Abstract
Background:
Sluggish cognitive tempo (SCT) is associated with—but distinct from—attention-deficit/hyperactivity disorder (ADHD). This study examined SCT rates in adult outpatients seeking an ADHD assessment, differences in rates based on ADHD status, impact of assessment method (i.e., reporting source, symptom count, and functional impairment), and age of SCT symptom onset.
Methods:
Outpatients (n=124) completed an SCT measure (n=120 other-reporters). SCT was based on reporting source (i.e., self-report, other-report, either reporting source [“or” rule], and both reporting sources [“and” rule]), symptom count (i.e., age-based norms at or near the 93rd percentile, and a higher symptom count threshold of five), and functional impairment (i.e., 0, 1, and ≥2 domains).
Results:
SCT rates varied based on assessment method for the full sample (26%-82%) and among those with (32%-91%) and without (16%-66%) ADHD. Rates decreased with stricter functional impairment and symptom count criteria. SCT was higher in the ADHD group than the non-ADHD group based on other-reporters and the “or” rule, but not the “and” rule. Functional impairment and symptom count criteria did not impact these comparisons. For self-reported SCT rates, ADHD/non-ADHD group comparisons did not differ based on age-based symptom count threshold, but did with a symptom count threshold of five. Self-reported SCT symptom onset was 13.36 years-old and was significantly younger for the ADHD group (11.69 years) than the non-ADHD group (16.36 years).
Conclusions:
Elevated SCT symptoms and related impairment are common among adults seeking an ADHD evaluation. These rates and ADHD/non-ADHD group differences vary substantially based on diagnostic methods.
Keywords: Sluggish Cognitive Tempo, Attention-Deficit/Hyperactivity Disorder
Introduction
Sluggish cognitive tempo (SCT) refers to a cluster of symptoms not represented in the Diagnostic and Statistical Manual of Mental Disorders (DSM; American Psychiatric Association [APA], 2013) that are separate from those identified in attention-deficit/hyperactivity disorder (ADHD), including excessive daydreaming, mental confusion, and sluggish-lethargic behavior (see Becker et al., 2016, for a review). In child and adult samples, SCT symptoms are associated with negative functional outcomes after controlling for ADHD symptoms. For example, in a population-based longitudinal sample of children followed for 10 years, SCT symptoms significantly predicted internalizing features, reading underachievement, and global social difficulties after considering the impact of inattentive ADHD symptoms (Becker, Burns, Leopold, Olson, & Willcutt, 2018).
Studies examining SCT in young adult and adult samples have primarily examined SCT symptoms dimensionally. Among young adults, SCT symptoms are associated with executive functioning problems and functional impairment after accounting for ADHD, anxiety, and depression symptoms (Flannery, Luebbe, & Becker, 2017; Wood, Lewandowski, Lovett, & Antshel, 2017). Other studies of young adults have demonstrated that SCT symptoms are associated with higher behavioral inhibition, neuroticism, and a facet of behavioral approach tendencies, and negatively associated with extraversion and conscientiousness (Becker, Schmitt, Jarrett, Luebbe, Garner et al., 2018). Among adult outpatients seeking an ADHD assessment, SCT symptoms are associated with functional impairment in multiple domains after considering ADHD symptoms (Lunsford-Avery, Kollins, & Mitchell, 2018). In one clinical sample of adults with ADHD, more severe SCT symptoms was associated with more depression, anxiety, and persistent inattention, as well as professional and relational impairment (Kamradt, Momany, & Nikolas, 2018). Other studies of adults with ADHD report similar findings. For example, SCT symptoms are associated with internalizing symptoms, inattentive symptoms, and executive functioning deficits—the latter only applied to a subset receiving stimulant treatment (Leikauf & Solanto, 2017).
