Tim Spector argues that studies of twins are the strongest evidence of a genetic basis for religiosity:

Twin studies conducted around the world in the U.S., the Netherlands and Australia as well as ours in the U.K. show a 40 to 50 percent genetic component to belief in God.

What is striking is that these findings of a genetic basis for belief are consistent even across countries like the U.S. and the U.K., with their huge differences in beliefs and church attendance. For example, in the latest surveys in the U.S., when asked, 61 percent of white Americans say they firmly (ie. without any doubt) believe in God, compared with only 17 percent of firm believers in similar populations in the U.K.–greater than a threefold difference. The opposite scenario of non-belief is also true–only a tiny 3 percent of the U.S. population report being firmly atheist compared with 18 percent in the U.K. As well as belief, participation follows separate trends in the two countries. Some form of weekly church attendance is now nearly three times higher in the U.S. than the U.K.

Skeptics among you might say that the twin studies showing similarity for belief are just reflecting some cultural or family influence that wasn’t properly corrected for in the study design. However in one study of adopted twins, the researchers looked at religious belief in a number of adopted twins raised apart. They found exactly the same result–greater similarity in identical twin pairs, even if raised apart. The conclusion is unavoidable: faith is definitely influenced by genes.