Religion and spiritualism may grow because religious people will spread a 'believers' gene' among the population at large, according to a Cambridge academic.
Robert Rowthorn, an economics professor at Cambridge University, said studies showed that more religious people tended to have more children.
This, coupled with the existence of a genetic predisposition in some towards belief, led him to speculate that religion could spread.
Writing in the journal Proceedings of the The Royal Society B, he cited a worldwide study showing that the more religious had more children.
The World Values Survey, which covered 82 nations from 1981 to 2004, found that adults who attended religious services more than once a week had 2.5 children on average; while those who went once a month had two; and those who never attended had 1.67.
Prof Rowthorn wrote: "The more devout people are, the more children they are likely to have."
Some religious sects had fertility rates three or four times the general population, he noted.
If people in these groups only married within them, he said "ultra-high fertility groups would rapidly outgrow the rest of the population and soon become a majority".
For example, the Amish in the US had grown from 123,000 in 1991 to 249,000 in 2010, and were forecast to increase to 44 million by 2150 if past trends continued.
In practice, however, many tended to leave these sects or marry outside them and consequently have less children than they might have done.
Such "defections" would "slow down the spread of the religiousity gene" but not stop it, he reasoned.
"Defections from such groups will spread religiousity genes to the rest of society," he concluded.
"There will be an increasing number of people with a genetic predisposition towards religion but who lead secular lives."