While SCT has been characterized as a dimensional construct, some have suggested that SCT may be a distinct disorder (Barkley, 2013) or syndrome (Becker & Barkley, 2017). Consistent with this, studies have attempted to classify SCT groups (e.g., Servera, Saez, Burns, & Becker, 2018; Unsel-Bolat et al., 2019) and have generally relied upon parent- or self-reported SCT symptoms that meet a symptom cut-off threshold. However, there are no widely agreed upon SCT classification or assessment criteria, which may result in variability in SCT assessment approaches and heterogeneity in samples across studies. Studies are needed to inform how different factors may impact SCT classification. Reporting source, symptom count threshold, and inclusion of functional impairment are likely relevant factors. Using the positively correlated yet distinct symptoms of ADHD as an example, discrepancies between self and other-reporters often result in significant differences in reporting ADHD symptoms among young adults (Sibley et al., 2012). Similarly, including different symptom count and functional impairment criteria impacts diagnostic rates (Sibley, Mitchell, & Becker, 2016). These factors may also be relevant for SCT assessment.
Another issue relevant to SCT classification is the extent to which it occurs in outpatient samples. In the general adult population, SCT prevalence is 5.1% (Barkley, 2012). However, no studies to date have examined rates in adult psychiatric outpatients. In clinical practice, it is unclear how often adult outpatients present with SCT, and therefore, how many adult outpatients have potentially unmet treatment needs associated with SCT. Relatedly, although factor analytic studies have demonstrated SCT and ADHD symptom clusters can be differentiated in adults (Becker, Burns, Garner, et al., 2018), studies are needed to examine to what extent SCT occurs in adults with and without ADHD.
Studies are also needed to understand age of SCT symptom onset. While SCT has been proposed to be similar to ADHD as a disorder characterized by inattentiveness (Barkley, 2012), it is unclear if they share certain defining features, particularly childhood symptom onset. Specifically, diagnostic criteria for ADHD (APA, 2013) requires the onset of at least “several” symptoms before age 12 years. However, no studies to our knowledge have reported on the age of onset for SCT symptoms.
The overall aims of the current study were to examine the following in an adult psychiatric outpatient sample seeking an ADHD assessment: (a) SCT rates in the full sample, (b) SCT rates between outpatients with and without ADHD; (c) impact of reporting source, symptom count threshold, and functional impairment on SCT rates; and (d) age of SCT symptom onset in the full sample and among those with and without ADHD. Based on findings reviewed above that demonstrate how SCT symptoms are related to ADHD symptoms yet form a distinct factor of symptoms, we predicted that SCT would be present in both groups, but that the ADHD group would yield higher SCT rates. We also predicted a high level of variability among SCT rates based on the inclusion of criteria involving reporting source, symptom count, and functional impairment.
Material and Methods
Participants and Procedure
Adults (n = 124) aged 18-67 years (M = 31.13, SD = 11.47; 50% female; 72% Caucasian; n = 80 ADHD and n = 44 non-ADHD clinical controls) seeking an ADHD evaluation from an outpatient university medical center clinic participated. Full details about the sample are reported in Lunsford-Avery et al. (2018). Self- and other-report versions of an SCT scale were completed prior to a diagnostic assessment. Other-report was available for 120 participants (43% were partners, 38% parents, 12% friends, 6% “other,” and 3% siblings).1 All participants completed the CAADID Parts I and II (Epstein et al., 2001) to assess for ADHD, a computerized psychiatric screener of symptoms required for a diagnosis with follow-up interviewing to assess full DSM (APA, 2013) criteria for the respective disorder (modules assessed for depressive, bipolar, anxiety, substance use, and eating disorders) adapted from the Mini International Neuropsychiatric Interview (Sheehan et al., 1998) for DSM-5 diagnoses, and review of medical/school records and prior psychiatric and/or psychoeducational reports as available (see Lunsford-Avery et al., 2018 for additional details).
Full details of the sample are listed in Lunsford-Avery et al. (2018). In brief, among the ADHD sample, 36 met criteria for combined presentation, 40 for inattentive presentation, 1 for hyperactive-impulsive presentation, and 3 for unspecified ADHD. Among the non-ADHD sample, 37 met criteria for other DSM diagnoses and 7 did not meet criteria for any DSM diagnosis. Rates of comorbidity were 53% (n = 42) for the ADHD group and 39% (n = 17) for the non-ADHD group.
This study was carried out in accordance with the latest version of the Declaration of Helsinki and was approved by a local IRB. Informed consent of participants was obtained after the nature of the procedures had been fully explained.
