by Peter Flom

Created on: September 04, 2009

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.

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 Wikipedia. But for those who don't want details, it's a disorder that is caused by a defect in a chromosone 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.....it'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. That's a statistical term. 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 chromosone 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.

When there is an interaction, the main effects 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.

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

• 1
• 2
• Next Page >>