Three part series on genotypes at the Mermaid's Tale.
Another chapter in the critique of twin studies by Jay Joseph. The problem of reflection for behavior and identity development as regards genetic inheritance claims is something I have tried to wrap my mind around. Joseph summarizes this: “The similar physical appearance and level of attractiveness of MZAs will elicit more similar behavior-influencing treatment by their social environments.” The idea being that if we hold a social environment steady, then some shared trait, like height, may encourage further shared behaviors and identity structures within those individuals. Across a given culture then a simpler shared trait will blossom into a more common shared trait, one that would hold across all niches of that society. And hence it will be questionable what a twin study would mean as regards that complex trait. Without understanding the social influence on that complex trait, it will then seem to make perfect sense that the complex trait is genetically shared. In some sense, of course, it is genetically shared, similar to how slavery often had a shared genetic component.
We can also see other complex factors. Take two twin babies, given (for arguments sake) a shared disposition for crying. The behavioral interaction of others towards those babies may go on to influence emotions and behaviors within those babies. However, us humans are complex behavioral machines. Given an emotional distaste of crying babies by adults, this will not mean that the response to crying babies has to be uniform. An adult behavioral response is capable of being tailored in many ways, such as by scientific thought of the day or by situational structures, such as care-givers that are more or less readily available. If across a given society we see rather shared behavioral responses to crying babies as well as care-givers rather readily on hand, some further emotional or behavioral characteristic may develop in these babies in response to those adult interactions. If more complex behavior and emotional structures arise out of those dynamics, what will then seem like rather correlated genetically structured responses will actually have a social dynamic that is embedded into that more complex trait but arising uniformly from a simple trait within a shared social response. This also throws the wrench into dizygotic versus monozygotic analysis.
As I have previously claimed, this is the problem with trying to pronounce shared genetic structures when we have no clue as to the mechanisms involved or even of good definitions and categorizations of the behaviors (e.g. see some of the wrangling over schizophrenia).
My first major recognition of this came in a popular book by Sam Wang and Sandra Aamodt called Welcome to Your Child's Brain. The book was enjoyable and informative on many levels, though I do have some critiques.