Psychiatric comorbidities of mild intellectual disability in children and adolescents in a clinical setting

Abstract

The aim of this study was to investigate the psychiatric disorders that accompany mild intellectual disability (ID) in school-aged children in a clinical setting. The Kiddie Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime Version interview was conducted with the children with mild ID and their parents to diagnose any comorbid disorders. The mean age of the 111 children that fulfilled the study criteria was 12.09 ± 3.28 years, 59 of them (53.2%) were males, and 80.2% had at least one lifetime comorbid psychiatric diagnosis. Attention deficit hyperactivity disorder (64.9%), oppositional defiant disorder (21.6%), anxiety disorders (18.0%), were the most common comorbidities. The correlates of exhibiting comorbid psychiatric disorder were being male and irritability symptoms in the clinical history. Being aware of the comorbid psychiatric disorders and planning treatment strategies toward all of the diagnoses may help in the adaptation and rehabilitation of children with mild IDs.

Introduction

Intellectual disability (ID) is a neurodevelopmental disorder characterized by mental ability deficits, like problem-solving, reasoning, planning, abstract thinking, judgment, academic learning, and learning from experience (American Psychiatric Association (APA) 2013). The prevalence of ID in the general population is 1–3% (Strømme and Diseth 2000). The majority of people with ID are classified as having mild ID (National Academies of Sciences, Engineering, and Medicine, 2015), representing 85% of those individuals (Sadock et al. 2014). A mild ID is a specifier of ID, and individuals with mild ID are slower in all areas of conceptual development and social and daily living skills (APA 2013). In the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) the ID severity levels were based only on the intelligence quotient (IQ) categories, and the approximate IQ range for mild ID was defined as 50–69 (American Psychiatric Association (APA) 1994). However, the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) has classified the severity based on the daily skills, and it describes the mild ID as an individual who “can live independently with minimum levels of support” (APA 2013).

Children and adolescents with ID are a high-risk group for mental health problems (Hatton et al. 2017), and previous studies have shown the diagnostic overshadowing of comorbid psychiatric diagnoses when an individual is diagnosed with an ID (Reiss et al. 1982, Manohar et al. 2016, Perry et al. 2018). However, today it is known that the co-occurrence rate of mental problems among children and adolescents with ID is as high as three to four times greater than the rate among their typically developing peers (APA 2013). A relatively recent systematic review regarding the comorbidity of IDs and mental disorders in children and adolescents reported comorbidity rates for children and adolescents between 30% and 50%, with a relative risk of a mental disorder associated with an ID ranging from 2.8 to 4.5 (Einfeld et al. 2011). A recent study by Platt et al. (2018) assessed the ID prevalence in a population-representative sample of US adolescents, and they found an ID prevalence of 3.2%. Among those adolescents with ID, 65.1% met the lifetime criteria for a mental disorder. However, those researchers took into account all of the IDs, including mild to severe.

The majority of the ID comorbidity studies have handled all types of IDs. Nevertheless, in child and adolescent psychiatry clinical settings, the youths with mild ID constituted a large part of the referrals, and most of the referral’s reason is for governmental issues. Especially in the Ministry of Health hospitals, the disability reports are handled by a committee of doctors that includes a child and adolescent psychiatrist. Sometimes, the parents only need to be gotten the disability report, but nothing else, which may cause diagnostic overshadowing.

For individuals with mild ID, there may be insignificant obvious conceptual differences (APA 2013).Sometimes, they are referred to child and adolescent psychiatry clinics with behavioral disturbances or learning difficulties, which are common among individuals with ID and their disability, is detected afterward. In their study, Pogge et al. (2014) investigated previously undetected intellectual disabilities in consecutively referred child and adolescent psychiatric inpatients over a 10-year period. Of the 2621 cases that had completed the Wechsler Intelligence Scale for Children (WISC)-IV, 16.1% earned a Full-Scale IQ of less than 70, and only one of them had a previous ID diagnosis. Therefore, it is obvious that comorbid psychiatric disorders increase the morbidity of IDs.

However, there is a need to examine outpatients with mild ID in terms of the frequency of psychiatric disorders. Therefore, in this study, we aimed to evaluate the comorbid psychiatric diagnoses of consecutively referred cases with mild ID by using a semi-structured interview, the Kiddie Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime Version (K-SADS-PL).