Measures
Self- and other-reported SCT symptoms over the past 6 months were assessed using the 9-item Barkley Adult ADHD Rating Scale (BAARS) SCT subscale (Barkley, 2011). Response options ranged from 1 (“never or rarely”) to 4 (“very often”). SCT symptom endorsements were based on ratings of 3 (“often”) or 4 (“very often”). Raters also indicated SCT symptom-related impairment in five different domains (i.e., home, social, school, community, and work) in a binary response format (i.e., “yes” or “no”) and retrospectively estimated age of SCT symptom onset. ADHD symptoms from the BAARS were removed so responses for all impairment and age-of-onset items would pertain to SCT symptoms.
Analyses
SCT rates were calculated based on symptom reporting source, symptom count threshold, and functional impairment. First, symptom reporting source was determined four different ways for each participant: self-report only, other-report only, symptom count threshold was met by either reporter (“or” rule), and symptom count threshold was met by both reporter (“and” rule).
Second, symptom count thresholds were determined two different ways for each participant: age-based norms and a symptom cutoff of ≥5. Symptom count threshold using age-based norms involved use of the 93rd percentile for self-reported symptoms for participants between the ages of 18-39 years and ≥60 years. In cases when the 93rd percentile was not available (i.e., there was no corresponding number of symptoms listed at the 93rd percentile), the 92nd percentile was used—this applied only for participants in the 40-59 years age range. This resulted in symptom count threshold scores of ≥4 symptoms for ages 18-59 years and ≥3 symptoms for ages ≥60 years. Age-based norms were not available for the other-report version of the scale, so the corresponding symptom count threshold for was applied. For symptom cutoff of ≥5, raters had to endorse at least 5 symptoms. This symptom threshold was based on a previous study examining rates of SCT in the general population (Barkley, 2012).
Third, functional impairment criterion was applied three different ways: impairment endorsements in 0, 1, or ≥2 domains. When self-report only for symptoms was considered (i.e., SCT rates when symptom count criterion based on self-report was met, plus impairment in 1 and ≥2 domains), self-report of functional impairment was used. Similarly, when other-report of symptoms was considered, other-report of functional impairment was used. When both reporting sources were used while applying the “or” rule, the same reporter who endorsed symptom count criterion also had to endorse the corresponding functional impairment criterion when SCT was classified when symptom count and impairment at 1 and ≥2 was met. When both reporting sources were used while applying the “and” rule, both reporting sources had to endorse impairment in 1 and ≥2 domains when that requirement for SCT classification was imposed. Figure 1 provides a summary.
Figure 1. Criteria to Determine SCT Status.
SCT rates were determined based on combinations of symptom reporting source (i.e., self-report only, other-report only, either reporting source, and both reporting sources), symptom threshold (i.e., age-based norms and symptom count ≥5), and functional impairment (i.e., no impairment criterion required, ≥1 domain impaired, ≥2 domains impaired). For symptom reporting source, SCT symptom count threshold for age-based calculations was based on the 93rd percentile. When there was no corresponding number of symptoms listed at the 93rd percentile, the 92nd percentile was used—this applied only for participants in the 40-59 years age range. For functional impairment when a domain of impairment was required to determine SCT, the reporting source for impairment had to be the same reporting source for symptom count.
SCT rates for the full sample according to these different methods is reported descriptively. Chi-square analyses were conducted to compare SCT rates between ADHD and non-ADHD clinical control groups. Group differences in SCT symptom onset was compared using ANOVA.
Results
Demographics
SCT rates2 among the full sample did not differ based on sex (SCT: 43% male, 57% female; without SCT: 55% male, 45% female; p = .20), race (SCT: 81% White, 19% non-White; without SCT: 67% White, 33% non-White; p = .098), or ethnicity (SCT: 9% Hispanic, 79% non-Hispanic, 12% not reported; without SCT: 10% Hispanic, 76% non-Hispanic, 14% not reported; p = .93). However, the SCT group tended to be younger than those without SCT (SCT: M age = 27.48, SD = 7.90; without SCT: M age = 32.91, SD = 12.30; p = .011).
