The first installment ended on the following note: Suppose you have many sets of same-sex non-identical twins raised together and many sets of same-sex identical twins raised together and find that the identical twins are on average more similar. It seems reasonable to conclude that is because they share all their genes whereas the non-identical twins share fewer of their genes. However, that conclusion doesn’t say that it’s the same nature—the same genes—or the same nurture that brings about the resemblance from one pair of twins to the next. Given this possibility of underlying heterogeneity where are you? What can you do?
You might take the results of the multiple twins study and transfer them back into thinking about your family. Suppose that the non-identical twins are very much less similar so that sharing fewer genes makes a big difference (skipping here the technicalities of getting the number, “heritability,” that quantifies that result). If that is so, you might say: “There’s nothing I could do as a parent to change the outcome for my offspring (for whatever trait you’re thinking about, e.g., IQ test score). I’m not to blame for the outcome (other than having passed on my genes). If that seems justified to you, you might then reason that the same is true for every other family, and thus society as a whole shouldn’t try to change what it’s doing for it won’t make a difference.
Now flip that scenario. Suppose that the non-identical twins are just as similar so that sharing fewer genes makes little difference. What can you do as a parent? Or, if your offspring are grown, what could you have done?—What then can you advise others for the future, or society at large? In this scenario, the possibility of underlying heterogeneity is a problem. Your study of twins has not shown you what environmental factors have had an effect so you don’t know what to change. And, if you can’t expect the factors to be the same from one family to the next, you might just give up on trying to identify those factors.
Notice an asymmetry in these scenarios. The possibility of underlying heterogeneity didn’t lead you to give up on looking for the genetic factors because your reasoning did not lead you to look for them at all. You simply concluded that you weren’t to blame for the outcome in your family and, by extrapolation, society shouldn’t try to change what it’s doing.
Now there’s a problem in the reasoning that says because sharing fewer genes makes a big difference there’s nothing a parent can do to make a difference. Any set of twins, call it set i, is raised together in family i—each nature i has a nurture i. There are lots of i’s. There is nothing in the average over many sets of nature i – nurture i pairs that says there cannot be a nurture j or k or l in which nature i in nurture j or k or l wouldn’t be different in interesting ways. Perhaps if you found that identical twins raised apart were just as similar on average as identical twins raised together, you’d doubt that such a nurture j or k or l could be found. You’d doubt, but not be sure. You could be surprised. Japanese offspring after WWII grew taller on average than their parents, but a comparison of twins in the previous generation woud have shown that sharing fewer genes makes a big difference (i.e., heritability for height was high).
Once you entertain the possibility that nature i varies across nurture i, j, k, l, … you can ask about what genetic factors and what environmental factors are involved and how they act together. Now there is a symmetry in how difficult it is to identify those factors if you cannot expect them to be common across families. Some responses to this difficulty are taken up in the next installment.