Method

Participants

Those patients with mild ID between the ages of 6 and 17 years old that were referred to the Faculty of Medicine for 6 months starting on 1 July 2017 were included in this study. The inclusion criteria were as follows: the patient was referred to the hospital with their mother or father, and a clinically detected mild ID was confirmed with a standardized test. The exclusion criteria were as follows: younger than 6 years old and older than 18 years old, did not refer to the hospital with his mother or father, and the standardized test applied was below a total score of 50 points or above 70 points. Patients with a history of cancer, progressive neurologic diseases or severe head injury were excluded.

Data collection tools

K-SADS-PL

The K-SADS-PL is a semi-structured interview developed by Kaufman et al. (1997) that aims to diagnose psychotic, affective, anxiety, elimination, disruptive behavior, substance use, eating, and tic disorders. Gökler et al. (2004) conducted the reliability and validity studies of the K-SADS-PL for the Turkish population.

WISC-revised

The WISC (Wechsler 1974) was revised (WISC-R) in 1974, and the scale is administered to children aged 6–16 years old. This scale was adapted to Turkish by Savaşır and Şahin (1995). It consists of 10 core subtests and 2 supplemental subtests organized into verbal scales (Information, Similarities, Arithmetic, Comprehension, Vocabulary, and Digit Span) and performance scales (Picture Completion, Picture Arrangement, Block Design, Object Assembly, Coding, and Mazes). The WISC-R provides scores for the Verbal IQ, Performance IQ, and Full-Scale IQ, as well as standard scores for these subtests.

Ankara Developmental Screening Inventory (AGTE)

The AGTE inventory is used for the evaluation of the developmental aspects of children aged 0–6 years old. It was tested for validity and reliability in Turkey (Savasır et al. 2005). In this inventory, speech development, cognitive skills, fine muscle development, gross muscle development, social development, and self-care skills are observed and scored.

Wechsler Adult Intelligence Scale-Revised (WAIS-R)

The Turkish standardization of the WAIS-R was conducted by Sezgin et al. (2014) as a pilot study. The WAIS-R is administered as 11 subtests. Information, Digit Span, Vocabulary, Arithmetic, Comprehension, and Similarities are the subscales of the verbal scale, and Picture Completion, Picture Arrangement, Block Design, Object Assembly, and Coding are the subtests of the performance scale.

Procedure

The case histories were taken from the mother and/or the father who presented with the child. The K-SADS interview was conducted with the child and their parents after the clinical pre-diagnosis of a mild ID was supported by standardized mental state examination tests. The developmental and intellectual evaluations of the cases are conducted by using WISC-R, WAIS or AGTE. The 6–16 years old patients that could answer the questions are evaluated with WISC-R, the olders are evaluated with WAIS. The children with poor language skills are evaluated with AGTE. The K-SADS questions were also addressed in those cases with good verbal skills. Thus, 104 (93.7%) children used language skills fluently, 3 of them (2.7%) could only produce short sentences, and 3 of them (2.7%) could only produce single words [two of them had autism spectrum disorder (ASD) diagnoses and one presented due to a language delay]. In total, six (5.4%) of the cases could not answer the K-SADS questions. Only the parents’ opinions were taken into account in diagnosing the comorbid psychiatric disorders in those cases.

Irritability was accepted as a tendency toward a negative affective state, usually anger, which was expressed as a temper outburst (Stringaris et al. 2012). The parents were asked if their children got angry very quickly and experienced temper outbursts in the interview conducted by the child and adolescent psychiatrist in this study.

The approval for this study was obtained from the Ethics Committee of the Ankara Yildirim Beyazit University Faculty of Medicine, Yenimahalle Training and Research Hospital in Ankara, Turkey. Informed consents are taken from the parents.

Statistical analyses

The age of the cases was compared in terms of the genders using the Mann-Whitney U test. The other clinical and sociodemographic characteristics were examined using the chi-squared test. The Spearman correlation test was used to analyze the correlation between the age and number of comorbidities. Finally, a binary logistic regression was conducted to identify the correlates of having a comorbid psychiatric diagnosis.

