[C]onsider the results of the study of multiple twins while thinking about your own family. Consider a scenario in which the nonidentical twins are much less similar than identical twins, which means that sharing fewer genes makes a big difference (skipping here the technicalities of getting the number—heritability—that quantifies that result…). If this were the case, for whatever trait we are thinking about, e.g., IQ test score, we might say: “There is nothing I could do as a parent to change the outcome for my offspring. I am not to blame for the outcome other than having passed on my genes.” If that conclusion seems justified, we might then reason that the same is true for every other family, and thus society as a whole should not try to change what it is doing because it will not make a difference.
Now flip that scenario. Suppose that the nonidentical twins are just as similar, so that sharing fewer genes makes little difference. What can we do as a parent to make a difference in IQ test scores? (If our offspring are grown, we might ask what could we have done, or what could we advise others to do, or what should be done by society at large?) Our study of twins has not shown us what environmental factors have an effect, so we do not know what to change. Moreover, we have to face the possibility of underlying heterogeneity, so that we cannot expect the factors to be the same from one family to the next. We might then just give up on trying to identify those factors.
Notice the asymmetry in these last two paragraphs. Although the possibility of underlying heterogeneity might lead us to give up looking for the relevant environmental factors, it did not lead us to give up on looking for the genetic factors. This is because our reasoning did not lead us to look for those factors at all. We simply concluded that we were not to blame for the outcome in our family and, by extrapolation, society should not try to change what it is doing. This asymmetry should make us suspect that there is a problem in such reasoning in thinking that, because sharing fewer genes makes a big difference, there is nothing a parent can do to change the outcome…
The quotes above are from page 158 in Taylor, Peter J. (2014) Nature-Nurture? No: Moving the Sciences of Variation and Heredity Beyond the Gaps.
(Introduction to this series of posts)