A Cambridge academic says that religion and spirituality grow because religious people spread a 'believers' gene' among the population around them.
Robert Rowthorn, who is an economics professor at Cambridge University, was writing in a scientific journal when he cited a global study that proves religious people have more children.
The World Values Survey, which covered 82 nations from 1981 to 2004, discovered that adults who attended worship activities on a weekly basis had 2.5 children on average; while those who went once a month had two; and those who never attended had 1.67.
Rowthorn says that this coupled with the existence of a genetic predisposition in some towards belief, lead him to speculate that religion will continue to spread.
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."
The Amish in the US are a good example of this as they have 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.
Although many tend to leave or marry outside of these groups, Rowthorn does not believe this will stop the spread of religion.
"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."