Results

Of the 111 patients who were referred during the 6-month period of the study, 59 (53.2%) of them were males. The mean age of the cases was 12.1 ± 3.2 years old (range: 6–17). When their reasons for presentation were analyzed, 49 of them (44.1%) presented due to repetitions in their disability reports, which they used to obtain special education funded by the government. Thirty-three of them (9.7%) were referred due to course failures, 20 (18.0%) due to irritability, 6 (5.4%) due to hyperactivity and inattentiveness, 2 (1.8%) due to delays in language skills, and 1 (0.9%) due to separation anxiety. Based on these ratios, just 26 of the cases (26.1%) were referred due to psychiatric complaints. Totally, 59.5% of the cases had irritability according to the parents report, 17.1% of them had self-injurious behaviors. The sociodemographic and clinical characteristics of the cases are presented in Table 1.

Table 1.

Sociodemographic, clinic, and referral characteristics of the cases with mild intellectual disability.

		Male (n = 59)	Female (n = 52)	Total (n = 111)	X2 or Z	p
Age		11.6 ± 3.3	12.6 ± 3.1	12.1 ± 3.2	−1.478	.13
Referral complaint	Renew the disability report	25 (42.4%)	24 (46.2%)	49 (44.1%)	0.160	.68
	Failure in school	15 (25.4%)	18 (34.6%)	33 (29.7%)	1.118	.29
	Irritability	11 (18.6%)	9 (17.3%)	20 (18.0%)	0.033	.85
	Hyperactivity	6 (10.2%)	0 (%)	6 (5.4%)	5.590	.01
	Language delay	2 (3.4%)	0	2 (1.8%)	0.033	1.00
	Separation anxiety	0	1 (1.9%)	1 (1.9%)	1.145	.28
Irritability		38 (64.4%)	28 (53.8%)	66 (59.5%)	1.279	.33
Self-injurious behaviors		8 (13.6%)	11 (21.2%)	19 (17.1%)	1.124	.32
Physical illness		10 (16.9%)	15 (28.8%)	25 (22.5%)	2.242	.17
Special education		47 (79.7%)	36 (69.2%)	83 (74.8%)	1.594	.27
Kindergarten history		31 (52.5%)	21 (40.4%)	52 (46.8%)	1.641	.25
Epilepsy		3 (5.1%)	3 (5.8%)	6 (5.4%)	0.025	.59
Literacy		36 (61.0%)	41 (78.8%)	77 (69.4%)	4.135	.04

Comorbid diagnoses

The most common comorbid psychiatric disorder was attention deficit and hyperactivity disorder (ADHD, n = 72, 64.9%). Oppositional defiant disorder (ODD, n = 24, 21.6%), anxiety disorder (n = 20, 18.0%), enuresis nocturna (n = 18, 16.2%), conduct disorder (CD) (n = 12, 10.8%), and depressive disorder (n = 7, 6.3%) were the other most common comorbid diagnoses. All of the comorbid psychiatric disorders are listed in Table 2.

Table 2.

The K-SADS diagnoses of the cases with mild ID.

Comorbid psychiatric disorder	Male (n = 59)	Female (n = 52)	Total (n = 111)	X2	p
No comorbidity	6 (10.2%)	16 (30.7%)	22 (19.8%)	7.381	.007
ADHD	46 (78.0%)	26 (50.0%)	72 (64.9%)	9.485	.003
ODD	17 (28.8%)	7 (13.5%)	24 (21.6%)	3.844	.05
Anxiety disorders	8 (13.6%)	12 (23.1%)	20 (18.0%)	1.695	.22
Enuresis nocturna	12 (20.3%)	6 (11.5%)	18 (16.2%)	1.576	.30
CD	9 (15.3%)	3 (5.8%)	12 (10.8%)	2.579	.13
Depressive disorders	2 (3.4%)	5 (9.6%)	7 (6.3%)	2.744	.14
Stuttering	4 (6.8%)	3 (5.8%)	7 (6.3%)	0.048	1.000
Encopresis	1 (1.7%)	2 (3.8%)	3 (2.7%)	0.486	.59
ASD	4 (6.8%)	0 (%)	4 (3.6%)	3.657	.05
OCD	3 (5.1%)	1 (1.9%)	4 (3.6%)	0.795	.37
PTSD	2 (3.4%)	2 (3.8%)	4 (3.6%)	0.017	.89
Psychotic disorders	1 (1.7%)	2 (3.8%)	3 (2.7%)	0.486	.48
Mania	1 (%)	(%)	1 (0.9%)	1.527	.21
Eating disorders	0 (%)	1 (1.9%)	1 (0.9%)	1.145	.28

K-SADS: Kiddie Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime Version; ADHD: attention deficit hyperactivity disorder; ODD: oppositional defiant disorder; CD: conduct disorder; ASD: autism spectrum disorder; OCD: obsessive-compulsive disorder; PTSD: post-traumatic stress disorder.

