics, describing the person who is empathic and makes an effort to establish positive relationships with others (α = .68). The Conscientiousness (versus Lack of Direction) scale captures the multiple elements of persistence and impulse control in task and achievement settings (α = .67). The Neuroticism (versus Emotional Stability) scale reflects multiple elements of negative emotionality, such as nervous tension, fearfulness, and brittleness under stress (α = .73). The Openness to Experience scale refers to persons who are imaginative, curious, and creative (α = .66). Each item was measured on a 6-point scale (1 = agree strongly to 6 = disagree strongly). All scales consisted of 6 items except the Neuroticism scale, which consisted of 5 items. Items from each scale were averaged, with higher scores indicating higher levels of the personality trait.

Methods of Data Analysis

The method of data analysis was two-way analysis of covariance. The first independent variable was disability group, specifically, whether the participant was a sibling of a person with mild intellectual deficits (ID), mental illness (MI), or a comparison sibling. The second independent variable was the gender of the target individual, that is, the brother or sister with ID, MI, or the randomly selected target individual in the comparison group. Preliminary analyses suggested that the gender of the non-disabled sibling had a main effect on many of the outcomes, but rarely interacted with disability group (analyses available from first author). Thus, we chose to statistically control for gender of the non-disabled sibling and not include it as a factor in the models. Type III sums of squares was chosen as the method for obtaining marginal means; this implies an unweighted average of the cell means and thus insures that the ID and MI groups, which have smaller sample sizes, will not be weighted less strongly than the comparison group.

Because we were primarily interested in how siblings of adults with ID or MI differed from the norm, planned pair-wise comparisons were conducted for all significant disability group main effects or disability group by gender interactions. For main effects, the ID group was contrasted with the comparison group and the MI group was contrasted with the comparison group. For disability group by target gender interactions, the following contrasts were tested: siblings of sisters with ID versus siblings of comparison sisters; siblings of brothers with ID versus siblings of comparison brothers; siblings of sisters with MI versus siblings of comparison sisters; and siblings of brothers with MI versus siblings of comparison brothers.

In order to disentangle the life course implications of having a brother or sister with ID or MI from the background characteristics for which group differences were found (see Table 1), statistical controls for mother's and father's education, father's occupational prestige, family income, population of 1957 town, and IQ of the non-disabled sibling were included in all subsequent analyses. Although it is not known whether the groups differed on other unmeasured variables, controlling for these background variables in effect makes the groups equivalent on those variables for which differences were observed. As noted earlier, statistical controls for the gender of the non-disabled sibling were also included.

Results

Are Sibling Relationships with Disabled Individuals Different from the Norm?

Analysis of covariance for the sibling relationship variables and adjusted group means can be found in Tables 2a and 2b. There was a main effect of disability group for all four of the sibling relationship variables. Follow-up tests indicated that significant differences for the proportion of siblings living in the same state and closeness were between the ID and comparison groups, ts = 2.53 and -2.41, respectively, ps < .05. Siblings of adults with ID were more likely to be living in the same state as their brother or sister with the disability (M = .59, SE = .03) than siblings of comparison adults (M = .50, SE = .02). However, despite this geographic proximity, siblings of adults with ID reported feeling less emotionally close to the target sibling than the comparison group (M = 3.02, SE = .05 for ID group; M = 3.16, SE = .03 for comparison group). The post hoc tests revealed that both the ID and MI groups had significantly fewer contacts with the target sibling than did the comparison group, ts = -2.08 and -2.05, respectively, ps < .05, with the ID group averaging 35.20 (SE = 5.17) contacts per year, the MI group reporting an average of 27.25 (SE = 9.56) contacts per year, whereas the comparison group averaged 47.72 (SE = 2.94) contacts. Furthermore, siblings in both the ID and MI groups reported a more dissimilar outlook on life to the brother or sister with the disability (M = 2.87, SE = .05 for ID group, M = 2.77, SE = .09 for MI group) than siblings in the comparison group (M = 3.04, SE = .03), ts = -3.20 and -2.94, respectively, ps < .01.

