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Friday, January 14, 2011

Tiger mothers and behavior genetics

Everyone seems to want to know what I think about the WSJ excerpt Why Chinese Mothers Are Superior, from Amy Chua's new book Battle Hymn of the Tiger Mother. See here for some discussion on Quora.

When I first read the piece I thought Chua sounded like a nutcase -- more extreme than even the nuttiest mothers of the other Chinese kids I had grown up with. Later I realized that the excerpt is not from a parenting book, but rather a memoir, and that Chua is making fun of her younger self in the excerpted chapter. See this interview with Leonard Lopate for more nuanced comments from Chua, and this NYTimes article.

The WSJ excerpt has been widely discussed, but I have yet to see anyone point out what the real data says about parents' ability to shape their children (other than by passing on their genes). See here for a long post I wrote just over a year ago on the subject:

In what may have been the most influential article ever written in the field of developmental behavior genetics, Plomin and Daniels (1987) reviewed evidence that a substantial portion of the variability in behavioral outcomes could not be explained by the additive effects of genotype or the environmental influences of families. They suggested that this residual term, which they called the nonshared environment, had been neglected by environmentally oriented researchers who assumed that the most important mechanisms of environmental action involved familial variables, like socioeconomic status [SES] and parenting styles, that are shared by siblings raised in the same home and serve to make siblings more similar to each other. Indeed, Plomin and Daniels argued, once genetic relatedness has been taken into account, siblings seem to be hardly more similar than children chosen at random from the population.

In other words, despite a lifetime of proximity, your adopted child may bear no more similarity to you (in terms of, e.g., intelligence) than someone selected at random from the general population. The shared family environment that your children (biological or adopted) experience has little or no measurable effect on their cognitive development. While there are environmental effects on intelligence (the highest estimates of heritability for adult IQ are around .8, and some would argue for a lower value; see here for Turkheimer's work suggesting low heritability in the case of severe deprivation), they seem to be idiosyncratic factors that can't be characterized using observable parameters such as the parents' SES, parenting style, level of education, or IQ. It is as if each child experiences their own random micro-environment, independent of these parental or family characteristics.

... The naive and still widely held expectation is that, e.g., high SES causes a good learning environment, leading to positive outcomes for children raised in such environments. However, the data suggests that what is really being passed on to the children is the genes of the parent, which are mainly responsible for, e.g., above average IQ outcomes in high SES homes ...

The implications are quite shocking, especially for two groups: high investment parents (because the ability of parents to influence their child's development appears limited) and egalitarians (because the importance of genes and the difficulty in controlling environmental effects seem to support the Social Darwinist position widely held in the previous century).

I have no doubt that certain narrow skill sets (like piano playing, swimming, baseball) can be transmitted through parental effort. But the evidence seems to point the other way for intelligence, personality, religiosity and social attitudes. Are Chinese moms such outliers that they constitute a counterexample to the large-statistics studies cited above? It was shown by Turkheimer that in cases of extreme deprivation heritability can decrease significantly. Can Tiger Moms have the same effect at the positive end of the spectrum?

As a parent myself I am used to the very sloppy epistemology that is par for the course: we did X to the kids when they were younger, which is why they are so Y now, or: if you do X to your kids now, they are more likely to be Y later. In reality, as with any complex system, it is nearly impossible to relate cause and effect in a remotely rigorous way. Nevertheless, the world is full of (often contradictory) parenting advice, and most parents are absurdly overconfident in their opinions.

See figure below (click to enlarge) from Sources of human psychological differences: Minnesota study of twins reared apart, Bouchard et al. (p. 142). MZA = MonoZygotic twins raised Apart, MZT = MonoZygotic twins raised Together. On the personality inventories the MZA and MZT correlations are almost identical -- even more so than for g. Shared family environment did not make twins more similar in personality, and only slightly more similar in IQ.

I stole this figure from Razib. It's from Turkheimer's paper showing that heritability of IQ increases with SES. Thus, Tiger Moms are fighting against diminishing returns: once the child's environment is already pretty good (say, 80th percentile SES), most variation is due to genetics. Note Turkheimer's results show more shared environmental variance than Plomin and Daniels found, but everything is consistent within errors. One objection to Turkheimer's results is that his measurements are of young children, and in other studies it is found that heritability increases with age -- perhaps because children gain more control over their lives and their genetic proclivities become more manifest.

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