Out of all of the cases, 22 of them (19.8%) had no comorbid psychiatric disorders [80.2% of the cases (n = 89) had at least one comorbid diagnosis (X² =7.381, p = .007)], 44 (39.6%) had one, 29 (26.1%) had two, 12 (10.8%) had three, 2 (1.8%) had four, and 2 (1.8%) had five comorbid diagnoses. There was no significant correlation between the age and the number of diagnosis (r = 0.011, p = .912).Seventy-two (64.9%) of the cases had comorbid ADHD, and 10 (13.9%) had an additional CD. Two of the CD cases had no ADHD. Specific phobia, social anxiety disorder, separation anxiety disorder, and generalized anxiety disorder made up the “anxiety disorders.”

Comorbid psychiatric diagnosis in children and adolescents with mild ID are examined according to age. There was no difference between the children <12 years old and ≥12 years old in terms of the comorbid psychiatric diagnoses (Table 3). The frequencies of the disorders were similar.

Table 3.

Comorbid psychiatric diagnosis in children and adolescents with mild ID according to age.

Comorbid psychiatric disorder	<12 years (n = 48)	≥12 years (n = 63)	Total (n = 111)	X2	p
ADHD	33 (68.8%)	39 (61.9%)	72 (64.9%)	0.560	.45
ODD	17 (35.4%)	7 (11.1%)	24 (21.6%)	9.497	.002
Anxiety disorders	8 (16.7%)	12 (19.0%)	20 (18.0%)	0.105	.74
Enuresis nocturna	10 (20.8%)	8 (12.7%)	18 (16.2%)	1.327	.24
CD	3 (6.3%)	9 (14.3%)	12 (10.8%)	1.824	.17
Depressive disorders	4 (8.3%)	3 (4.8%)	7 (6.3%)	0.160	.68
Stuttering	2 (4.2%)	5 (7.9%)	7 (6.3%)	0.655	.41
Encopresis	1 (2.1%)	2 (3.2%)	3 (2.7%)	0.123	.72
ASD	2 (4.2%)	2 (3.2%)	4 (3.6%)	0.077	.78
OCD	0 (%)	4 (6.3%)	4 (3.6%)	3.162	.07
PTSD	0 (%)	4 (6.3%)	4 (3.6%)	3.162	.07
Psychotic disorders	0 (%)	3 (4.8%)	3 (2.7%)	2.349	.12
Bipolar disorder	0 (%)	1 (1.6%)	1 (0.9%)	–	N/A
Eating disorders	1 (2.1%)	0 (%)	1 (0.9%)	–	N/A

K-SADS: Kiddie Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime Version; ADHD: attention deficit hyperactivity disorder; ODD: oppositional defiant disorder; CD: conduct disorder; ASD: autism spectrum disorder; OCD: obsessive-compulsive disorder; PTSD: post-traumatic stress disorder.

A binary logistic regression was conducted to identify the correlates of having a comorbid psychiatric diagnosis. Maleness [odds ratio (OR)=0.22, p = .01] and irritability (OR = 66.24, p < .0001) in the case history were identified as predictors of comorbid psychiatric disorders for mild ID in the children and adolescents (Table 4).

Table 4.

The correlates of having a comorbid disorder in cases with a mild Intellectual disability.

	B	OR	p	CI
Age	−0.012	0.988	.906	0.809–1.207
Gender	−1.515	0.220	.018	0.063–0.770
Irritability	4.193	66.24	<.0001	7.917–554.279
Physical disorder	−0.898	0.407	.109	0.098–1.700

OR: odds ratio; CI: confidence interval.