SCT Rates: Full Sample
SCT rates ranged from 26% to 82% among the full sample across all assessment methods (Table 1). Regarding reporting source when using age-based norms, SCT rates tended to be highest using the “or” rule (75% to 82%), followed by self-report only (63% to 73%) and other-report only (48% to 54%), and lowest using the “and” rule (35% to 44%). The same trend followed using a symptom cutoff of ≥5, although rates were descriptively lower in comparison to the use of age-based norms. That is, SCT rates were highest using the “or” rule (64% to 68%), followed by self-report only (53% to 59%) and other-report only (37% to 40%), and lowest using the “and” rule (26% to 31%). When looking at age-based norms, SCT rates within each method of symptom assessment decreased going from 0 domains required for SCT to ≥2 domains by a range of six to 10 percentage points. When looking at a symptom cutoff of ≥5, SCT rates within each method of symptom assessment decreased going from 0 domains required for SCT to ≥2 domains by a range of three to six percentage points. .
Table 1.
SCT Rates Based on Reporting Source, Symptom Count Threshold, and Impairment
| Full Sample | Group Comparisons | |||
|---|---|---|---|---|
| ADHD Group | Control Group | p | ||
| Age-Based Norms Symptom Count | ||||
| Self-report Only | ||||
| Symptoms Only | 73% | 78% | 64% | .098 |
| Symptoms and 1 domain impaired | 68% | 71% | 61% | .260 |
| Symptoms and ≥2 domains impaired | 63% | 69% | 52% | .069 |
| Other-report Only | ||||
| Symptoms Only | 54% | 63% | 39% | .009 |
| Symptoms and 1 domain impaired | 51% | 59% | 36% | .016 |
| Symptoms and ≥2 domains impaired | 48% | 55% | 34% | .025 |
| Self-report or Other-report | ||||
| Symptoms Only | 82% | 91% | 66% | <.001 |
| Symptoms and 1 domain impaired | 80% | 88% | 66% | .004 |
| Symptoms and ≥2 domains impaired | 75% | 84% | 59% | .002 |
| Self-report and Other-report | ||||
| Symptoms Only | 44% | 49% | 36% | .190 |
| Symptoms and 1 domain impaired | 38% | 42% | 32% | .264 |
| Symptoms and ≥2 domains impaired | 35% | 40% | 27% | .177 |
| Symptom Count of ≥5 | ||||
| Self-report Only | ||||
| Symptoms Only | 59% | 66% | 46% | .024 |
| Symptoms and 1 domain impaired | 55% | 60% | 46% | .119 |
| Symptoms and ≥2 domains impaired | 53% | 60% | 41% | .041 |
| Other-report Only | ||||
| Symptoms Only | 40% | 49% | 25% | .011 |
| Symptoms and 1 domain impaired | 38% | 46% | 25% | .022 |
| Symptoms and ≥2 domains impaired | 37% | 45% | 23% | .016 |
| Self-report or Other-report | ||||
| Symptoms Only | 68% | 78% | 50% | .002 |
| Symptoms and 1 domain impaired | 65% | 73% | 50% | .012 |
| Symptoms and ≥2 domains impaired | 64% | 73% | 48% | .006 |
| Self-report and Other-report | ||||
| Symptoms Only | 31% | 37% | 21% | .061 |
| Symptoms and 1 domain impaired | 28% | 33% | 21% | .145 |
| Symptoms and ≥2 domains impaired | 26% | 32% | 16% | .059 |
Notes. N = 124 self-report (80 ADHD, 44 control) and 120 other-report (76 ADHD, 44 control). Age-based norms were based on the 93rd percentile. When there was no corresponding number of symptoms listed at the 93rd percentile), the 92nd percentile was used—this applied only for participants in the 40-59 years age range.
SCT Rates: ADHD vs. Non-ADHD Clinical Controls
When parsed into those with and without ADHD, SCT rates tended to be higher in the ADHD group (Table 1). That is, using age-based norms for symptom threshold, SCT rates ranged from 40% to 91% of those with ADHD across all methods of assessment, compared to 27% to 66% of the non-ADHD clinical controls. Statistically significant effects emerged when rates between ADHD and non-ADHD groups were considered using other-reporters across all ways of applying functional impairment criterion (ADHD: 55% to 63%, non-ADHD: 34% to 39%; p’s ranging from .009 to .025). Similarly, group differences emerged when applying the “or” rule regardless of which functional impairment criterion was adopted (ADHD: 84% to 91%, non-ADHD: 59% to 66%; p’s ranging from <.001 to .004). Differences in SCT between ADHD and non-ADHD groups did not differ when self-report or the “and” rule was applied (across all methods of applying functional impairment criterion).
