Until about 25 years ago, scientists assumed that religious behaviour was simply the product of a person's socialisation - or "nurture". But more recent studies, including those on adult twins who were raised apart, suggest genes contribute about 40% of the variability in a person's religiousness.

But it is not clear how that contribution changes with age. A few studies on children and teenagers - with biological or adoptive parents - show the children tend to mirror the religious beliefs and behaviours of the parents with whom they live. That suggests genes play a small role in religiousness at that age.

Now, researchers led by Laura Koenig, a psychology graduate student at the University of Minnesota in Minneapolis, US, have tried to tease apart ...