Discussion

The comorbid psychiatric diagnoses of the cases with mild ID that were referred over 6 months were investigated in this study. Of all the patients, 80.2% of the cases had at least one lifetime comorbid psychiatric diagnosis. Several studies investigated the prevalence of comorbidities; for example, in a systematic review conducted by Einfeld et al. (2011), the studies were divided into two groups according to whether they had a non-ID control group or not. In Dekker et al.’s study (2002), the prevalence of having a comorbid psychiatric disorder was 50% in the ID sample, while it was 18% in the non-ID sample. This prevalence was 36% to 8%, respectively, in Emerson and Hatton’s study (2007). A study conducted in Norway indicated the ID frequency as 0.62% in the general population, and the comorbidity ratio was 37% for the ID sample (Strømme and Diseth 2000). In those studies, the sample selections, assessment devices, and methodology varied. It would be expected that the clinical populations were more likely to exhibit psychiatric comorbidities than the general population. After all, Platt et al. (2018) found that among the adolescents with IDs, 65.1% met the lifetime criteria for a mental disorder in their population-based study. The rate in our study was higher, probably because of the instrument that was used and because of the sample that was ascertained from the clinical setting.

The correlates of having at least one comorbid psychiatric disorder were maleness and irritability symptoms in the clinical history. Thus, it seems important to ask about irritability in ID cases. Irritability was not a referral complaint in most of our cases, but 59.5% of the parents reported irritability symptoms in their children with mild ID when the clinician asked about such a history. Irritability is defined as a low frustration tolerance characterized by anger and temper outbursts (Brotman et al. 2006); however, it is not assumed to be a specific ID symptom. It is also seen in depression, anxiety, ADHD, and mood disorders (Ceylan et al. 2012). In our study, those psychiatric disorders that were comorbid with mental retardation may have caused the irritability.

The referral reasons for most of the cases were to obtain a disability report in order to benefit from government, educational, financial, or tax-related support. The second most common referral reason was course failure. Just 26 of the cases (26.1%) were referred due to psychiatric complaints. However, when asked, irritability was present in 59.5% of the cases. This situation confirms the diagnostic overshadowing not only for the clinicians but also for the parents. The parents may believe that the behavioral disturbances they encounter are due to the ID, and they can accept this situation. Thus, clinicians should be aware of the high psychiatric comorbidity in mild ID children and adolescents in the clinical setting, and they should ask about the presence of psychiatric symptoms.

The most common comorbid diagnosis was ADHD for both boys and girls with mild ID. However, the boys had significantly higher ADHD rates since 78% of the boys fulfilled the diagnostic criteria for ADHD. One former study suggested that ADHD is more common in children with ID, and that the risk increases with an increasing ID severity (Voigt et al., 2006). Those researchers reported that 30% of the borderline to mild ID individuals fulfilled the ADHD criteria; this ratio was 6.4% for the non-ID individuals and 66.6% of the individuals with ADHD and ID were males. ADHD is another developmental disorder that frequently accompanies ID. Overall, the girls with mild IDs were more literate than the boys, which may be due to the higher ADHD rates of the boys with mild IDs.

The diagnosis of ODD is broadly based on a frequent and persistent anger or irritable mood, argumentativeness/defiance, and vindictiveness (APA 2013). ODD was present in 35% of the children under the age of 12 years and 11% of the children over 12 years of age. The ability of young children with ID to express themselves is low, but it grows as their age rises. For this reason, there may have been a decrease in these negative behaviors. ADHD and ODD comorbidities have been seen at high rates in several studies (Park et al. 2017).Another reason for the high ODD comorbidity in the young children with mild ID was the high level of the ADHD comorbidity in our study.

Anxiety disorder was the third highest comorbid disorder among the mild ID cases. In Dekker and Koot’s study (2003) of 474 random ID students from 7–20 years old, 22% of the ID sample also met the criteria for an anxiety disorder. Any distress disorder, including social anxiety disorder (SAD), post-traumatic stress disorder (PTSD), major depressive disorder/dysthymia, and generalized anxiety disorder, was diagnosed in 22.6% of the ID sample in Platt et al.’s study (2018). Stoddard et al. (2014) showed that the parent or self-reported irritability was higher in the youths with anxiety disorders than in the healthy controls. Thus, irritability symptoms may be associated with anxiety, especially in ID children and adolescents. However, in the future, higher sample-sized studies are needed to reveal this association in ID individuals.