Table 2.

Table 2a F-tests from Analysis of Covariance of 2004/5 Sibling Relationship Variables by Disability Group and Gender

Table 2b Group Means with Standard Errors for 2004/5 Sibling Relationship Variables by Disability Group and Gender

Variable Disability Group Target Gender Disability Group × Target Gender
Proportion that live in same state 3.52* .84 .49
Closeness 3.46* 8.54** .96
Number of contacts in past year 3.77* 3.65 .80
Similarity in outlook on life 8.35** .14 1.89
Siblings of Adults with ID Siblings of Adults with MI Siblings of Comparison Adults
Variable Target Sister Target Brother Target Sister Target Brother Target Sister Target Brother
Proportion that live in same state .58
(.05)		 .60
(.04)		 .42
(.07)		 .53
(.09)		 .50
(.03)		 .50
(.02)
Closeness 3.18
(.07)		 2.86
(.07)		 3.04
(.11)		 3.01
(.15)		 3.29
(.04)		 3.02
(.04)
Number or contacts in past year 39.73
(7.58)		 30.68
(6.88)		 32.70
(11.07)		 21.81
(15.57)		 59.20
(4.23)		 36.24
(4.06)
Similarity in outlook on life 2.88
(.07)		 2.85
(.06)		 2.71
(.10)		 2.83
(.14)		 3.13
(.04)		 2.96
(.04)
Open in a new tab
*

p < .05

**

p < .01

Note. ID = Mild Intellectual Deficits; MI = Mental Illness. Target gender refers to the gender of the brother or sister with the disability (or the target sibling in the case of the comparison group). Non-disabled sibling's IQ and gender, mother's and father's education, father's job prestige, family income in 1957, and size of 1957 town are controlled in all analyses.

Patterns of Family Formation and Involvement with Family of Origin

Results from analysis of covariance for family formation and involvement are presented in Table 3a and adjusted sub-group means are reported in Table 3b. Regarding family formation, there were no significant differences between the groups in the proportion of siblings who were currently married, had been married more than one time by their mid 60's, or the number of children they had. However, regarding involvement with the family of origin, analyses indicated statistically significant main effects of disability group for the number of visits with relatives and ability to talk with siblings. Follow-up tests revealed that the differences were between siblings of adults with ID and siblings of comparison adults. Siblings of adults with ID visited with relatives nearly once a week, which is about one more visit per month than the comparison group (M = 3.78, SE = .20 for ID group; M = 3.07, SE = .12 for comparison group), t = 2.99, p < .01. Although they had more visits with relatives, individuals who had a brother or sister with ID were significantly less likely to report being able to talk with siblings (M = .44, SE = .03) than individuals in the comparison group (M = .54, SE = .02), t = -2.58, p < .05.

Table 3.

Table 3a F-tests from Analysis of Covariance of 2004/5 Family Formation and Involvement Variables by Disability Group and Gender

Table 3b Group Means with Standard Errors for 2004/5 Family Formation and Involvement Variables by Disability Group and Gender

Variable Disability Group Target Gender Disability Group × Target Gender
Proportion currently married 1.98 .02 .07
Proportion married more than once 1.68 .04 .50
Total number of kids 1.07 .56 .34
Number of visits with relatives 4.60** .17 1.12
Proportion can talk with any sibling 3.50* 6.35* .39
Siblings of Adults with ID Siblings of Adults with MI Siblings of Comparison Adults
Variable Target Sister Target Brother Target Sister Target Brother Target Sister Target Brother
Proportion currently married .78
(.04)		 .77
(.03)		 .81
(.05)		 .84
(.07)		 .84
(.02)		 .83
(.02)
Proportion married more than once .12
(.04)		 .16
(.03)		 .24
(.05)		 .17
(.07)		 .18
(.02)		 .20
(.02)
Total number of kids 2.92
(.15)		 2.66
(.14)		 3.10
(.22)		 3.11
(.31)		 2.95
(.09)		 2.86
(.08)
Number of visits with relatives 4.12
(.30)		 3.44
(.27)		 3.16
(.45)		 3.67
(.61)		 3.17
(.17)		 2.98
(.16)
Proportion can talk with any sibling .48
(.05)		 .40
(.04)		 .54
(.07)		 .42
(.09)		 .61
(.03)		 .47
(.03)
Open in a new tab
*