When applying the symptom count threshold of ≥5, a similar trend emerged: SCT rates ranged from 32% to 78% for the ADHD group across all methods of assessment, compared to 16% to 50% of the non-ADHD group. Also similar to the analysis using age-based norms, when using symptom cutoff of ≥5, SCT rates were consistently higher for the ADHD group when using other-reporters (ADHD: 45% to 49%, non-ADHD: 23% to 25%; p’s ranging from .011 to .022) and the “or” rule (ADHD: 73% to 78%, non-ADHD: 48% to 50%; p’s ranging from .002 to .012), but did not differ using the “and” rule—these results were consistent regardless of using functional impairment criterion. The only statistically significant effects between ADHD and non-ADHD groups that emerged when using the symptom threshold ≥5 (that did not emerge for age-based norms) involved use of self-reported SCT symptoms. Rates were higher in the ADHD group when no functional impairment criterion was imposed (66% ADHD, 46% non-ADHD; p = .024) and when ≥2 domains of impairment were required (60% ADHD, 41% non-ADHD; p = .041).
Age of SCT Symptom Onset
Self-reported age of SCT symptom onset was provided by 101 participants (65 ADHD, 36 non-ADHD). The mean age was 13.36 years-old (SD = 8.93). The ADHD group reported an earlier onset than the non-ADHD group (ADHD: M age = 11.69 years, SD = 7.24; non-ADHD: M age = 16.36 years, SD = 10.85; p = .011). Regarding the distribution of SCT symptom onset, 66% of cases in the ADHD group reported childhood onset (≤12 years-old), 12% in adolescence (13 to 17 years-old), and 22% in adulthood (≥18 years-old), whereas 39% in the non-ADHD group reported childhood onset, 28% in adolescence, and 33% in adulthood. Because 39 other-reporters provided an SCT onset estimate (M = 13.59 years, SD = 12.50), we did not examine ADHD and non-ADHD differences.
Discussion
This is the first study, to our knowledge, to establish SCT rates in an adult outpatient psychiatric sample seeking an ADHD assessment and demonstrate how SCT rates vary based on reporting source, symptom count, and functional impairment. Further, SCT occurred at a significantly higher rate in the ADHD group according to some assessment approaches, but not others. This study also demonstrated that adult psychiatric outpatients seeking an ADHD assessment report the onset of SCT symptoms in early adolescence on average, though adults with ADHD report significantly earlier onset (11.69 years-old) than those without ADHD (16.36 years-old). Taken together, these findings can inform SCT assessment and treatment in future research.
SCT rates were descriptively higher with the “or” rule (i.e., either reporting source meeting symptom count threshold). For example, using age-based norms for symptom count thresholds without the functional impairment criterion, SCT rates using the “or” rule were 82% in comparison to relying on self-report only (73%), other-report only (54%), and the “and” rule combining self and other-report (44%). Across the four different combinations for reporting source to determine SCT, SCT rates decreased by up to ten percent when a functional impairment criterion was imposed using age-based norms for symptom count threshold—a similar trend emerged when a symptom count threshold of ≥5 was imposed, although the decrease in percentage points when functional impairment criterion was imposed was smaller. SCT is an increasingly examined topic and as this research literature continues to expand, generalization across studies will be hindered by heterogeneity in how SCT is determined. Building on these findings, future work is needed to establish an empirically-informed consensus on SCT assessment methods. Given findings that other-reported SCT symptoms is a better predictor of impairment than self-reported (Lunsford-Avery, Kollins, & Mitchell, 2018), we recommend inclusion of other reporting sources. In addition, given the high rates of SCT in our study—the lowest rate was 16% using the “and” rule, with a symptom count threshold of ≥5, and ≥2 domains impaired—perhaps a more stringent approach to reporting source combination (i.e., the “and” rule) is warranted to avoid potentially artificially inflated SCT rates.
