In which our intrepid hero nitpicks on genetics and smoking.

Yesterday, I noticed a story in the Sydney Morning Herald, citing a New Zealand scientist who believed that he’d found a genetic basis for Maori criminality. From the SMH‘s sister paper from Melbourne, The Age:

New Zealand Maori carry a "warrior" gene which makes them more prone to violence, criminal acts and risky behaviour, a scientist has controversially claimed.

New Zealand researcher Dr Rod Lea and his colleagues have told an Australian genetics conference that Maori men have a "striking over-representation" of monoamine oxidase—dubbed the warrior gene—which they say is strongly associated with aggressive behaviour. […]

"Obviously, this means they are going to be more aggressive and violent and more likely to get involved in risk-taking behaviour like gambling," Dr Lea said ahead of his presentation to the International Congress of Human Genetics in Brisbane. […]

"There are lots of lifestyle, upbringing-related exposures that could be relevant here so, obviously, the gene won’t automatically make you a criminal."

He said the same gene was linked to high rates of alcoholism and smoking among Maori. […]

"[The Maori alcohol metabolism gene] influences their drinking behaviour so they’re much more likely to binge drink than other groups which are more likely to moderate their drinking."

Nitpick one: monoamine oxidase isn’t a gene—it’s a pair of enzymes. The enzyme in question here is actually monamine oxidase A (MAO-A). Whilst there are sites on the X chromosome to code for MAO-A production, some effort should be made to distinguish the enzyme from the gene.

Nitpick two: "Lifestyle, upbringing-related exposures" account for at least 50% of outcomes. A Maori child raised in a loving, healthy environment is unlikely to be criminally violent, no matter how much MAO-A is coursing through their brains.

[Update: According to this story on the ABC Science site, genetic factors account for only 19% of many personality traits. So it seems my second nitpick may have been on the conservative side.]

Nitpick three: The gene doesn’t confer criminal violence—merely a predisposition to emotional disquiet. How an individual deals with that emotional disquiet is largely up to them.

Nitpick four: The link between MAO-A overexpression and smoking and binge-drinking is not a direct one. The nature of this link is not immediately clear.

See this collection of slides at the US National Insitute of Health, and pay particular attention to the last two. The slide titled "Brain MAO A and Smoking Status" shows a marked difference between the top row (a non-smoker) and the middle (a blacklung par excellence). And the "Epidemiology of Smoking" slide shows incidence rates amongst the mentally ill.

Let’s look at the epidemiology first:

  • 88% of schizophrenics smoke;
  • 75% of depressives smoke;
  • 80% of alcoholics smoke; and
  • 29% of the general population smoke.

This would seem to suggest that smoking is a major cause of mental illness.

However, when you look at the "Brain MAO A" slide, something else becomes apparent: the lack of MAO A in the smoker’s brain. Nicotine is a monoamine oxidase inhibitor, just like many antidepressants on the market.

Schizophrenics and depressives (and probably a fair number of alcoholics) smoke to self-medicate. In fact, MAO inhibitors are prescribed to treat smokers who want to quit.

Many alcoholics also drink to self-medicate, and there are proven interactions between alcohol and nicotine. The fact that alcohol is itself a depressant means that self-medication of mental illness with alcohol is a particularly nasty downward spiral; from a mental health perspective, it’s actually much better to smoke than to drink.

Which brings us back to Maoris and MAO-A overexpression. If you’re predisposed to be unhappy, whether through genetic or social factors, then you’re more likely to indulge in chain-smoking and binge drinking, to attempt to alleviate the symptoms of mental illness. MAO-A overexpression doesn’t automatically make you a drunk or to smoke like a chimney.

Maoris aren’t necessarily aggressive due to a single gene; they’re probably just unhappy. It’s a complex situation, one not helped by broad generalisations such as Dr Lea’s.