p < .05

**

p < .01

Note. ID = Mild Intellectual Deficits; MI = Mental Illness. Target gender refers to the gender of the brother or sister with the disability (or the target sibling in the case of the comparison group). Non-disabled sibling's IQ and gender, mother's and father's education, father's job prestige, family income in 1957, and size of 1957 town are controlled in all analyses.

Psychological Distress, Well-being, and Personality

The results of the analysis of covariance for psychological distress, well-being, and personality are presented in Table 4a; adjusted group means are presented in Table 4b. A significant main effect of disability group was found for lifetime history of depression; follow-up analyses revealed that the prevalence of ever having had a depressive episode was higher for siblings of individuals with MI (M = .31, SE = .06) than in the comparison group (M = .19, SE = .01), t = 2.19, p < .05. There were no differences among the groups in current depressive symptoms (as measured by the CES-D). Regarding psychological well-being of the sibling, there was a significant interaction between disability group and the gender of the target brother or sister (group means are presented in Table 4b). Follow-up tests indicated that siblings of brothers with MI had significantly lower well-being scores than siblings of brothers in the comparison group, t = -2.99, p < .01. In contrast to the effects for the MI group, siblings of individuals with ID did not differ from the norm in psychological distress or well-being.

Table 4.

Table 4a F-tests from Analysis of Covariance of 2004/5 Distress, Well-being, and Personality Variables by Disability Group and Gender

Table 4b Group Means with Standard Errors for 2004/5 Distress, Well-being, and Personality Variables by Disability Group and Gender

Variable Disability Group Target Gender Disability Group × Target Gender
Any lifetime history of depression 3.60* .47 .69
CES-D 2.52 .50 2.34
Psychological well-being 2.82 3.66 3.73*
Extraversion 5.56** 1.31 2.91
Agreeableness 1.33 4.01* 3.10*
Conscientiousness 3.17* 3.77 1.20
Neuroticism .84 2.54 5.73**
Openness .24 .32 .22
Siblings of Adults with ID Siblings of Adults with MI Siblings of Comparison Adults
Variable Target Sister Target Brother Target Sister Target Brother Target Sister Target Brother
Any lifetime history of depression .25
(.04)		 .23
(.04)		 .25
(.06)		 .37
(.09)		 .19
(.02)		 .18
(.02)
CES-D 7.66
(.57)		 5.77
(.51)		 7.11
(.82)		 8.25
(1.15)		 6.39
(.31)		 5.97
(.30)
Psychological well-being 4.84
(.06)		 4.87
(.05)		 4.85
(.08)		 4.48
(.11)		 4.81
(.03)		 4.83
(.03)
Extraversion 3.69
(.08)		 3.84
(.07)		 3.61
(.12)		 3.21
(.17)		 3.76
(.05)		 3.74
(.04)
Agreeableness 4.74
(.06)		 4.75
(.06)		 4.80
(.09)		 4.41
(.13)		 4.73
(.04)		 4.74
(.03)
Conscientiousness 4.81
(.06)		 4.75
(.06)		 4.71
(.09)		 4.43
(.13)		 4.78
(.03)		 4.76
(.03)
Neuroticism 2.99
(.08)		 2.90
(.07)		 2.68
(.12)		 3.27
(.16)		 2.94
(.05)		 2.82
(.04)
Openness 3.57
(.07)		 3.64
(.06)		 3.59
(.10)		 3.63
(.14)		 3.64
(.04)		 3.64
(.04)
Open in a new tab
*

