The research found that religious people tend to have more children than atheists, to extent that non-religious people will one day disappear.
Quoted by The Times in the Evolutionary Psychological Science journal, scientists said: "It is ironical that effective birth control methods were developed primarily by secularists, and that these methods are serving to slowly diminish the proportional representation of secularists in forthcoming generations."
More than 4,000 students in the United States and Malaysia were asked about their beliefs and how many siblings they had.
The experts concluded non-religious children from the US, on average, came from parents who had 3.04 children, compared with the 3.2 in the general population.
The difference was starker in the Malaysian participants, with non-religious children typically having 1.5 less siblings compared to the average.
The scientists concluded: "By the mid-19th century, scientific discoveries had moved to a point that human reproduction was sufficiently well understood that fertility rates began to be impacted, especially in the emerging industrial countries."