Our results suggest that different intervention policies should be considered in different socioeconomic areas. In socioeconomically advantaged areas, it might be more fruitful to focus on individually based preventions and treatments. In socioeconomically disadvantaged areas, intervention and prevention policies might be more effective on a community level, to account for shared environmental risk factors.

Environmental factors shared by co-twins affect BMI in childhood, but little evidence for their contribution was found in late adolescence. Our results suggest that genetic factors play a major role in the variation of BMI in adolescence among populations of different ethnicities exposed to different environmental factors related to obesity.

Marijuana is one of the most commonly used drugs in the United States, and use during adolescence-when the brain is still developing-has been proposed as a cause of poorer neurocognitive outcome. Nonetheless, research on this topic is scarce and often shows conflicting results, with some studies showing detrimental effects of marijuana use on cognitive functioning and others showing no significant long-term effects. The purpose of the present study was to examine the associations of marijuana use with changes in intellectual performance in two longitudinal studies of adolescent twins (n = 789 and n = 2,277). We used a quasiexperimental approach to adjust for participants' family background characteristics and genetic propensities, helping us to assess the causal nature of any potential associations. Standardized measures of intelligence were administered at ages 9-12 y, before marijuana involvement, and again at ages 17-20 y. Marijuana use was self-reported at the time of each cognitive assessment as well as during the intervening period. Marijuana users had lower test scores relative to nonusers and showed a significant decline in crystallized intelligence between preadolescence and late adolescence. However, there was no evidence of a dose-response relationship between frequency of use and intelligence quotient (IQ) change. Furthermore, marijuana-using twins failed to show significantly greater IQ decline relative to their abstinent siblings. Evidence from these two samples suggests that observed declines in measured IQ may not be a direct result of marijuana exposure but rather attributable to familial factors that underlie both marijuana initiation and low intellectual attainment.M arijuana is one of the most commonly used drugs in the United States, with a lifetime use prevalence of 50% (1, 2). Despite this, research on the long-term effects of marijuana use is scarce compared with that of other illicit substances. Changing cultural attitudes toward marijuana have recently led to social and legal acceptance of recreational use (3, 4), making research on the potential consequences particularly salient.Previous research has shown that marijuana use can have a high societal cost through increased unemployment, absenteeism, decreased productivity, and increased rates of crime and incarceration (5-8). Given that about 19% of youth and young adults (ages 18-25 y) in the United States have used marijuana in the past month (9), the potential impact is nontrivial. There is evidence to suggest that the adolescent brain may be particularly vulnerable, especially with regard to neurocognitive functioning (10, 11). Marijuana use in adolescence, when the brain is still undergoing major developmental changes, has been associated with decreased intelligence (12, 13), reduced memory (13-15), poorer attention (16-18), and lower verbal ability (19-21). However, these findings come from cross-sectional studies, where the temporal ordering of cause and effect is uncertain. For longitudinal studies examining marijuana use and ch...

Height variation is known to be determined by both genetic and environmental factors, but a systematic description of how their influences differ by sex, age and global regions is lacking. We conducted an individual-based pooled analysis of 45 twin cohorts from 20 countries, including 180,520 paired measurements at ages 1–19 years. The proportion of height variation explained by shared environmental factors was greatest in early childhood, but these effects remained present until early adulthood. Accordingly, the relative genetic contribution increased with age and was greatest in adolescence (up to 0.83 in boys and 0.76 in girls). Comparing geographic-cultural regions (Europe, North-America and Australia, and East-Asia), genetic variance was greatest in North-America and Australia and lowest in East-Asia, but the relative proportion of genetic variation was roughly similar across these regions. Our findings provide further insights into height variation during childhood and adolescence in populations representing different ethnicities and exposed to different environments.

