A shift of potentially historic significance is under way, perhaps without many people noticing. Last week, after an official inquiry, Sweden moved to ban cousin marriage, following in the footsteps of Norway, which announced the same intention earlier this year.

The day after the Swedish announcement, Denmark did so too. In the United States, Tennessee banned cousin marriage in April, and other states lacking the edict have placed it under review.

Alert readers may remember that I wrote on this subject 18 months ago, arguing that we in the UK should ban consanguineous marriage. The column became one of the most-read stories of the year, not because of any journalist merit but because readers instantly spotted the logic. Friends in Scandinavia said the piece had been picked up there, and an article I cited by the brilliant scholar Patrick Nash started to trend. A movement seemed to be spreading.

Why is this significant? Why do I think a ban could not only help western nations but transform the developing world by boosting growth and reducing bloodshed? Well, permit me to offer a bit of context. Humankind has been tribal for much of the past 12,000 years (since the agricultural revolution). This form of social organisation made sense because cohesive groups built on kinship are good at defending territory. And how are the tribes glued together internally? By cousin marriage. People marry within the group, the unions typically arranged by patriarchs, ensuring a clear demarcation with outsiders.