A case in point is provided by the recent study of regular tobacco use among SATSA's twins (24). Heritability was estimated as 60% for men, only 20% for women. Separate analyses were then performed for three distinct age cohorts. For men, the heritability estimates were nearly identical for each cohort. But for women, heritability increased from zero for those born between 1910 and 1924, to 21% for those in the 1925-39 birth cohort, to 64% for the 1940-58 cohort. The authors suggested that the most plausible explanation for this finding was that "a reduction in the social restrictions on smoking in women in Sweden as the 20th century progressed permitted genetic factors increasing the risk for regular tobacco use to express themselves." If purportedly genetic factors can be so readily suppressed by social restrictions, one must ask the question, "For what conceivable purpose is the phenotypic variance being allocated?" This question is not addressed seriously by MISTRA or SATSA. The numbers, and the associated modeling, appear to be ends in themselves.
The idea that culture, itself, is an environmental exposure does shed some serious doubt on twin studies as the gold standard to separate genetic and environmental influences on phenotypes. Tyler Cowen says it well here:
"Culture" and "genes" are two major factors determining individual outcomes, toss in parenting, and if you wish call parenting and culture two parts of "environment." It is obvious that culture matters a great deal, and this comes from knowledge which existed prior to rigorous behavioral genetic studies.
I say "soda" and people in Nebraska say "pop." Singapore vs. southern China. German musical tastes in 1780 vs. today. Rural Africa vs. urban Africa. Most concretely, if I meet someone I want to know what country he came from and grew up in; in fact that is the first thing I wish to know. "The culture word" may be overused and abused, but still the power of culture is evident.
I think that we should think carefully about how quick we are to ascribe behaviors to genetics (once we account for within culture variability) without considering between culture variability. Even worse, it is not clear that the modern world has a sufficient degree of cultural variation (given our connectivity in the modern world) to even measure this parameter properly.
Mark also has another insight as to a limitation of twin studies that I hope he posts at some point.