The influence of sibship composition on language development at 2 years of age in the ELFE birth cohort study.

The number of older siblings a child has is negatively correlated with the child's verbal skills, an effect that is well known in the literature. However, few studies have examined the effect of older siblings' sex, of the age gap between siblings, of having foreign-speaking parents, as well as the mediating role of parental interaction. Using data from 12,296 children (49.3% female) from the French ELFE birth cohort, we analyzed the effect of these characteristics of the siblings and their family on children's expressive vocabulary measured using the French MacArthur-Bates Communicative Development Inventory. Children's vocabulary at age 2 years was negatively associated with the number of older siblings (-0.08 SD per additional sibling), and this effect was partly mediated by parental interactions. In analyses restricted to children with one older sibling, the vocabulary score was negatively correlated with the age gap between the target child and their older sibling. The vocabulary score was not correlated to their sibling's sex, contrary to the result of a previous study. In addition, the effect of the number of siblings was less negative in foreign speaking families that in French speaking families, suggesting that older siblings might partly compensate for the effect of having foreign-speaking parents. Overall, our results are consistent with the resource dilution (stating that parents have limited resources to distribute among their children) and inconsistent with the confluence model (stating that a child's cognitive ability is correlated to the mean cognitive ability of the family). RESEARCH HIGHLIGHTS: Our results are consistent with the resource dilution model and inconsistent with the confluence model The negative effect of the number of siblings on expressive vocabulary is partly mediated by parental interactions Larger age gaps between a child and their older sibling are associated with lower expressive vocabulary score.

Assessing consistency in children's and monkeys' performance across computerized and manual detour problem tasks.

The detour test is a widely used test in comparative psychology that assesses various cognitive abilities, including inhibitory control. The goal of this study was to assess the degree of consistency in the performance of human children and capuchin monkeys on a manual detour test and on a computerized equivalent. Because computerized testing is more common in comparative research, it is important to determine whether a computerized version of the detour problem can provide measures of general inhibition that align with measures obtained from the same subjects when given a manual version. We found that human children and capuchin monkeys approached both tasks very differently and also that there was no correlation between the manual and the computerized task within each species. This suggests caution is needed when considering one format or the other when the goal is to use a single task to provide a general measure of inhibitory control.