GENES that increase an individuals reproductive output will be preserved and spread from generation to generation. That is the process of evolution by natural selection. More subtly, though, in species that have the sorts of learnable, and thus transmissible, behaviour patterns known as culture, cultural changes that promote successful reproduction are also likely to spread. This sort of cultural evolution is less studied than the genetic variety, but perhaps that should change, for a paper published this week in Nature Ecology and Evolution, by Janet Howard and Mhairi Gibson of the University of Bristol, in England, suggests understanding it better may help to wipe out a particularly unpleasant practice, that of female genital mutilation.

FGM, as it is known for short, involves cutting or removing part or all of a females external genitaliausually when she is a girl or just entering puberty. Unlike male circumcision, which at least curbs the transmission of HIV, the AIDS-causing virus, FGM brings no medical benefit whatsoever. Indeed, it often does harm. Besides psychological damage and the inevitable risk that is associated with any sort of surgery (especially when not conducted in clinical conditions), FGM can cause subsequent obstetric complications and put a woman at risk of future infections. All these seem like good reasons why it would harm reproductive output and thus be disfavoured by evolution, whether biological or cultural. Yet the practice persists, particularly in parts of Africa and among migrant populations originating in these places. Ms Howard and Dr Gibson wanted to understand why.

To do so they drew on data from five national health surveys carried out in west Africa (specifically, Burkina Faso, Ivory Coast, Nigeria, Mali and Senegal) over the past ten years. These provided data on the FGM-statusmutilated or otherwiseof more than 60,000 women from 47 ethnic groups. That enabled Ms Howard and Dr Gibson to establish the prevalence rates of mutilation in each of these groups, and to search for explanations of any variation.

They first established formally what common sense would suggest is truethat the daughters of a mother belonging to an ethnic group where the practice is widespread are, themselves, more likely to have undergone mutilation than those of a mother not belonging to such a group. But there was more to the pattern of those results than mere correlation. The average rates of mutilation in the groups the researchers looked at tended to cluster towards the ends of the distribution, near either 0% or 100%, rather than being spread evenly along it.

In the argot of statistics, then, the distribution is U-shaped. This suggests something is pushing behaviour patterns away from the middle and towards the extremes. What that something might be is in turn suggested by the two researchers second finding: the consequences of mutilation for a womans reproductive output.

For convenience, Ms Howard and Dr Gibson defined a womans reproductive output as the number of her children still living when she reached the age of 40. Just over 10,000 women in the five pooled surveys were over this age, and it was from these that the researchers drew their data. Their analysis showed that in ethnic groups where mutilation was common, mothers who were themselves mutilated had more children over their reproductive lifetimes than did the unmutilated. In groups where mutilation was rare, by contrast, it was the other way around. At the extremes, in groups where mutilation was almost ubiquitous or almost unheard of, the average difference amounted to a third or more of an extra child per lifetime. That is a strong evolutionary pressure to conform to the prevailing social norm, whatever it is.

What causes this difference Ms Howard and Dr Gibson cannot say for sure, but they suggest that conforming to whichever norm prevails might let a woman make a more advantageous marriage, and also give her better access to support networks, particularly of members of her own sex. Cultural evolution, in other words, is generating conformity in the same sort of way that biological evolution does when, say, the plumage of a male bird has to conform to female expectations of what a male looks like if that male is to mate successfully, even though the particular pattern of his plumage brings no other benefit.

All this does, though, offer a lever to those who are trying to eradicate female genital mutilation, for unlike genetic norms, cultural ones can be manipulated. The distributions shape suggests that, if mutilation rates in societies where FGM is now the norm could somehow be pushed below 50%, then positive feedback might continue to reduce them without further effort (though such effort could well speed things up).

One thing that is known to push in the right direction is more and better educationand not just for girls. That is desirable for reasons far wider than just the elimination of FGM, however. In a companion piece to Ms Howards and Dr Gibsons paper, Katherine Wander of Binghamton University, in New York state, offers a thought inspired directly by the new research. She wonders if fostering social connections between cut and uncut women in a community might reorganise support networks specifically in a way that reduces the advantages of mutilation.

More widely, the method Ms Howard and Dr Gibson have pioneered, of looking for unanticipated reproductive advantages that help explain the persistence of other undesirable behaviours, might be applied elsewhere. So-called honour killings would be a candidate for such a study, as would the related phenomena of daughter neglect, and the selective infanticide and selective abortion of females. On the face of things these might be expected to be bad for total reproductive output. But perhaps, as with FGM, that is not always the case. And, if it is not, such knowledge would surely help in the fight against them.

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Cultural evolution and the mutilation of women - The Economist