Parents and teachers make different contributions to a shared perspective on hyperactive-impulsive and inattentive symptoms: a multivariate analysis of parent and teacher ratings on the symptom domains of ADHD

Understanding the effects of neighborhood disadvantage on youth psychopathology

BACKGROUND

In 1942, Shaw and McKay reported that disadvantaged neighborhoods predict youth psychopathology (Shaw & McKay, ). In the decades since, dozens of papers have confirmed and extended these early results, convincingly demonstrating that disadvantaged neighborhood contexts predict elevated rates of both internalizing and externalizing disorders across childhood and adolescence. It is unclear, however, how neighborhood disadvantage increases psychopathology.

METHODS

Our study sought to fill this gap in the literature by examining the Area Deprivation Index (ADI), a composite measure of Census tract disadvantage, as an etiologic moderator of several common forms of psychopathology in two samples of school-aged twins from the Michigan State University Twin Registry (N = 4815 and 1030 twin pairs, respectively), the latter of which was enriched for neighborhood disadvantage.

RESULTS

Across both samples, genetic influences on attention-deficit hyperactivity problems were accentuated in the presence of marked disadvantage, while nonshared environmental contributions to callous-unemotional traits increased with increasing disadvantage. However, neighborhood disadvantage had little moderating effect on the etiology of depression, anxiety, or somatic symptoms.

CONCLUSIONS

Such findings suggest that, although neighborhood disadvantage does appear to serve as a general etiologic moderator of many (but not all) forms of psychopathology, this etiologic moderation is phenotype-specific.

The Role of Preclinical Models in Creatine Transporter Deficiency: Neurobiological Mechanisms, Biomarkers and Therapeutic Development

Creatine (Cr) Transporter Deficiency (CTD) is an X-linked metabolic disorder, mostly caused by missense mutations in the SLC6A8 gene and presenting with intellectual disability, autistic behavior, and epilepsy. There is no effective treatment for CTD and patients need lifelong assistance. Thus, the research of novel intervention strategies is a major scientific challenge. Animal models are an excellent tool to dissect the disease pathogenetic mechanisms and drive the preclinical development of therapeutics. This review illustrates the current knowledge about Cr metabolism and CTD clinical aspects, with a focus on mainstay diagnostic and therapeutic options. Then, we discuss the rodent models of CTD characterized in the last decade, comparing the phenotypes expressed within clinically relevant domains and the timeline of symptom development. This analysis highlights that animals with the ubiquitous deletion/mutation of SLC6A8 genes well recapitulate the early onset and the complex pathological phenotype of the human condition. Thus, they should represent the preferred model for preclinical efficacy studies. On the other hand, brain- and cell-specific conditional mutants are ideal for understanding the basis of CTD at a cellular and molecular level. Finally, we explain how CTD models might provide novel insight about the pathogenesis of other disorders, including cancer.

Psychometric properties and contextual appropriateness of the German version of the Early Development Instrument

BACKGROUND

Assessing the early development of children at a population level in educational settings, may be useful for public health and policy decision making. In this study, we evaluated the psychometric properties and the contextual appropriateness of a German language version of the Early Development Instrument (EDI), a survey-based instrument originally developed in Canada, which assesses developmental vulnerability for children in preschool settings.

METHODS

Sixty preschool teachers from six preschool organizations (22% of organizations contacted) in three cities in southwest Germany participated. They administered a German version of the EDI (GEDI) to 225 children (51% of eligible children). We assessed internal consistency, test-retest and interrater reliability. Preschool teachers assisted in determining face-validity by reviewing item coverage and comprehensibility. Exploratory factor analysis (EFA) was used to evaluate convergent validity. Concurrent validity was measured using correlations and agreements (Bland-Altman plots) between GEDI and other validated instrument scores. Additionally, we compared associations between GEDI domain scores and sociodemographic characteristics with similar associations in EDI studies worldwide.

RESULTS

GEDI domains showed good to excellent internal consistency (0.73 < α > 0.99) and moderate to good test-retest and interrater reliability (0.50 to 0.81 and 0.48 to 0.71, respectively [p-value < 0.05]). Face validity was considered acceptable. EFA showed a factor structure similar to the original EDI. Correlations (range: 0.32 to 0.67) and agreements between GEDI scores and other German language instruments suggested good external reliability. Scoring within the lowest 10th percentile was strongly associated with age.