p < .05

**

p < .01

Note. ID = Mild Intellectual Deficits; MI = Mental Illness. Target gender refers to the gender of the brother or sister with the disability (or the target sibling in the case of the comparison group). Non-disabled sibling's IQ and gender, mother's and father's education, father's job prestige, family income in 1957, and size of 1957 town are controlled in all analyses.

There were statistically significant effects of disability group for four out of the five factors of personality. Specifically, there were main effects of disability group for extraversion and conscientiousness, and significant interactions between disability group and the gender of the target brother or sister for agreeableness and neuroticism. Follow-up tests for all of these variables revealed significant differences between the MI and comparison groups; there were no differences in personality measures for siblings of adults with ID. Consistent with expectations, siblings of individuals with MI had lower extraversion scores (M = 3.41, SE = .10) and conscientiousness scores (M = 4.57, SE = .08) than siblings in the comparison group (M = 3.75, SE = .03 for extraversion; M = 4.77, SE = .02 for conscientiousness), ts = -3.27 and -2.47, respectively, ps < .05. Group means for the interactions are found in Table 4b. Siblings of brothers with MI had significantly lower agreeableness scores and higher neuroticism scores than siblings of brothers in the comparison group, ts = -2.51 and 2.66, respectively, ps < .05. However, contrary to expectations, siblings of sisters with MI had lower neuroticism scores than siblings of sisters in the comparison group, t = -2.07, p < .05.

Discussion

As expected, the results from the present study demonstrated a differential pattern of effects depending on the type of disability of the target sibling. For siblings of adults with ID, we observed an altered pattern in the quality of the relationship with the brother or sister with the disability as well as their involvement with their family of origin. Interestingly, siblings of adults with mild intellectual deficits were more likely than the norm to live in the same state as their brother or sister with the disability and had more contact with their extended family, but appeared to be less emotionally close to their extended family members (including the brother or sister with mild intellectual deficits). These findings provide evidence for an “obligatory” type of relationship between the non-disabled sibling and the brother or sister with mild intellectual deficits. Bengtson (2001) defines an obligatory relationship as one that has a high level of structural connectedness, that is, high levels of proximity and interaction. However, in obligatory relationships, structural connectedness is offset by a lower level of emotional attachment. Although we had hypothesized that high levels of family solidarity for siblings of adults with mild intellectual deficits would contribute to normative emotional closeness between siblings and greater emotional closeness among extended family members, our results suggest that in older age the non-disabled siblings' relationships with their brother or sister and extended family may better be characterized by norms of obligation rather than feelings of closeness.

Differences were also observed in the sibling relationship for siblings of adults with mental illness who, compared to the normative group, reported less contact with their brother or sister with the disability as well as a more dissimilar outlook on life. This pattern is corroborated by previous research in which siblings of young adults with mental illness coped by psychologically distancing themselves from their sibling (Seltzer et al., 1997). These findings may also be related to the downward mobility that is often experienced by individuals with disabilities (Jenkins, 1991), leading to a considerable gap in socioeconomic status among siblings. Disparities in social status may explain why non-disabled siblings rate their brother or sister with mental illness as being dissimilar to them, and may also be related to declining contact as individuals tend to prefer to spend time with others who have similar achievements (Verbrugge, 1977).

We found that siblings of adults with mild intellectual deficits did not differ from the norm on any of the measures of psychological well-being, distress, or personality during later adulthood, nor did their patterns of family role acquisition (i.e., marriage, childbearing) differ from the norm. These patterns are mostly consistent with our hypotheses and with data from volunteer samples, which have generally found few negative psychological effects of having a sibling with an intellectual disability and marital patterns that appear similar to the norm during later adulthood (Burton & Parks, 1994; Cleveland & Miller, 1977; Hodapp & Urbano, 2007).

