Nature or nurture? It’s a zombie debate

In 1997, the one-hit wonder White Town song, Your

Woman, rocketed to the top of the charts all over the world. In a preview of the Me Too movement, the singer asks about the reasons for a man’s bad behaviour: ‘‘Is it in your genes?’’

If something is in our genes, it is surely fundamental to our being. According to this view, genes determine our fate – we are the inevitable consequence of our genetics. It is hardly the man’s fault, his genes made him bad.

But this isn’t how genes work. The view that genes determine our behaviour (or anything else about us) is not supported by scientific evidence. This is not to say, of course, that genes are unimportant. As a geneticist, I am well aware that genes are crucial to every aspect of what makes us human.

Genes do not act alone. Environment is critical as well. Indeed, some commentators would have us think that environment is all that matters in explaining things like bad behaviour.

Maybe environmental exposure to ‘‘high-brow Marxist ways’’ was the root cause of the man’s naughtiness in the White Town song. It wasn’t his fault, it was his upbringing. But, just as it isn’t all genes, it isn’t all environment, either.

You may recognise this genes versus environment argument as the latest incarnation of the old nature-nurture controversy. This seemingly un-killable debate rages on like a zombie, blundering into letters to the editor, coffee-shop discussions and sophisticated dinner party conversations.

Why does the zombie not die? After all, we know that genes and environment – nature and nurture – both matter.

A recent article I co-wrote with Professor Marlene Zuk of the University of Minnesota in this month’s issue of the journal

BioScience tries to explain what scientists have long known.

In short, no trait, whether behavioural or otherwise, is caused by either genes or the environment or even by the sum of the two. The interaction between genes and environment is the important feature.

What we mean by interaction is that the effect of genes depends on the environment, just as much as the effect of environment depends on the genes.

Here’s an example. Most parents will recall that, soon after birth, their new baby had a heel-prick blood test for a number of diseases, one of which was phenylketonuria (commonly known as PKU). Babies with two copies of a defective PAH gene cannot properly metabolise the amino acid phenylalanine.

Levels of phenylalanine build up in their bloodstream and eventually lead to severe intellectual disabilities.

This condition occurs, however, only when the baby’s diet contains phenylalanine. In its near-absence, the baby will develop quite normally. (Babies need a very small amount of phenylalanine; none at all is, in fact, fatal.)

So, the effect of the PAH genes depends on the diet (the environment), but also the environmental effect of the diet depends on whether the babies have two defective PAH genes.

Importantly, it isn’t enough to say, even in the case of PKU, that it is both genes and environment. This view invites the question, ‘‘Well, which is more important, genes or environment?’’ And the zombie is back, having us argue about nature versus nurture again.

This choice between genes and environment is wrong: genes work in the context of environment and environment forges in the context of genes. Interaction has decapitated the zombie.

Dr Hamish G Spencer is Sesquicentennial Distinguished Professor in the Department of Zoology at the University of Otago.