CONCLUSIONS

Our psychometric assessment suggests good reliability and consistency of the GEDI. Differences in the age distribution of children, pedagogical objectives and educational system features of German preschools require future work to determine score thresholds indicative of vulnerability. Aside from dropping selected items from the original EDI that were inconsistent with features of the German educational system, the distribution of values in the language and cognitive development domain also suggested that context-specific cut-offs must be established for the German version. Such efforts are needed to account for relevant contextual differences between the educational systems.

Peer relationships and prosocial behaviour differences across disruptive behaviours

It is unclear if impairments in social functioning and peer relationships significantly differ across common developmental conditions such as attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), conduct disorder (CD), and associated callous-unemotional traits (CU traits). The current study explored sex differences and symptoms of parent- and teacher-reported psychopathology on peer relationships and prosocial behaviour in a sample of 147 referred children and adolescents (aged 5-17 years; 120 m). The results showed that increases in parent-reported ADHD Inattentive symptoms and teacher-reported ADHD Hyperactive-Impulsive symptoms, CD, ODD, and CU traits were significantly associated with peer relationship problems across sex. At the same time, teacher-reported symptoms of ODD and both parent- and teacher-reported CU traits were related to difficulties with prosocial behaviour, for both boys and girls, with sex explaining additional variance. Overall, our findings show a differential association of the most common disruptive behaviours to deficits in peer relationships and prosocial behaviour. Moreover, they highlight that different perspectives of behaviour from parents and teachers should be taken into account when assessing social outcomes in disruptive behaviours. Given the questionable separation of conduct problem-related constructs, our findings not only point out the different contribution of those aspects in explaining peer relationships and prosocial behaviour, but furthermore the variance from different informants about those aspects of conduct problems.

Genetic and environmental links between motor activity level and attention problems in early childhood

Cross-lagged biometric models were used to examine genetic and environmental links between actigraph-assessed motor activity level (AL) and parent-rated attention problems (AP) in 314 same-sex twin pairs (MZ = 145, DZ = 169) at ages 2 and 3 years. At both ages, genetic correlations between AL and AP were moderate (ra2 = .35; ra3 = .39) indicating both overlap and specificity in genetic effects across the two domains. Within- and across-age phenotypic associations between AL and AP were entirely due to overlapping genetic influences. There was a unidirectional effect of AL at age 2 predicting later AP. For AP, genetic and environmental influences from age 2 were transmitted to age 3 via stability effects and from AL. For AL, across-age effects were transmitted only via stability. These results suggest that overactivity in late infancy may impact the later development of problems related to inattention, and that genetic factors explain the association between the two domains.

Investigating error-related processing in incarcerated adolescents with self-report psychopathy measures

Disparate results have been found in previous reports when incorporating both interview-based and self-report measures of psychopathic traits within the same sample, suggesting such assessments should not be used interchangeably. We previously found Total and Facet 4 scores from Hare's Psychopathy Checklist: Youth Version (PCL:YV) were negatively related to amplitude of the error-related positivity (Pe) event-related potential (ERP) component. Here, we investigated using the same previously published sample whether scores on four different self-report measures of adolescent psychopathic traits (the Antisocial Process Screening Device [APSD], Child Psychopathy Scale [CPS], Inventory of Callous-Unemotional Traits [ICU], and Youth Psychopathic Traits Inventory [YPI]) were similarly associated with reduced Pe amplitude. Unlike our previous results, adolescent self-report psychopathy scores were not associated with reduced Pe amplitude in multiple regression analyses. Results obtained in the current report support previous research observing incongruent findings when incorporating different assessment types within the same sample.

