Vocabulary knowledge was tested in a native (Cantonese-Chinese) and foreign (English) language in 150 twins and 150 singletons aged 6-11 years, matched on age, gender, grade level, nonverbal intelligence, parents' education, family income, and number of siblings and household members. The singletons clearly outperformed the twins on the native…

Reviews of behavioral genetic studies note that "control" aspects of parenting yield low estimates of heritability, while "affective" aspects (parental feelings) yield moderate estimates. Research to date has not specifically considered whether positive and negative aspects of parenting--for both feelings and control--may…

Youth who experience adverse environments in early life initiate sexual activity at a younger age, on average, than those from more advantaged circumstances. Evolutionary theorists have posited that ecological stress precipitates earlier reproductive and sexual onset, but it is unclear how stressful environments interact with genetic influences on…

Lexical access tasks are designed to measure efficiency of lexical access, but task demands and methods vary greatly. Many lexical access tasks do not account for confounding factors including competence in other linguistic abilities. In this study, preschoolers were given two lexical access tasks. In the single-category naming (SCN) task,…

The prevalence of reading difficulties is typically higher in males than females in both referred and research-identified samples, and the ratio of males to females is greater in more affected samples. To explore possible gender differences in reading performance, we analysed data from 1133 twin pairs in which at least one member of each pair had…

This study examined (a) the predictive link between peer victimization and children's reactive and proactive aggression, and (b) the potential moderating effect of reciprocal friends' reactive and proactive aggression in this context. The study also examined whether these potential moderating effects of friends' characteristics were stronger with…

Previous studies have suggested that among affected sib pairs with autism there is an increase in the frequency of twins over what would be expected in comparison to the prevalence of twins in the general population. In this study we sought to determine whether "sub-threshold" autistic traits were more pronounced in twins than in non-twins. The…

Background: It is not known if the prevalence of parental psychiatric disorders is higher in stepfather than intact families, or if parental alcoholism is differentially associated with risk for conduct disorder (CD) symptoms in stepfather families versus intact families. Method: The sample comprised 839 girls and 741 boys from 792 intact families…