Saturday, May 16, 2009

A question about twin studies

A relatively obvious objection to twin and adoption studies -- I feel like I've seen this mentioned from time to time but don't remember where -- is that adoptive families don't really have much socio-economic variation, esp. in terms of education, settledness, marital status etc.; in samples without a lot of environmental variation, the remaining differences are genetic because that's all they could be; therefore, twin and adoption studies don't prove any general claims about heritability. This argument strikes me as correct if you grant the premise, so either citing adoption studies is disingenuous or the premise is false. So my question is whether anyone (Dice? Grob? James?) knows of studies testing the premise that adoptive families are all alike.


Grobstein said...

The obvious defense of adoption twin studies is that they wash out some social / cultural variation even if all the families are in a narrow band of economic circumstance. But I have no idea whether the premise is correct.

Westcott said...

I was under the impression that adoptive parents are a pretty diverse bunch (religion, socioeconomic background, etc.) but this is just anecdotal evidence and it's possible there are some skews I'm not aware of.

But even if adoptive parents were all quite alike in those areas (education, socioeconomic status, marital status), it seems like some studies would still be valid. Like if you had twins that had genes for schizophrenia or social anxiety - ones that grew up in families where parents exhibited those characteristics vs. ones that didn't would tell you something about the heritability of those traits.

Zed said...

Dave -- maybe, but I'm not sure how much that helps. Hardly anyone thinks that fine cultural details determine the broad sorts of outcomes (re IQ, wages, etc.) that these studies are often cited for.

Dice -- I suppose twin studies can prove that traits are not _entirely_ environmental (there are usually easier ways of seeing this) but I'm not sure about your anxiety example. Seems like the fact of being adopted might make you likelier to be anxious no matter what.