Testing for the mediating role of endophenotypes using molecular genetic data in a twin study of ADHD traits

Family and twin studies have identified endophenotypes that capture familial and genetic risk in attention-deficit/hyperactivity disorder (ADHD), but it remains unclear if they lie on the causal pathway. Here, we illustrate a stepwise approach to identifying intermediate phenotypes. First, we use previous quantitative genetic findings to delineate the expected pattern of genetically correlated phenotypes. Second, we identify overlapping genetic associations with ADHD-related quantitative traits. Finally, we test for the mediating role of associated endophenotypes. We applied this approach to a sample of 1,312 twins aged 7-10. Based on previous twin model-fitting analyses, we selected hyperactivity-impulsivity, inattention, reading difficulties (RD), reaction time variability (RTV) and commission errors (CE), and tested for association with selected ADHD risk alleles. For nominally significant associations with both a symptom and a cognitive variable, matching the expected pattern based on previous genetic correlations, we performed mediation analysis to distinguish pleiotropic from mediating effects. The strongest association was observed for the rs7984966 SNP in the serotonin receptor gene (HTR2A), and RTV (P = 0.007; unadjusted for multiple testing). Mediation analysis suggested that CE (38%) and RTV (44%) substantially mediated the association between inattention and the T-allele of SNP rs3785157 in the norepinephrine transporter gene (SLC6A2) and the T-allele of SNP rs7984966 in HTR2A, respectively. The SNPs tag risk-haplotypes but are not thought to be functionally significant. While these exploratory findings are preliminary, requiring replication, this study demonstrates the value of this approach that can be adapted to the investigation of multiple genetic markers and polygenic risk scores. © 2016 The Authors. American Journal of Medical Genetics Part B: Neuropsychiatric Genetics Published by Wiley Periodicals, Inc.

Prenatal Smoke Exposure Predicts Hyperactive/Impulsive but Not Inattentive ADHD Symptoms in Adolescent and Young Adult Girls

We examined the longitudinal associations between prenatal tobacco smoke exposure (PSE) and attention-deficit hyperactivity disorder (ADHD) symptom domains in adolescence and young adulthood. A sample of girls with ADHD combined presentation (N=93), ADHD predominantly inattentive presentation (N=47), and matched comparisons (N= 88) was assessed prospectively. Symptoms of hyperactivity/impulsivity (HI), inattention (IA), and oppositionality (oppositional defiant disorder) were measured via multiple informants 5 (M age =14 years; retention rate =92%) and 10 years (M age =20 years; retention rate =95%) following childhood ascertainment. PSE was captured via maternal self-report. We used linear regressions to examine the prediction from PSE to both HI and IA in adolescence and early adulthood after stringent control of relevant confounding variables. PSE significantly predicted HI during adolescence and young adulthood across multiple informants but did not predict IA at either wave. Symptoms of HI may have partial etiological independence from IA symptoms.

Heritability of attention-deficit hyperactivity disorder in adults

Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder. Symptoms often persist into adulthood, with a prevalence of 2.5-5% in adult populations. Twin studies in childhood consistently report high heritabilities of 70-80%, while studies in adult samples show only moderate heritability of 30-40% when estimated from self-ratings. This review summarizes the available research on the heritability of ADHD in adults. Three key findings are outlined: (i) self-ratings lead to relatively low heritability estimates of ADHD, independent of age and whether ratings refer to current or retrospective symptoms; (ii) studies relying on different informants to rate each twin within a pair (i.e., self-ratings and different parents/teachers rating each twin in a pair) consistently yield lower heritability estimates than studies relying on ratings from a single informant; (iii) studies using cross-informant data via either combined parent and self-ratings or clinical diagnoses information suggest that the heritability of ADHD in adults could be as high as 70-80%. Together, the reviewed studies suggest that the previously reported low heritability of ADHD in adults is unlikely to reflect a true developmental change. Instead, the drop in heritability is better explained by rater effects related to a switch from using one rater for both twins in a pair (parent/teacher) in childhood, to relying on self-ratings (where each twin rates themselves) of ADHD symptoms in adulthood. When rater effects are addressed using cross-informant approaches, the heritability of ADHD in adults appears to be comparable to the heritability of ADHD in childhood. © 2015 Wiley Periodicals, Inc.

