Sarah Blaffer Hardy, an evolutionary psychologist, has joked that we might be genetically programmed to set nature against nurture. The competing claims of biology and culture to explain human behaviour have certainly triggered some of the most bitter intellectual battles of the past few decades.

Recent scientific insights, however, have now called something of a truce. Last week’s study showing how genetic and environmental factors combine to affect people’s risk of post-traumatic stress disorder (PTSD) is only the latest to show that most aspects of health and personality are governed by both.

A team at Emory University in Atlanta found that people with a certain genetic profile were more likely to develop PTSD after harrowing experiences, but only if they also had abusive childhoods. Neither the gene, nor the abuse, made a difference alone.

These findings are of a piece with research from the Institute of Psychiatry in London. A variant of a gene called MAOA is linked to antisocial behaviour, but like the PTSD gene, only when accompanied by child abuse. People with another genetic variant are more likely to become depressed after divorce, bereavement or unemployment, and to develop psychosis after smoking cannabis, while a third gene affects whether breastfeeding will improve IQ.

The new consensus is proving uncomfortable for some. Since the heyday of eugenics, no serious scientist in the “nature” camp has argued that genes exclusively control behaviour . Cultural determinists, however, have been much freer with the extreme claim that genes do not matter at all.

The observation that genetics is important to conditions such as PTSD, however, can still be misinterpreted. One American newspaper, for instance, suggested that the Emory results could be used by the military to screen recruits, so that those with vulnerable genetic profiles might be excluded from combat.

Such a test would not only discriminate unfairly, by judging invididuals on the basis of statistics that apply only to groups, its results would also be meaningless. Genes, by themselves, say nothing about susceptibility to post-traumatic stress.

Though I support the Human Fertilisation and Embryology Bill, which will boost medical research of great promise, I sympathise with the Roman Catholic bishops’ campaign to allow Labour MPs a free vote. This is important legislation that must last, and that prospect would be enhanced by frank discussion. The cardinals’ insistence that Catholic MPs “have got to act according to their Catholic convictions” and oppose the Bill, however, has something of the pot and kettle about it. While opposing a political three-line whip, they are essentially imposing a religious one.

Mark Henderson is Science Editor of The Times

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