The current study also demonstrated that SCT rate differences based on ADHD status depend on assessment method. That is, regardless of whether functional impairment is required using age-based norms, SCT was significantly higher in the ADHD group based on other-reporters and use of the “or” rule, but not self-report and use of the “and” rule. A similar trend emerged when a symptom count threshold of ≥5 was imposed (as opposed to age-based norms) with the only exception being differences in SCT rates between those with and without ADHD when using self-report only. While future studies are needed that establish SCT rates that also account for whether another disorder better accounts for SCT symptoms (a common criterion for DSM diagnoses), and how this affects ADHD/non-ADHD group differences, these findings demonstrate that at least 16% to 27% of a non-ADHD clinical control group—depending on the symptom count threshold—exhibit elevated SCT symptoms on a standardized self-report scale. This rate of SCT in non-ADHD adults supports other findings that SCT is unique from ADHD (see Becker et al., 2016, for a review). Future studies are needed to examine the unique functional impact of SCT symptoms and whether treating these symptoms contributes to improved outcomes.
Finally, the current study suggests that onset of self-reported SCT symptoms generally occurs prior to adulthood when assessed retrospectively and that adults with ADHD may report an earlier onset than non-ADHD peers. SCT is not a formal diagnostic entity, though this is currently a consideration (Becker & Barkley, 2017). Defining features of SCT—such as whether it requires a childhood age of onset similar to ADHD and the clinical relevance of such a criterion—will need to be addressed. According to our findings, a majority report that symptoms emerge in childhood or adolescence. Given that SCT has been conceptualized as similar to ADHD in that both involve attentional aspects, we recommend future studies consider age of onset in analyses of SCT samples to inform this issue and the utility of this as a diagnostic criterion.
This study had several limitations. First, we did not assess whether SCT was better accounted for by other psychiatric diagnoses. While the rate of elevated SCT symptoms may be important for treatment planning regardless of SCT diagnostic status, SCT rates in this study are likely impacted by not taking this limitation into account. This limitation also highlights the need for SCT diagnostic guidelines for consideration of other psychiatric disorders in diagnostic decision-making. Second, this study included a heterogeneous clinical control group. Future studies should consider more homogeneous psychiatric control groups and non-clinical control groups. Relatedly, while this study focused on the presence of SCT among those with and without ADHD, future studies should consider SCT’s association with other clinical groups (e.g., mood disorders). Third, the sample for this study was adults seeking an ADHD evaluation and may not generalize to a general outpatient psychiatric sample. Finally, the sample size limited our ability to provide a more fine-grained analysis of racial and ethnic differences in SCT.
In conclusion, SCT rates vary as a function of reporting source, how those reporting sources are combined, and consideration of symptom count threshold and functional impairment. Further, SCT occurs at a higher rate in adults with ADHD in comparison to non-ADHD adults according to some methods of assessment, but not others. For example, other-reporters indicate group differences whereas self-report did not using age-based norms, but this differentiation changes when a symptom count criterion of ≥5 is imposed. Finally, SCT symptom onset occurs in childhood or adolescence for the majority of adult outpatients seeking an ADHD evaluation. Future research is necessary to inform how to best assess SCT and consider the role of these symptoms in treatment.
Supplementary Material
Acknowledgements
Thank you to Kayla McKay and Michelle Lepsch-Halligan for their assistance with data management and to Maggie Sweitzer, Ph.D., and Julia Schechter, Ph.D., for serving as assessment clinicians. We also express our gratitude to the participants and their families for their valuable contribution to research.
Footnotes
Conflicts of Interest
J.T.M. has consulted with Avanir. N.O.D. has received research support from Akili. S.H.K. has received research support and/or consulting fees from the following sources: Adlon Pharmaceuticals, Akili Interactive, Bose Corporation, OnDosis AB, Sana Health, Tris Pharmaceuticals. SHK has stock options in Akili Interactive and equity ownership of Behavioral Innovations Group, LLC. J.R.L.A has no conflicts of interest to report.
The sample of 124 participants reported in Lunsford-Avery et al. (2018) are reported here as well. In Lunsford-Avery et al. 119 other-reporters were used, whereas 120 were included in this study. This involved an other-reporter who did not have a fully completed scale measuring SCT symptoms who was included in this study.
These rates are based on age-based norms, use of the “and” rule, and functional impairment in two or more domains.
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