Common prefrontal cortical gene expression profiles between adolescent SHR/NCrl and WKY/NCrl rats which showed inattention behavior

Factor analyses of attention-deficit/hyperactivity (ADHD) symptoms divide the behavioral symptoms of ADHD into two separate domains, one reflecting inattention and the other, a combination of hyperactivity and impulsivity. Identifying domain-specific genetic risk variants may aid in the discovery of specific biological risk factors for ADHD. In contrast with data available on genes involved in hyperactivity and impulsivity, there is limited information on the genetic influences of inattention. Transcriptional profiling analysis in animal models of disorders may provide an important tool to identify genetic involvement in behavioral phenotypes. To explore some of the potential genetic underpinnings of ADHD inattention, we examined common differentially expressed genes (DEGs) in the prefrontal cortex of SHR/NCrl, the most validated animal model of ADHD and WKY/NCrl, animal model of ADHD-inattentive type. In contrast with Wistar rats, strain representing the "normal" heterogeneous population, SHR/NCrl and WKY/NCrl showed inattention behavior in the Y-maze task. The common DEGs in the PFC of SHR/NCrl and WKY/NCrl vs. Wistar rats are those involved in transcription (e.g. Creg1, Thrsp, Zeb2), synaptic transmission (e.g. Atp2b2, Syt12, Chrna5), neurological system process (e.g. Atg7, Cacnb4, Grin3a), and immune response (e.g. Atg7, Ip6k2, Mx2). qRT-PCR analyses validated expression patterns of genes representing the major functional gene families among the DEGs (Grin3a, Thrsp, Vof-16 and Zeb2). Although further studies are warranted, the present findings indicate novel genes associated with known functional pathways of relevance to ADHD which are assumed to play important roles in the etiology of ADHD-inattentive subtype.

Benefits of a working memory training program for inattention in daily life: a systematic review and meta-analysis

BACKGROUND

Many common disorders across the lifespan feature impaired working memory (WM). Reported benefits of a WM training program include improving inattention in daily life, but this has not been evaluated in a meta-analysis. This study aimed to evaluate whether one WM training method has benefits for inattention in daily life by conducting a systematic review and meta-analysis.

METHODS

We searched Medline and PsycINFO, relevant journals and contacted authors for studies with an intervention and control group reporting post-training estimates of inattention in daily life. To reduce the influence of different WM training methods on the findings, the review was restricted to trials evaluating the Cogmed method. A meta-analysis calculated the pooled standardised difference in means (SMD) between intervention and control groups.

RESULTS

A total of 622 studies were identified and 12 studies with 13 group comparisons met inclusion criteria. The meta-analysis showed a significant training effect on inattention in daily life, SMD=-0.47, 95% CI -0.65, -0.29, p<.00001. Subgroup analyses showed this significant effect was observed in groups of children and adults as well as users with and without ADHD, and in studies using control groups that were active and non-adaptive, wait-list and passive as well as studies using specific or general measures. Seven of the studies reported follow-up assessment and a meta-analysis showed persisting training benefits for inattention in daily life, SMD=-0.33, 95% CI -0.57 -0.09, p=.006. Additional meta-analyses confirmed improvements after training on visuospatial WM, SMD=0.66, 95% CI 0.43, 0.89, p<.00001, and verbal WM tasks, SMD=0.40, 95% CI 0.18, 0.62, p=.0004.

CONCLUSIONS

Benefits of a WM training program generalise to improvements in everyday functioning. Initial evidence shows that the Cogmed method has significant benefits for inattention in daily life with a clinically relevant effect size.

Where are the strongest associations between autistic traits and traits of ADHD? evidence from a community-based twin study

Autism spectrum conditions (ASC) and attention-deficit/hyperactivity disorder (ADHD) regularly co-occur. Twin studies increasingly indicate that these conditions may have overlapping genetic causes. Less is known about the degree to which specific autistic traits relate to specific behaviours characteristic of ADHD. We hence tested, using the classical twin design, whether specific dimensional autistic traits, including social difficulties, communication atypicalities and repetitive behaviours, would display differential degrees of aetiological overlap with specific traits of ADHD, including hyperactivity/impulsivity and inattention. Parents of approximately 4,000 pairs of 12-year-old twins completed the Childhood Autism Spectrum Test and Conners' Parent Rating Scale. These measures were divided into subscales corresponding to different types of autistic and ADHD behaviours. Twin model fitting suggested that the degree of genetic overlap was particularly strong between communication difficulties and traits of ADHD (genetic correlations = .47-.51), while repetitive behaviours and social difficulties showed moderate (genetic correlations = .12-.33) and modest (.05-.11) genetic overlap respectively. Environmental overlap was low across all subscales (correlations = .01-.23). These patterns were also apparent at the extremes of the general population, with communication difficulties showing the highest genetic overlap with traits of ADHD. These findings indicate that molecular genetic studies seeking to uncover the shared genetic basis of ASC and ADHD would benefit from taking a symptom-specific approach. Furthermore, they could also help to explain why studies of the communication abilities of individuals with ASC and ADHD have produced overlapping findings.

