Additional Evidence Against Shared Environmental Contributions to Attention-Deficit/Hyperactivity Problems

Department of Psychology, Michigan State University, 107D Psychology Building, East Lansing, MI, 48824, USA, .
Behavior Genetics (Impact Factor: 3.21). 05/2012; 42(5):711-21. DOI: 10.1007/s10519-012-9545-y
Source: PubMed


A recent meta-analysis "Burt (Psychol Bull 135:608-637, 2009)" indicated that shared environmental influences (C) do not contribute to Attention-Deficit/Hyperactivity Disorder (ADHD). Unfortunately, the meta-analysis relied almost exclusively on classical twin studies. Although useful in many ways, some of the assumptions of the classical twin model (e.g., dominant genetic and shared environmental influences do not simultaneously influence the phenotype) can artifactually decrease estimates of C. There is thus a need to confirm that dominant genetic influences are not suppressing estimates of C on ADHD. The current study sought to do just this via the use of a nuclear twin family model, which allows researchers to simultaneously model and estimate dominant genetic and shared environmental influences. We examined two independent samples of child twins: 312 pairs from the Michigan State University Twin Registry and 854 pairs from the PrE School Twin Study in Sweden. Shared environmental influences were found to be statistically indistinguishable from zero and to account for less than 5 % of the variance. We conclude that the presence of dominant genetic influences does not account for the absence of C on ADHD.

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    • "Although largely genetically determined, both shared and non-shared environmental factors also impact ADHD etiology (Burt et al., 2012). Non-shared environment is non-genetic factors that result in differences between individuals growing up in the same family. "
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    ABSTRACT: Introduction: The study of differences between monozygotic (MZ) twin pairs with respect to ADHD may provide novel leads to disentangle the environmental contribution driving its phenotypes. Objectives: To examine non-shared environmental influences on executive function in dimensionally defined ADHD. Methods: This study included 27 MZ twin pairs (7 female) aged 11-20 years being moderately to substantially discordant for ADHD traits as assessed by the Attention Problem (AP) scale of the Child Behavior Checklist/Adult Behavior Checklist. The twins completed the Wisconsin Card Sorting Test (WCST) for cognitive flexibility and the Tower Test (TT) for foresighted planning. Two statistical approaches were used to analyze the data. First, correlations between ADHD trait intra-pair differences and WCST and TT scores were calculated. Second, the significance of those intra-pair differences on WCST and TT, using ADHD as categorical variable in clinically discordant pairs, was tested. Results: Both analysing strategies revealed a link between ADHD on one hand, and foresighted planning and inhibitory control on the other hand mediated by non-shared environmental factors. The first statistical approach yielded positive correlations between intra-pairs differences on the AP scale and intra-pair differences on two subscales of the TT: total rule violation (r s=0.41) and rule-violation-per-item-ratio (rs=0.38). Findings in categorically discordant pairs were consistent, showing within-pair differences on the same subtests (z-1.63, P=0.05, one-tailed and z=-1.60, P=0.05, one-tailed). Conclusions: Findings confirm previous research suggesting ADHD to be a quantitative extreme on a continuum with executive functions being a cognitive marker of ADHD traits. Non-shared environmental factors appear to influence planning skills and inhibitory control.
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    ABSTRACT: The primary aim of the Michigan State University Twin Registry (MSUTR) is on understanding developmental changes in genetic, environmental, and neurobiological influences on internalizing and externalizing disorders, with antisocial behavior and disordered eating representing our particular areas of interest. The MSUTR has two broad components: a large-scale, population-based registry of child, adolescent, and adult twins and their families (current N ~20,000) and a series of more focused and in-depth studies drawn from the registry (current N ~4,000). Participants in the population-based registry complete a family health and demographic questionnaire via mail. Families are then recruited for one or more of the intensive, in-person studies from the population-based registry based on their answers to relevant items in the registry questionnaire. These in-person assessments target a variety of biological, genetic, and environmental phenotypes, including multi-informant measures of psychiatric and behavioral phenotypes, census and neighborhood informant reports of twin neighborhood characteristics, buccal swab and salivary DNA samples, assays of adolescent and adult steroid hormone levels, and/or videotaped interactions of child twin families. This article provides an overview of the MSUTR and describes current and future research directions.
    Twin Research and Human Genetics 10/2012; 16(1):1-7. DOI:10.1017/thg.2012.87 · 2.30 Impact Factor
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    ABSTRACT: It is not clear whether sibling's gender ratio is associated with attention deficit hyperactivity disorder (ADHD). This study examines whether inattentiveness severity and hyperactivity/impulsivity severity are associated with birth order of children with ADHD. Participants are a clinical sample of 173 children and adolescents with ADHD and 43 ones without ADHD. Diagnoses were made using Diagnostic and Statistical Manual of Mental Disorders forth edition-Text Revision (DSM-IV-TR), diagnostic criteria according to face-to-face interview with the children and their parents. ADHD DSM-IV checklist was used to measure inattentiveness and hyperactivity/impulsivity scores. The association of birth order and diagnosis of ADHD was not statistically significant after adjusting for covariate factors. The gender ratio of siblings is not associated with ADHD. Birth order and siblings gender ratio are independent of ADHD diagnosis. The results of this study support the fact that genetic factors rather than environmental factor of birth order is associated with ADHD. Moreover, contrary to autism, the current results do not suggest the androgen theory for ADHD.
    Iranian Journal of Psychiatry 11/2012; 7(3):109-13.
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