In sociology there is something called the ‘selection effect’. For example, sociologists wonder whether religious  people are less likely to divorce because they are religious or is it because the sort of people who are religious are less likely to divorce anyway?

The same question applies to marriage. Are married men less likely to abuse drugs, alcohol, to get into fights, to commit crime because they are married, or it is because men who are less anti-social are more likely to get married anyway?

If the latter, then it isn’t marriage as such that makes men better behaved, it’s that well-behaved men ‘select’ themselves into marriage, and badly behaved men don’t get married because no-one will have them. In other words, badly behaved men are ‘selected’ out of marriage.

But now a very important study has been published that goes a long way towards answering this question. It examines 289 pairs of male twins (more than half were identical) and it finds the answer is a bit of both, that is, there is a selection effect and a marriage effect.

The more pro-social men were more likely to marry, but once married they became still more pro-social.

This study has practical consequences because among the other reasons family diversity advocates insist there is no need for the State to favour marriage is that its alleged positive effects are illusory due to the selection effect.

But if there is also a marriage effect per se, that is, if marriage itself also makes men more civilised, then that is just one more reason for the State to encourage marriage, quite apart from its pro-child character.

This study of twins indicates that there is indeed a marriage effect.