The co-occurrence of autistic and ADHD dimensions in adults: an etiological study in 17,770 twins

Autism spectrum disorder (ASD) and attention deficit/hyperactivity disorder (ADHD) often occur together. To obtain more insight in potential causes for the co-occurrence, this study examined the genetic and environmental etiology of the association between specific ASD and ADHD disorder dimensions. Self-reported data on ASD dimensions social and communication difficulties (ASDsc), and repetitive and restricted behavior and interests (ASDr), and ADHD dimensions inattention (IA), and hyperactivity/impulsivity (HI) were assessed in a community sample of 17,770 adult Swedish twins. Phenotypic, genetic and environmental associations between disorder dimensions were examined in a multivariate model, accounting for sex differences. ASDr showed the strongest associations with IA and HI in both sexes (r(p) 0.33 to 0.40). ASDsc also correlated moderately with IA (females r(p) 0.29 and males r(p) 0.35) but only modestly with HI (females r(p) 0.17 and males r(p) 0.20). Genetic correlations ranged from 0.22 to 0.64 and were strongest between ASDr and IA and HI. Sex differences were virtually absent. The ASDr dimension (reflecting restricted, repetitive and stereotyped patterns of behavior, interests and activities) showed the strongest association with dimensions of ADHD, on a phenotypic, genetic and environmental level. This study opens new avenues for molecular genetic research. As our findings demonstrated that genetic overlap between disorders is dimension-specific, future gene-finding studies on psychiatric comorbidity should focus on carefully selected genetically related dimensions of disorders.

Searching for the best approach to assess teachers' perception of inattention and hyperactivity problems at school

Although major guidelines in the field and current diagnostic criteria clearly demand an assessment of children's attention deficit/hyperactivity disorder (ADHD) symptoms at school, few studies address the fundamental question of which is the best approach for clinicians to get this information from teachers. Three screening strategies for ADHD were applied to teachers of 247 third grade students. They were asked (1) an overt question about potential cases of ADHD in their classroom; (2) to complete a broad-band questionnaire assessing common child mental health problems; (3) to rate ADHD-specific symptoms in a narrow-band questionnaire. Based on the overt question, teachers identified one in five students (21.1 %) as having ADHD; 28 cases (11.3 %) were identified using standard cut-offs for the narrow-band, and 13 (5.3 %) using a standard threshold for the sub-scale of hyperactivity from the broad-band questionnaire. Agreement among strategies was low (k = 0.28). A subsample of students, clinically assessed to confirm screenings, showed modest agreement with final diagnosis. The narrow-band questionnaire had the best diagnostic performance. Multivariate analysis indicated that the presence of a comorbid externalizing disorder was the only variable associated with teachers' ascertainment of ADHD caseness or non-caseness. Choice of screening strategy significantly affects how teachers report on ADHD symptoms at school. The halo effect of externalizing behaviors impacts the correct identification of true cases of ADHD in the school setting. Clinicians can rely on narrow-band instruments like the SNAP-IV to get information on ADHD symptoms at school from teachers.

Assessing genetic influences on behavior: informant and context dependency as illustrated by the analysis of attention problems

Assessment of genetic influences on behavior depends on context, informants, and study design: We show (analytically) that, conditional on study design, informant specific genetic variance is included in the genetic variance component or in the environmental variance component. To aid the explanation, we present an illustrative empirical analysis of data from the Netherlands Twin Register. Subjects included 1,571 monozygotic and 2,672 dizygotic 12-year-old twin pairs whose attention problems (AP) were rated by their parents, teachers, and themselves. Heritability estimates (h(2)) of AP were about ~0.75 for same informant ratings (mother, father, and same teacher ratings) and ~0.54 for different informants' ratings (different parents', different teachers', and two twins' self-ratings). Awareness of assessment effects is relevant to research into psychiatric disorders. Differences in assessment can account for age effects, such as a drop in heritability of ADHD symptoms. In genome-wide association studies, effects of rating specific genetic influences will be undetectable.