In contrast to the ID group, the most striking pattern of differences from the norm when one member had mental illness was in the psychological realm. Siblings of adults with mental illness had a higher likelihood of ever having a depressive episode, were less extraverted, and less conscientious. These findings are corroborated by previous research (Farmer et al., 2002; Farmer et al., 2000; Kendler & Prescott, 1999). In addition, siblings of brothers with mental illness (but not siblings of sisters) had lower levels of psychological well-being, scored lower on agreeableness, and had higher levels of neuroticism than siblings of brothers without disabilities. Although we had hypothesized that the gender of the adult with mental illness would influence the sibling relationship, its main influence was instead on the psychological functioning of the non-disabled sibling. It may be that the psychological well-being and personality dimensions are affected more for siblings of males because of the tendency for males with mental illness to have more severe symptoms than females (Thorup et al., 2007).

Although the differences between siblings of individuals with MI and the comparison group might reflect underlying genetic vulnerability rather than the reactive effects of having a sibling with a disability, these gender-specific findings argue against a completely genetic explanation of the psychological effects of having a sibling with mental illness. Farmer and colleagues (2002) drew a similar conclusion; using a sibling pair design, they determined that both genetic and non-genetic factors contributed to the personality of siblings of individuals with major depression.

It is important to note that the cross-sectional nature of this study does not permit us to draw the conclusion that the adult with the disability is the cause of the group differences that emerged. As pointed out by Stoneman (2005), there are many other factors that influence the development of the sibling separate from the person with the disability him or herself. In addition to genetically linked vulnerabilities, siblings of brothers or sisters with disabilities likely grow up in more strained family environments than normative sibling pairs, because of the stress of caring for a child with disabilities on the whole family. Although this study is an important first step in describing the later life development of siblings of adults with disabilities, behavioral genetics and adoption studies are needed to tease apart the impacts of genetic predispositions and the family environment on the functioning of siblings.

There are three main limitations in the present study. As with any study, the generalizability of findings is limited by the characteristics of the sample. All participants in this sample attended high school in the same state and all were European-American, reflecting Wisconsin's population at mid-century. Although this sample is likely more representative of the population than the siblings from self-selected convenience samples, the finding are nonetheless limited by the geographic and age cohort from which the sample was drawn. Second, adults with mild intellectual deficits and their siblings came from families of origin that consistently differed from the norm. Families who had a member with ID tended be poorer, lived in smaller towns, and parents tended to have less education than the comparison group. Although these family of origin factors were statistically controlled in all of our analyses, they may be indicative of other unmeasured, pre-existing dissimilarities between the groups that help contribute to the observed differences in sibling later life outcomes. Third, beyond basic gender comparisons, the group-comparison design used in this paper masks heterogeneity in individual patterns of resilience and vulnerability within the sibling population. The next step in our program of research will be to examine the pathways leading to more or less favorable sibling relationships in later life and psychological well-being among aging individuals who have a brother or sister with a disability.

Acknowledgments

Support for this paper was provided by the National Institute on Aging (R01 AG20558, P01 AG20179) and the Eunice Kennedy Shriver National Institute of Child Health and Human Development (P30 HD03352)

Footnotes

Publisher's Disclaimer: The following manuscript is the final accepted manuscript. It has not been subjected to the final copyediting, fact-checking, and proofreading required for formal publication. It is not the definitive, publisher-authenticated version. The American Psychological Association and its Council of Editors disclaim any responsibility or liabilities for errors or omissions of this manuscript version, any version derived from this manuscript by NIH, or other third parties. The published version is available at http://www.apa.org/journals/fam/

Contributor Information

Julie Lounds Taylor, Waisman Center, University of Wisconsin-Madison.

