Continuing the series:
The authors investigate the interaction between child abuse and MAOA (gene) activity on future antisocial behavior.
FYI: MAOA is a gene on the X chromosome-- so males only have one copy. It makes MAO-A enzyme, which metabolizes serotonin, dopamine, and norepinephrine, so having low MAOA gene activity probably means less MAO-A, and thus more serotonin, norepinephrine, and dopamine. MAO-A is in CNS, liver, and GI tract; MAO-B (metabolizes mostly dopamine and phenethylamines (e.g. amphetamines)) is in CNS and platelets.
Importantly, having been maltreated in childhood predisposed you to becoming antisocial; having the MAO-A deficiency, by itself, did not. This is important: MAO activity has no effect in the absence of child abuse. Having low MAOA activity does not predisose you to violence. The abuse is the primary determinant.
What about the interaction between the environment (abuse) and biology (MAOA)? This interaction is very significant, but how you explain this interaction makes all the difference. Here's the figure:
The easy (and wrong) explanation, the one that jumps out at you, is this: if you were maltreated, having low MAO-A predisposes you to becoming antisocial.
But that's not what the figure shows. What it shows is that having high MAOA mitigated, i.e. lessened, the effect of being abused on future criminality.
What you see is that when MAO-A is high, you are protected against the effects of abuse. When it is low, abuse matters.
It may seem like the distinction between MAO being protective vs. being a risk factor is only semantics, but it isn't. How we define the problem actually generates different problems. "Having low MAOA increases your risk of being antisocial" is a very different social problem than "having high MAOA lessens the effect of child abuse." So if you're a lawyer, don't go concocting a "low MAOA made me do it" defense.
As an aside, it might be helpful if someone could explain how having low MAOA is a risk factor for agression and violence, but taking antidepressants (and MAO inhibitors especially) are supposed to make you less violent?