Religiously observant people have more children than other people do, according to demographic studies. Assuming there's a genetic predisposition for religion, this means the religion gene could spread relatively quickly throughout a population. Research using new mathematical models demonstrates just how quickly this could happen.
A religious group that makes up only 0.5 percent of a population could make up 50 percent within 10 generations, according to one of the models.
"All people who work in this area know there is a genetic basis to being religious, in the sense there is a genetic basis to all human behavior," said Robert Rowthorn, a professor emeritus of economics at Kings College in Cambridge, who developed the models.
There are two possibilities, Rowthorn told LiveScience: Either we all have the same genetic foundation that predisposes us to religion, or, alternatively, some people have a genetic makeup that makes them more receptive to religion than others.
For various reasons, religious people around the world have more children than secular people do: While on average, nonreligious women have one or two children, ultra-Orthodox Jewish women, for example, average six, according to Rowthorn. [Read: Teen Birth Rates Higher in Highly Religious States]
If members of religious groups do carry a gene that predisposes them to religion, it follows that the gene will become more common with every generation. In fact, for very high-fertility groups like ultra-Orthodox Jews, members will compose half of the wider population within 10 generations, the model predicts.
The gene also could become more widely distributed if members of the group who carry it leave and join the secular population, he added.
If many leave, the pace of change slows and the eventual share of the population that is religious is much lower. Even so, the gene will spread to the majority of the population with the defecting members.
"This is a purely speculative exercise," Rowthorn said of his research. He noted that the differences in fertility are variable and likely to change. In fact, up until about 150 years ago, this contrast in the size of religious and nonreligious families did not exist at all. Then birth rates fell globally, and the transition affected some groups more than others.
Even controlling for income and education, religious people have more children than secular people do, according to others' research that Rowthorn cited. In some cases, such as with mainstream churches, the difference is small, while others, such as the Amish and ultra-Orthodox Jews, the total fertility rates are three or four times higher than the secular population.
The transition to lower birth rates was driven, in part, by individualistic values of self-fulfillment that run against traditional religious teaching, Rowthorn wrote in his study, which will be published in the Jan. 12 issue of the journal Proceedings of the Royal Society B.
It is possible that by depressing birth rates, secularism could ultimately lead to the human population becoming more religiously inclined, although this would depend on the balance between fertility and defection from the religious group, according to Rowthorn.
"This is currently the situation with Islam in Europe, but it is uncertain whether it will persist," he said in an e-mail. "Muslim fertility may fall, or there may be more defections."
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You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry.
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