Enuresis nocturna was the fourth most common comorbid disorder among the cases with mild ID, and 16.2% of the cases had comorbid enuresis. The average age of the ID children in our study was about 12 years old, and the incidence of enuresis nocturna in the normal population of a similar age group was 2–3% (Spee-van der Wekke et al. 1998). Enuresis is a frequent disorder in children with special needs, and coexisting incontinence can have an additive negative effect on the well-being and quality of life of the patient, affecting his or her daily and family functioning (von Gontard et al. 2011). In Basiri et al.’s study (2017), the boys with primary nocturnal enuresis had lower IQs when compared with the healthy control group, but only in a low-income district. Thus, they suggested adjusting the results of the IQ assessment in these children according to their socioeconomic status.

CDs were also frequent in the mild ID cases that were often accompanied by ADHD. Only two of the cases had CDs but no ADHD diagnoses. There are problems of impulse control in ADHD (American Psychiatric Association (APA) 2013), with impulsiveness being the most crucial personality dimension that predicts antisocial behavior (Lipsey and Derzon 1998). Platt et al. (2018) concluded that those individuals with IDs had higher CD rates than those without IDs (9.1% vs. 4.2%, respectively). These rates were 20.5% vs. 4.3% in Emerson and Hatton’s study (2007) (ID sample vs. non-ID sample).This comorbidity may cause social interaction failures and may contribute negatively to the social acceptance of these individuals.

Based on the cases with no comorbidities, the rates of the girls with mild IDs were significantly higher. When the comorbid diagnoses were interpreted in terms of the difference between the genders, ADHD and ASD were significantly more frequent in boys. Strømme and Diseth (2000) found significantly higher psychiatric disorder prevalence rates among the males with ID. However, Molteno et al. (2001) suggested no difference between the genders in terms of comorbidity. This difference between the studies may due to the instruments that were used for the diagnoses or the characteristics of the samples.

There was no statistically significant difference between the children and adolescents in terms of the comorbidities that we evaluated with the K-SADS-PL, with the exception of ODD. School-aged children and adolescents with mild IDs have poor academic degrees. In children and adolescents with milder forms of ID, a negative self-image, low self-esteem, poor frustration tolerance, interpersonal dependence, and rigid problem-solving style are common (Sadock et al. 2014). All of these may increase the frequency of depressive disorders and anxiety disorders in individuals with mild IDs. Emerson and Hatton (2007) reported that older children with IDs (aged 11–16 years) were more likely to be identified as having emotional disorders, and younger children (aged 5–10 years) were more likely to exhibit hyperactivity. However, this was not true for our study. ADHD is a lifelong disorder; thus, there was no difference between the children and adolescents with regard to ADHD. The frequency of ADHD was significantly higher than the frequencies of all of the other disorders. This discrepancy between the studies may have been due to the sample sizes.

Identifying the comorbid psychiatric disorders in children with mild ID could help us to support them better by selecting the appropriate psychotherapeutic method (Collins 2015, Oshodi and Turk 2017). Additionally, increasing the parental well-being and coping capacity are essential in approaching to ID cases (Horsley and Oliver 2015). Determining and treating the comorbid psychiatric disorders in mild ID cases should help in increasing the coping capacity of the parents.

In the case of mild ID-diagnosed patients, detailed structured psychiatric interviews with both the family and children could increase the reliability of our work, even if the families have no complaints. Along with this, our study did have a few limitations. First of all, the sample size was small. Second, despite the fact that most of the patients presented in order to obtain a report, this was not a population-based study. Thus, another limitation was the inclusion of patients referred for outpatient evaluations. The study was dependent on referrals and did not compare with children having borderline I.Q./normal children. Also, the severity of behavioral problems is not evaluated in this study, thus, the correlation of the severity of behavior problems with IQ scores did not evaluated.

Conclusion

Psychiatric comorbidities can be seen at a rate as high as 80% in mild mental retardation cases. It seems that the referral complaints do not reflect the comorbid psychiatric diagnoses, and most likely, the parents are ignoring the psychiatric symptoms. However, if the clinician asks, the parents do talk about the psychiatric symptoms; therefore, the clinicians should ask about the most common psychiatric symptoms among the mild ID cases, even if there are no complaints. Relieving the comorbid psychiatric disorders may help individuals with mild ID to better adapt to social and occupational environments.

Disclosure statement

The authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Author contributions

STH and MFC conceived of the study, acquired the data, performed the statistical analysis and interpreted the data. MK and CPY were involved in the conception and design of the study. All authors read and approved the final manuscript.