The Swedish Twin study of CHild and Adolescent Development (TCHAD) is a longitudinal study of how genes and environments contribute to development of health and behavioral problems from childhood to adulthood. The study includes 1480 twin pairs followed since 1994, when the twins were 8 to 9 years old. The last data collection was in 2005 when the twins were 19 to 20 years old. Both parents and twins have provided data. In this article we describe the sample, data collections, and measures used. In addition, we provide some key findings from the study, focusing on antisocial behavior, criminality, and psychopathic personality.

A meta-analysis of twin, family and adoption studies was conducted to estimate the magnitude of genetic and environmental influences on impulsivity. The best fitting model for 41 key studies (58 independent samples from 14 month old infants to adults; N = 27,147) included equal proportions of variance due to genetic (0.50) and non-shared environmental (0.50) influences, with genetic effects being both additive (0.38) and non-additive (0.12). Shared environmental effects were unimportant in explaining individual differences in impulsivity. Age, sex, and study design (twin vs. adoption) were all significant moderators of the magnitude of genetic and environmental influences on impulsivity. The relative contribution of genetic effects (broad sense heritability) and unique environmental effects were also found to be important throughout development from childhood to adulthood. Total genetic effects were found to be important for all ages, but appeared to be strongest in children. Analyses also demonstrated that genetic effects appeared to be stronger in males than in females. Method of assessment (laboratory tasks vs. questionnaires), however, was not a significant moderator of the genetic and environmental influences on impulsivity. These results provide a structured synthesis of existing behavior genetic studies on impulsivity by providing a clearer understanding of the relative genetic and environmental contributions in impulsive traits through various stages of development.

Previous studies examining the covariation among Attention Deficit Hyperactivity Disorder (ADHD), Oppositional Defiant Disorder (ODD) and Conduct Disorder (CD) have yielded inconsistent results. Some studies have concluded that the covariation among these symptoms is due to common genetic influences, whereas others have found a common environmental overlap. The present study investigated the genetic and environmental correlations among these three childhood disorders, based on a sample of 1,241 twins, age 9-10 years. A latent externalizing behavior factor was found to explain the covariance among ADHD, ODD and CD symptoms. Genetic influences explained more than half of the variance in this externalizing factor in both boys and girls. There were also unique genetic and environmental influences in each set of symptoms, suggesting some etiological independence of the three disorders. Our findings have implications for molecular genetic studies trying to identify susceptibility genes, as well as for possible prevention of externalizing disorders through identification of at-risk children early in life. KeywordsADHD; CD; ODD; covariation; genetic influences; twins Attention Deficit Hyperactivity Disorder (ADHD), Oppositional Defiant Disorder (ODD) and Conduct Disorder (CD) are three of the most prevalent disruptive behavior disorders in children and adolescents. They affect approximately 1-15% of all school age children, and account for a large amount of childhood referrals to mental health clinics (Brown et al., 2001;Maughan et al., 2004). They often lead to stress and frustration in the affected children, their families, teachers and peers. Numerous studies have reported comorbidity among these disruptive behavior disorders in both epidemiological and clinical samples (Angold, Costello & Erkanli, 1999;Biederman, Newcorn & Sprich, 1991;Faraone et al., 1998b;Maughan et al., 2004), often making them difficult to isolate and understand individually. Such high levels of comorbidity among supposedly different domains of externalizing problems, such as inattention, hyperactivity, aggression and defiance make these various problem behaviors complex and particularly hard to treat. To date, the genetic and environmental etiology that underlies the comorbidity among these disruptive behavior disorders is unclear.ADHD is characterized by pervasive and impairing symptoms of inattention, hyperactivity and impulsivity (American Psychiatric Association, 2004). The disorder is associated with academic underachievement, substance use and dependence, psychosocial problems and social NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript maladjustment, including unemployment in adulthood (Biederman & Faraone, 2006;Kuperman et al., 2001).Like ADHD, ODD is also a common psychiatric disorder in children (Maughan et al., 2004). The disorder typically occurs in early childhood and is characterized by a pattern of negativistic, disobedient, hostile and defiant behaviors. In contrast to ODD, CD is a more severe d...

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