Jan S. Greenberg, Waisman Center, University of Wisconsin-Madison

Marsha Mailick Seltzer, Waisman Center, University of Wisconsin-Madison.

Frank J. Floyd, Georgia State University

References

  1. Begun AL. Sibling relationships involving developmentally disabled people. American Journal on Mental Retardation. 1989;93:566–574. [PubMed] [Google Scholar]
  2. Bengtson VL. Beyond the nuclear family: The increasing importance of multigenerational bonds. Journal of Marriage and the Family. 2001;63:1–16. [Google Scholar]
  3. Bengtson VL, Roberts REL. Intergenerational solidarity in aging families: An example of formal theory construction. Journal of Marriage and the Family. 1991;53:856–870. [Google Scholar]
  4. Burton SL, Parks AL. Self-esteem, locus of control, and career aspirations of college-age siblings of individuals with disabilities. Social Work Research. 1994;18:178–185. [Google Scholar]
  5. Cicirelli VG. Sibling relationships across the life span. New York: Plenum; 1995. [Google Scholar]
  6. Cicirelli VG. Sibling relationships in middle and old age. In: Brody GH, editor. Sibling relationships: Their causes and consequences. Norwood, NJ: Ablex; 1996. pp. 47–74. [Google Scholar]
  7. Cleveland DW, Miller N. Attitude and life commitments or older siblings of mentally retarded adults: An exploratory study. Mental Retardation. 1977;15:38–41. [PubMed] [Google Scholar]
  8. Cuskelly M, Gunn P. Sibling relationships of children with Down syndrome: Perspectives of mothers, fathers, and siblings. American Journal on Mental Retardation. 2003;108:234–244. doi: 10.1352/0895-8017(2003)108<234:SROCWD>2.0.CO;2. [DOI] [PubMed] [Google Scholar]
  9. Deary IJ, Whalley LJ, Lemmon H, Crawford JR, Starr JM. The stability of individuals differences in mental ability from childhood to old age: Follow-up of the 1932 Scottish Mental Survey. Intelligence. 2000;28:49–55. [Google Scholar]
  10. Duncan OD. A socio-economic index for all occupations. In: Reiss AJ Jr, editor. Occupations and social status. New York: Free Press; 1961. pp. 109–138. [Google Scholar]
  11. Edgerton RB. The cloak of competence. Berkeley: University of California Press; 1967. [Google Scholar]
  12. Esbensen AJ, Seltzer MM, Krauss MW. Stability and change in health, functional abilities and behavior problems among adults with and without Down syndrome. American Journal on Mental Retardation. doi: 10.1352/0895-8017(2008)113[263:SACIHF]2.0.CO;2. in press. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Farmer A, Harris T, Redman K, Sadler S, Mahmood A, McGuffin P. Cardiff Depression Study: A sib-pair study of life events and familiarity in major depression. British Journal of Psychiatry. 2000;176:150–155. doi: 10.1192/bjp.176.2.150. [DOI] [PubMed] [Google Scholar]
  14. Farmer A, Mahmood A, Redman K, Harris T, Sadler S, McGuffin P. A sib-pair study of the Temperament and Character Inventory Scales in Major Depression. Archives of General Psychiatry. 2003;60:490–496. doi: 10.1001/archpsyc.60.5.490. [DOI] [PubMed] [Google Scholar]
  15. Farmer A, Redman K, Harris T, Mahmood A, Sadler S, Pickering A, et al. Neuroticism, extraversion, life events, and depression: The Cardiff Depression Study. British Journal of Psychiatry. 2002;181:118–123. doi: 10.1017/s0007125000161823. [DOI] [PubMed] [Google Scholar]