The separation of ADHD inattention and hyperactivity-impulsivity symptoms: pathways from genetic effects to cognitive impairments and symptoms

Both shared and unique genetic risk factors underlie the two symptom domains of attention deficit hyperactivity disorder (ADHD): inattention and hyperactivity-impulsivity. The developmental course and relationship to co-occurring disorders differs across the two symptom domains, highlighting the importance of their partially distinct etiologies. Familial cognitive impairment factors have been identified in ADHD, but whether they show specificity in relation to the two ADHD symptom domains remains poorly understood. We aimed to investigate whether different cognitive impairments are genetically linked to the ADHD symptom domains of inattention versus hyperactivity-impulsivity. We conducted multivariate genetic model fitting analyses on ADHD symptom scores and cognitive data, from go/no-go and fast tasks, collected on a population twin sample of 1,312 children aged 7-10. Reaction time variability (RTV) showed substantial genetic overlap with inattention, as observed in an additive genetic correlation of 0.64, compared to an additive genetic correlation of 0.31 with hyperactivity-impulsivity. Commission errors (CE) showed low additive genetic correlations with both hyperactivity-impulsivity and inattention (genetic correlations of 0.17 and 0.11, respectively). The additive genetic correlation between RTV and CE was also low and non-significant at -0.10, consistent with the etiological separation between the two indices of cognitive impairments. Overall, two key cognitive impairments phenotypically associated with ADHD symptoms, captured by RTV and CE, showed different genetic relationships to the two ADHD symptom domains. The findings extend a previous model of two familial cognitive impairment factors in combined subtype ADHD by separating pathways underlying inattention and hyperactivity-impulsivity symptoms.

Different heritabilities but shared etiological influences for parent, teacher and self-ratings of ADHD symptoms: an adolescent twin study

BACKGROUND

Parent and teacher ratings of attention deficit hyperactivity disorder (ADHD) symptoms yield high estimates of heritability whereas self-ratings typically yield lower estimates. To understand why, the present study examined the etiological overlap between parent, teacher and self-ratings of ADHD symptoms in a population-based sample of 11-12-year-old twins. Method Participants were from the Twins Early Development Study (TEDS). ADHD symptoms were assessed using the Strengths and Difficulties Questionnaire (SDQ) hyperactivity scale completed by parents, teachers and children. Structural equation modeling was used to examine genetic and environmental contributions to phenotypic variance/covariance.

RESULTS

The broad-sense heritability of ADHD symptoms was 82% for parent ratings, 60% for teacher ratings and 48% for self-ratings. Post-hoc analyses revealed significantly higher heritability for same-teacher than different-teacher ratings of ADHD (76% v. 49%). A common pathway model best explained the relationship between different informant ratings, with common genetic influences accounting for 84% of the covariance between parent, teacher and self-rated ADHD symptoms. The remaining variance was explained by rater-specific genetic and non-shared environmental influences.

CONCLUSIONS

Despite different heritabilities, there were shared genetic influences for parent, teacher and self-ratings of ADHD symptoms, indicating that different informants rated some of the same aspects of behavior. The low heritability estimated for self-ratings and different-teacher ratings may reflect increased measurement error when different informants rate each twin from a pair, and/or greater non-shared environmental influences. Future studies into the genetic influences on ADHD should incorporate informant data in addition to self-ratings to capture a pervasive, heritable component of ADHD symptomatology.

What have we learned from recent twin studies about the etiology of neurodevelopmental disorders?

PURPOSE OF REVIEW

The relative influence of genes and environment on the liability to neurodevelopmental disorders (NDDs) can be investigated using a twin design. This review highlights the results of the most recent twin studies of NDDs.

RECENT FINDINGS

Recent twin studies have confirmed that NDDs show moderate-to-high heritability, and that from an etiological viewpoint both autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) are best regarded as the extremes on a continuous liability distribution. Both ASD and ADHD show high heritability in childhood and a substantial drop in heritability in adulthood, which is likely explained by the use of different assessment strategies in childhood versus adulthood, or by a complex mechanism of gene-by-environment interaction. NDDs show substantial comorbidity among each other, and with other mental health problems, which is partly because of a shared genetic etiology between different disorders.

SUMMARY

The findings of twin studies implicate substantial heritability of NDDs, and warrant large-scale molecular genetic studies for such traits.