Turkheimer, Emery, and their students have analyzed the similarity of offspring of monozygotic twins with the aim of clarifying the relationship between parental traits, especially divorce, and the behavior of their offspring. Turkheimer (2008, 4) describes the logic of their analyses in two scenarios:
[I]f a genetic propensity to be aggressive makes parents more likely to get divorced, and those same genes when passed to the children make them more likely to be aggressive on the playground, then one will observe an association between divorce and playground aggressiveness that will not really be a causal consequence of divorce….But in identical twin parents…none of the differences between the children can arise from differences in the genes of their twin parent, so if the children do differ, we can (almost…) rule out a genetic explanation of the association.
Suppose poor families are more likely to be divorced than well-off families, and children raised in poor families are more likely to be delinquent. [We could] observe an association between divorce and delinquency that doesn’t have any causal relationship to divorce. But twin parents share their family history of poverty, so if the children of the divorced twin are more likely to be delinquent than the children of the nondivorced twin, the parental poverty isn’t a plausible alternative explanation…
Turkheimer (2008, 5) reports “a rich variability of outcomes”—some indicating a genetic association, some an association with the social factor, and some ruling out hypothesized associations.
Turkheimer follows up the parenthetical “almost” with caveats concerning, for example, the contribution of the other nontwin parent. Taylor (2014, 119) lists complications not included among Turkheimer’s caveats (complications that are elaborated on earlier in Taylor 2014):
a) shortcomings in estimation of human heritability, presuming that heritability is what “genetic propensity” refers to;
b) unreliability of the heuristic that all other things being equal, similarity in traits for relatives is proportional to the fraction shared by the relatives of all the genes that vary in the population;
c) heritability (“genetic propensity”) as a measure of similarity in the trait does not translate in any direct way to hypotheses that invoke underlying genes or genetic factors; and
d) asymmetry in conceptualization of genetic and environmental (or social) factors—the latter are measurable; the former unknown and potentially heterogeneous.
We do not know, however, how much the rich variability of outcomes Turkheimer points to is generated by the unreliable heuristic (issue b. above). Even then suppose that, for the purposes of discussion, we had results accompanied by an analysis of sensitivity to variation away from the unreliable heuristic value, or even that the heuristic were dispensed with because we had data on the necessary classes of relatives. Could we then interpret similarity or dissimilarity of offspring of monozygotic twins as a “reflection of the environmental and genetic developmental processes that underlie complex human behavior” (Turkheimer 2008, 5)?
My answer: Not readily. Turkheimer’s phrases “genetic propensity” and “genetic explanation” suggest an equivalence or a direct translation between measures of similarity in the trait and hypotheses that invoke genetic factors underlying the trait, but this is not so. “Genetic explanation,” moreover, suggests a symmetry with environmental or social explanation, and this is not so either (see Taylor 2014, 120-123). In this analysis of the similarity of offspring of monozygotic twins, the environmental factors are measurable and point to interventions, but the genetic factors are unknown, potentially heterogeneous, and informative only for advising close relatives, even in the thought-experiment where issues a. and b. have been overcome.
Adapted from page 118ff of Taylor, P. J. (2014) Nature-Nurture? No: Moving the Sciences of Variation and Heredity Beyond the Gaps.
Turkheimer, E. (2008). “A better way to use twins for developmental research.” LIFE Newsletter(Max Planck Institute for Human Development)(Spring): 2-5.
(Introduction to this series of posts)