The above figure is from a paper I stumbled upon, Genetic and environmental influences on impulsivity: a meta-analysis of twin, family and adoption studies:
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.
Shared environmental effects basically means the parents. It seems that parents have minimal impact on a child's impulsivity. This seems about right. Some parents can control their children through incentives and punitive measures, but this doesn't seem to result in a fundamental change in personal orientation. But the non-shared environment, the "unique environmental effects" in their terminology, is of some interest to me. Though that be simply be an error fact which collapses various forms of stochasticity, it may also point to cultural and subcultural norms. The incentives and punitive measures of parents are always limited to some extent. Children do spend time away from their parents at school. And, the reality is that at some point children will no longer be children, and parental influence diminishes. But cultural norms and expectations are much more difficult to evade. How many times have you known of children who refuse to eat something when urged on by their parents, but change their tune when they find out that all the kids at school like it?