Another individual trait which follows the 50-0-50 rule is the age of puberty, in particular, the age at which girls experience menarche (onset of menstruation).
In a recent study published in the Journal of Research on Adolescence, Xiaojia Ge, Misaki N. Natsuaki, Jenae M. Neiderhiser, and David Reiss use the twin study design to estimate the relative contributions of genes (heritability), shared environment, and nonshared environment to the age of puberty for both boys and girls. With the Nonshared Environment in Adolescent Development data, they estimate the breakdown to be 45-01-55 for boys, and 50-01-49 for girls. With the National Longitudinal Study of Adolescent Health data, they estimate it to be 40-05-55 for boys, 46-06-48 for girls. These estimates are very close to the 50-0-50 rule.
Other studies converge on the estimate of heritability around 50-80%, which means that the environment determines somewhere between 20-50% of the variance in pubertal timing.
This is a very good point to reiterate what shared and nonshared environment means for behavior genetics. In particular, it is very important not to conflate “shared environment” with “family environment” (even though all of the shared environment indeed happens within the family) and not to conflate “nonshared environment” with “nonfamily environment” (even though a lot of nonshared environment indeed happens outside of the family). Recall that the precise definition of shared environment is “all the influences that happen within the family that make siblings from one family similar to each other but different from those from another family” and the precise definition of nonshared environment is “all the influences that happen within and outside the family that make siblings from one family different from each other.” One of the key elements in the nonshared environment that strongly influences the timing of puberty, especially for girls, is parental divorce.
At first sight it may seem odd that parental divorce is part of the nonshared environment for the siblings in the family. After all, all the siblings share the same pair of parents, the parents divorce only once, and it happens at the same time for all siblings in the family; all the siblings experience the parents’ divorce at exactly the same time. Why, then, would it be part of their nonshared environment?
It is because siblings experience parental divorce differently depending upon their age. Except for twins (and higher-order multiple births), siblings are by definition at different ages when a singular event (like their parents’ divorce) takes place. The behavior genetic work and twin studies clearly show that the age of the girls when their parents divorce is crucial in determining its consequences for their pubertal timings, and later sociosexual orientation (as I discuss in earlier posts: I, II, III). If the girl is younger than about five years old when her parents get a divorce, she will experience puberty significantly earlier, and she will later have significantly more unrestricted sociosexual orientation (by starting to have sex at an earlier age and having a larger number of sex partners). For her older sister, who is only a few years older than her, shares half of her genes, and experiences the divorce of the same set of parents at exactly the same time, the consequences for her own pubertal timing and sociosexual orientation can be minimal.
The example of the effect of parental divorce on pubertal timing points out that sometimes a singular event that happens at a singular point in time within the family and is experienced by all siblings at exactly the same time, nonetheless constitutes part of their nonshared environment, not because the event is objectively different for them, but because the siblings themselves are different when it happens and as a result subjectively experience it differently.