Just because it’s not hereditary does not mean you can affect it

A comment below from Andrew:

Love to see a post about which human traits worth caring about are notable for having little or no hereditary component. It is all good and well to know what we cannot change, but it makes more sense to focus personally and as a parent on those things that aren’t genetically preordained.

This is a common sentiment I’ve seen. If you haven’t read The Nurture Assumption, please do so. I’d say a substantial reason I think that The Blank Slate is a good book is that Steven Pinker promoted Judith Rich Harris’ work.

With that out of the way: the implication in the comment above is that hereditary traits are the ones you have least control over, so you should focus on the non-hereditary traits. To some extent there is truth in this. Micronutrients are important. You don’t want to turn you children into cretins.

But a major problem with the idea that we can impact environmental impacts on characteristics is that on many traits we don’t know what those environmental impacts are. You can take a behavior genetic model and come to the following conclusion: within the population 50% of the variation is due to genes, 40% of the variation is due to non-shared environment, and 10% of the variation is due to shared environment (parents). We don’t really usually know what the non-shared environment means. It might be just developmental noise. It might be epistatic genetic effects. Or, in relation to behavior, it might be peer group, as Judith Rich Harris asserts.

We just don’t know. What that means is that the hereditary components are what you have legitimate effective control over through mate choice. And shared environment. These two combined are not nothing. And of course there is the impact of nation or community on the environmental in which propensities are expressed.

Addendum: The non-shared environmental variance was once explained to me as a “noise” factor. Just to give you a sense of how well we understand it.

3 thoughts on “Just because it’s not hereditary does not mean you can affect it

  1. I think the main reason why people focus on the non-hereditary effects is that these sort of questions (i.e. the relative important of hereditary/non-hereditary traits) only come into focus when people are older and have already made mate choices. So at that point, heredity factors are already set and beyond one’s control. Therefore to the extent that one could consciously influence traits, it’s really only the non-hereditary ones that are relevant.

  2. Shared environment is not just parents, it’s anything siblings share (neighborhood, schools etc.), including each other. Intersibling effects look the same in the classic twin design (CTD; MZ-DZ), but they have different consequences. Only parental (vertical environmental) effects can act to continue familial environments such as language, specific religion or culturally transmitted human capital traits. The existence of sibling effects can be seen in studies of parents and offspring. Usually the parent-offspring relationships are weaker than the (full) sibling-sibling relationships. E.g. for crime, I’d expect there to be substantial intersibling effects, but I have not seen a specific study.

    The extended twin design (ETD) can differentiate between these variance components. See e.g.:


    In general, there is a strong need for more extended twin designs to estimate the various variance components as well as taking assortative mating into account (big problem for CTD).

  3. Not “John” but I appreciate you addressing my comment and the point you make is a valid one, although as Fabrizio notes, mate choice is a horse that’s already out of the barn in many contexts that matter.

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