Nature nurture nonsense

The question “Is it nature or nurture?”, sometimes called nature vs. nurture, or genes vs. environment is meaningless

Take a human trait. Almost any human trait. Some of that trait is almost certainly caused by nature – by one’s genes. Some of that trait is almost certainly caused by nurture – by one’s environment.

Nature vs. nurture with a well understood trait

Let’s take a trait that we understand well: Phenylketonuria. It’s 100% environmental AND 100% genetic. How’s that again? Well, for details, see the link. But for those who don’t want details, it’s a disorder that is caused by a defect in a chromosome that leads to an absence of a certain enzyme. As a result, the body can’t metabolize a certain amino acid and there are dire consequences, including early death.

OK…’s all nature. If you have this defect, you have the disease.

But….if you avoid the amino acid that your body can’t process, there are no symptoms. So, it’s 100% environment.

The reason for such nonsensical statements being nonetheless true is that, in PKU, as in many traits, genes and environment interact in a statistical sense. I’ll explain.

Suppose you have one variable – we’ll call it DV – (here, dying from PKU) that is affected by two other variables – we’ll call them IVs – (here, having the faulty chromosome and eating the amino acid). An interaction occurs when the effect of one IV on the DV is different at different levels of the other IV. Here, if you don’t have the gene, the food doesn’t matter, and if you don’t have the food, the gene doesn’t matter.

Interactions make nature vs. nurture meaningless

In statistics, when you are studying interactions, one of the first things you learn is that when there is an interaction, the main effects (here, the effect of the chromosome and the effect of eating the amino acid) are meaningless on their own. What’s the effect of the food on dying? Impossible to say. It depends on the gene. What’s the effect of the gene on dying? Impossible to say, it depends on the food.

Nature vs. nurture and less understood traits

OK, now let’s take another trait, one we understand less well. Let’s take a personality trait like being a bully. I haven’t done any research on bullying, but I’d bet that there are genetic causes. And I’d bet that there are environmental ones. And I’d bet they interact. Level of adrenaline is probably related to bullying behavior, and that is, in turn, partly caused by genetic factors. But I’d be stunned if parenting didn’t affect bullying, and I’d be stunned if other environmental factors didn’t also affect it. How might an interaction work?

Well, the effect of parenting on bullying probably depends on the personality of the child. If the parent and the child ‘match’ in some sense, all may be well. But the same parenting style with a different child might be terrible. A father who is, say, a former marine who is into football and Nascar might be a great match for a child who likes similar things, but might have trouble with a child who is interested in art and poetry and hates physical activity. That could lead to bullying.

When the trait is something even more complex, like a learning disability, the issues are compounded. But, as long as there is any genetic effect and any environmental effect and an interaction between them, nature nurture is nonsense. One example would be dyslexia. Here, the environmental cause is the fact that, in our society, reading is essential. The genetic causes are …. well, whatever they are. There is an interaction much like the one in PKU.

That’s a  first reason nature nurture is nonsense.

There are more.

Measurement issues and nature vs. nurture

To determine how much of something is something, we need to be able to measure the things. If we talk about genetic and environmental effects on height, we know how to measure height. And we have a good idea how to measure the environment (we can measure calorie intake and intake of various nutrients). And we know how to measure the environment (take the average height of parents).

But we don’t really know how to measure bullying  very well; nor do we know how to measure most learning disabilities well at all. And, for these complex traits, we don’t know how to measure environment well. And we don’t really know how to measure the genes because, for complex traits like learning disabilities, we don’t know which genes are involved.

So we are saying that there is a relationship between X, Y and Z, when we don’t know how to measure X or Y or Z. Hmmmmm…… we might be able to say that there IS a relationship. But determining the relative importance of X and Y on Z is not possible.

Twin studies and nature vs. nurture

There’s yet another reason why it’s nonsense. Take the studies of twins reared apart. Then you can say (for identical twins) that there is NO genetic difference. So, any difference in the trait MUST be due to environment. But….well, sorry, it’s more problematic. First, identical twins reared TOGETHER aren’t the same (damn humans messing up these nice theories). Second, we don’t know how to tell how different the different environments are. When children are raised by people other than their biological parents, it if often by people who are similar in various ways to their parents. But how do we tell how different two homes are? We don’t know what to measure!

Nature is important
Nurture is important

The rest is nonsense

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