  16. Fujiura GT. Continuum of intellectual disability: Demographic evidence for the “Forgotten Generation. Mental Retardation. 2003;41:420–429. doi: 10.1352/0047-6765(2003)41<420:COIDDE>2.0.CO;2. [DOI] [PubMed] [Google Scholar]
  17. Gatz M, Hurowicz ML. Are old people more depressed? Cross-sectional data on Center for Epidemiological Studies Depression Scale factors. Psychology and Aging. 1990;5:284–290. doi: 10.1037//0882-7974.5.2.284. [DOI] [PubMed] [Google Scholar]
  18. Greenberg JS, Seltzer MM, Orsmond GI, Krauss MW. Siblings of adults with mental illness of mental retardation: Current involvement and expectations of future caregiving. Psychiatric Services. 1999;50:1214–1219. doi: 10.1176/ps.50.9.1214. [DOI] [PubMed] [Google Scholar]
  19. Gregory RJ. Psychological testing: History, principles, and applications. 2nd. Boston: Allyn & Bacon; 2004. [Google Scholar]
  20. Hauser RM, Sewell WH. Birth order and educational attainment in full sibships. American Education Research Journal. 1985;32:1–23. [Google Scholar]
  21. Hauser RM, Sheridan J, Warren JR. CDE Working Paper # 98-02. University of Wisconsin-Madison: Center of Demography and Ecology; 1998. Socioeconomic achievements of siblings in the life course: New findings from the Wisconsin Longitudinal Study. [Google Scholar]
  22. Hauser RM, Warren JR. Socioeconomic indexes for occupations: a review, update, and critique. In: Rafferty AE, editor. Sociological methodology. Cambridge, MA: Basil Blackwell; 1997. pp. 177–298. [Google Scholar]
  23. Henmon VAC, Nelson MJ. Educational Progress Bulletin. Vol. 13. Boston: Houghton Mifflin; 1937. The measurement of intelligence. [Google Scholar]
  24. Hodapp RM, Glidden LM, Kaiser AP. Siblings of persons with disabilities: Toward a research agenda. Mental Retardation. 2005;43:334–338. doi: 10.1352/0047-6765(2005)43[334:SSOSEB]2.0.CO;2. [DOI] [PubMed] [Google Scholar]
  25. Hodapp RM, Urbano RC. Kaiser A. Adult siblings of persons with disabilities: Findings from a national survey. Siblings of Individuals with Developmental Disabilities across the Life Span; Symposium conducted at the Gatlinburg Conference on Research and Theory in Mental Retardation and Developmental Disabilities; Annapolis, MD. 2007. Mar, Chair. [Google Scholar]
  26. Jenkins R. Disability and social stratification. British Journal of Sociology. 1991;42:557–580. [PubMed] [Google Scholar]
  27. John OP, Donahue EM, Kentle RL. The “Big Five” inventory: Versions 4a and 5a. Berkeley: University of California, Berkeley, Institute of Personality Assessment and Research; 1991. Jul, [Google Scholar]
  28. Kendler KS, Prescott CA. A population-based twin stuffy of lifetime major depression in men and women. Archives of General Psychiatry. 1999;56:39–44. doi: 10.1001/archpsyc.56.1.39. [DOI] [PubMed] [Google Scholar]
  29. Kessler RC, Berglund PA, Zhao S, Leaf PJ, Kouzis AC, Bruce ML, et al. The 12-month prevalence and correlates of serious mental illness. In: Manderscheid RW, Sonnenschein MA, editors. Mental health, United States, 1996 (DHHS Publication No (SMA) 96-3098. Washington, DC: U.S. Government Printing Office; 1996. pp. 59–70. [Google Scholar]
  30. Marsh DT. Families and mental illness: New directions in profession practice. New York: Praeger; 1992a. [Google Scholar]
  31. Marsh DT. Families and mental retardation: New directions in profession practice. New York: Praeger; 1992b. [Google Scholar]
