Comments on: Somatic mutations make twins’ brain less similar

By: The Amateur Thinker » Links of the day

The Amateur Thinker » Links of the day — Tue, 12 Jul 2011 00:08:22 +0000

[...] Somatic mutations make twins’ brain less similar. A new study indicates that one of the reasons that even identical twins differ in their [...]

By: ben g

ben g — Thu, 26 May 2011 09:26:35 +0000

kjmtchl,

We’re on the same page that shared effects are relatively small in the middle class populations they sample for these studies.

To get a bit deeper, assuming the shared environmental figures are insignificant as you say, why do you think natural selection would favor such a high degree of biological determinism? Wouldn’t there be a selective advantage to organisms who psychologically adapt– not just in beliefs/values, but also in emotional and cognitive abilities tendencies– to the conditions they’re raised in? Of course, too much malleability would be a liability because your psychological development would be at the whims of other individuals, as well as environmental flukes. But still, don’t you think there would be a selective advantage to malleability beyond “give me the right biological inputs (nutrition, etc.) and I’ll develop my genetically programmed personality”?

By: kjmtchl

kjmtchl — Thu, 26 May 2011 07:54:45 +0000

Thanks ben, for your comments and your interesting post. My take on the shared environment data for IQ, even taking the caveats and assumptions of the study designs into account, is that if an effect exists at all it is fairly small. Does this mean family environment doesn’t matter for IQ? Not at all – it only means that differences in family environments across the ranges within each study do not contribute largely to differences in IQ across each study. The Flynn effect clearly indicates environmental effects on IQ (most likely nutritional but possibly also educational) and trans-national differences in average IQ likely reflect the same factors. So the lack of a contribution of family environmental variance to phenotypic variance in any given population (especially Western ones) may just mean that the variance across families is not that great for the kinds of factors that affect IQ.

By: ben g

ben g — Thu, 26 May 2011 00:08:54 +0000

Correction to what I just wrote.. “nonshared environmental variation” should be “additive genetic variation”. There is of course no assumption in the calculation of e^2 because its just 1 – rMZ.

The reason this matters is, once you have significant shared environmental variation, systematic causes of the nonshared environmental variation (e.g. peer groups, status competition, etc.) become plausible.

By: ben g

ben g — Wed, 25 May 2011 23:35:27 +0000

Great post..

I think part of the “nonshared environmental variation” is actually shared environmental variation that is ignored because of assumptions inherent in the behavioral genetic methods. See here for my discussion of this with regards to IQ: http://www.gnxp.com/blog/2009/11/does-family-matter-for-adult-iq.php