Parenting and the Non-Shared Environment

There is a major thread in the parenting literature that claims, in short, that parenting (short of outright abuse) has little to no effect on adult outcomes that we care about. Two exemplars in this category include The Nurture Assumption and Selfish Reasons to Have More Kids. This claim is largely based on twin and adoption studies, which allow us to attribute the observed variance in traits to genetic, shared environment, and non-shared environment factors. The general pattern is that lots of traits are about half genetic and half non-shared environment, with little contribution from shared environment. There is a major embedded assumption in this line of reasoning: that parenting effects are mainly in the shared environment. It turns out that this assumption is not a particularly good one.

A review article on research into the non-shared environment was released about two years ago, and it provides some fascinating data on this subject. I will quote some sections at length, but the entire paper is worth reading. Here is the first excerpt of interest:

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