  32. Masi G, Mucci M, Favilla L, Brovedani P, Millepiedi S, Perugi G. Temperament in adolescents with anxiety and depressive disorders and their families. Child Psychiatry and Human Development. 2003;33:245–259. doi: 10.1023/a:1021408714741. [DOI] [PubMed] [Google Scholar]
  33. Maughan B, Collishaw S, Pickles A. Mild mental retardation: Psychosocial functioning in adulthood. Psychological Medicine. 1999;29:251–366. doi: 10.1017/s0033291798008058. [DOI] [PubMed] [Google Scholar]
  34. Neisser U, Boodoo G, Bouchard TJ, Boykin AW, Brody N, Ceci SJ, et al. Intelligence: Knowns and unknowns. American Psychologist. 1996;51:77–101. [Google Scholar]
  35. Orsmond GI, Seltzer MM. Brothers and sisters of adults with mental retardation: The gendered nature of the sibling relationship. American Journal on Mental Retardation. 2000;105:486–508. doi: 10.1352/0895-8017(2000)105<0486:BASOAW>2.0.CO;2. [DOI] [PubMed] [Google Scholar]
  36. Radloff LS. The CES-D Scale: A self-report depression scale for research in the general population. Applied Psychological Measurement. 1977;1:385–401. [Google Scholar]
  37. Richardson SA, Koller H. Twenty-two years: Causes and consequences of mental retardation. Cambridge: Harvard University Press; 1996. pp. 240–268. [Google Scholar]
  38. Ryff CD. Happiness is everything, or is it? Explorations on the meaning of psychological well-being. Journal of Personality and Social Psychology. 1989;57:1069–1081. [Google Scholar]
  39. Seltzer GB, Begun A, Seltzer MM, Krauss MW. Adults with mental retardation and their aging mothers: Impacts of siblings. Family Relations. 1991;40:310–317. [Google Scholar]
  40. Seltzer MM, Greenberg JS, Krauss MW, Gordon RM, Judge K. Siblings of adults with mental retardation or mental illness: Effects on lifestyle and psychological well-being. Family Relations. 1997;46:395–405. [Google Scholar]
  41. Seltzer MM, Floyd F, Greenberg J, Lounds J, Lindstrom M, Hong J. Life course impacts of mild intellectual deficits. American Journal on Mental Retardation. 2005;110:451–468. doi: 10.1352/0895-8017(2005)110[451:LCIOMI]2.0.CO;2. [DOI] [PubMed] [Google Scholar]
  42. Stoneman Z. Siblings of children with disabilities: Research themes. Mental Retardation. 2005;43:339–350. doi: 10.1352/0047-6765(2005)43[339:SOCWDR]2.0.CO;2. [DOI] [PubMed] [Google Scholar]
  43. Stoneman Z, Brody GH, Davis CH, Crapps JM. Role relations between children who have mental retardation and their older siblings: Observations in three in-home contexts. Research in Developmental Disabilities. 1989;10:61–76. doi: 10.1016/0891-4222(89)90029-2. [DOI] [PubMed] [Google Scholar]
  44. Thorup A, Petersen L, Jeppesen P, Ohlenschlaeger J, Christensen T, Krarup G, et al. Gender differences in yougn adults with first-episode schizophrenia spectrum disorders at baseline in the Danish OPUS study. Journal of Nervous and Mental Disorders. 2007;195:396–405. doi: 10.1097/01.nmd.0000253784.59708.dd. [DOI] [PubMed] [Google Scholar]
  45. Valliant GE, Davis JT. Socio/emotional intelligence and midlife resilience in schoolboys with low tested intelligence. American Journal of Orthopsychiatry. 2000;70:215–222. doi: 10.1037/h0087783. [DOI] [PubMed] [Google Scholar]
  46. Verbrugge LM. The structure of adult friendship choices. Social Forces. 1977;56:576–597. [Google Scholar]
  47. Zetlin AG. Mentally retarded adults and their siblings. American Journal of Mental Deficiency. 1986;91:217–225. [PubMed] [Google Scholar]

RESOURCES