Many people say Nature vs. Nurture is an ill-framed formulation, but the challenge is to explain why in a way that accounts for the persistent popularity of that formulation.

We know, for example, that both genes and environment are involved even in the cases in which there is a single gene with a major and direct effect, such as phenylketonuria (PKU).  In PKU the development of individuals having two copies of a non-functioning allele for the enzyme phenylalanine hydroxylase (PAH) is extremely impaired by the level of phenylalanine present in normal diets, but much much less impaired if a special diet is maintained.  OK, but one might say that genes are primary here: Without the genetic condition, we don’t need to worry about the diet (the environment).  Knowing the genetics, or at least, the biochemistry associated with the genetic condition, points to the appropriate environment to alter.  In fact, all kinds of changes in upbringing of individuals with the genetic condition would have no effect.  In short, nature interacts with nurture, but it’s most important to know about the genetics and biochemistry.  And, if that’s the case for PKU, you might well suspect that there are many other genes, perhaps of smaller and less direct effect, for which the same primacy would hold.  And, why decide in advance that certain traits, such as IQ test scores, are not amenable to genetic study?  The response sketched in this blog and follow-up installments is that the issue is more than primacy of genes as sketched above; there is a conflation of family and population involved in the persistence of the Nature vs. Nurture formulation.

Starting within a family, it’s very easy for someone to see that children physically resemble their (birth) parents (where resemble means more than look like their parents—it is look more like their parents than any two people randomly picked from the population).  In most cases parents pass onto their offspring environment as well as genes, so one might well ask how important is each?  That’s hard to say when parents pass on both so you imagine identical twins raised apart from birth and ask: Does the one raised at home resemble the parents more than the one raised away?  But, of course, no offspring resembles both parents well—(generally) a child is male or female and lots of characteristics (e.g., height, hips) go along with that.  So, perhaps you look at resemblance between offspring and same-sex parent for physical traits and between offspring and the average of the parents for other traits?  That doesn’t quite work for sexual characteristics.  Conversely, the average of the parents might also capture resemblance for physical traits such as height.  You simply expect there to be variation around that average, e.g., female offspring end up on the shorter side of the average and male offspring end up on the taller side.

Once you start talking about variation and averages, and stop expecting a clean picture of resemblance in any one family, you can shift your sense of resemblance from the family to averages over many offspring-parent pairs.  Going back to the identical twins, if you imagine many such pairs of twins, on average does the one raised at home resemble the parents more than the one raised away?  You could also ask, if you have many sets of same-sex non-identical twins raised together and many sets of same-sex identical twins raised together, on average do the does one identical twin resemble the other twin more than one non-identical twin resemble the other?  (Shifting to same-sex twins means that we don’t have the complication of differences between an offspring and its other-sex parent.  And shifting to twins raised together means you don’t have to search for the rare cases of twins separated and raised in truly independent families.)  If the answer in the twin-resemblance study is yes, it seems reasonable to conclude that the identical twins are on average more similar 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.  So then where are you?  Stay tuned for the next installment.