Even as atheism appears to be surprisingly popular in the United States, scientists warn that non-believers could hurt their own numbers by not having enough kids.

Psychologists Edward Dutton and Dimitri Van de Linden analyze why atheism is associated with high intelligence in a new article in Evolutionary Science and conclude that that humans have an instinctive tendency to be religious. To be atheist, meanwhile, requires an intellectual capacity that allows someone to move beyond those instincts. Surpassing that “evolved instincts” toward religion may lead atheists to look past other instincts too — like the drive to reproduce.

In an interview with Newsweek, Dutton, a researcher at the Ulster Institute for Social Research, pointed to the related trend of lower birth rates among intelligent people, and the possibility that this will drive down global IQ: Because intelligence is 80 percent genetic, and intelligent people have fewer children, then intelligence among the general population will decline as well. A similar trend could happen with atheists, and together they could have drastic effects.

“It was commented on at the end of Rome, that the upper class weren’t having any children,” says Dutton. “It’s the same now.”

It’s a provocative argument — and one that, for now, is deeply hypothetical. Other academics have argued that atheists have been around since antiquity, countering the idea that atheism is a modern projection that denies our evolutionary hardwiring.

In any case, the effect predicted by Dutton and Linden is showing up: Atheists are having fewer children. The Pew Research Center announced in April that religiously unaffiliated people make up 16 percent of the global population and that only 10% of the world’s babies birthed between 2010 and 2015 were born to religiously unaffiliated mothers. Religious people, however, continue to pop out infants at record numbers: Pew projects that by 2009, nine percent of babies will be born to parents without religion, while 70 percent will either have Muslim or Christian parents.

While President Donald Trump may believe that in America “we do not worship government, we worship God,” that’s obviously isn’t true of the entire country. In fact, another new study in Social Psychology and Personality Science suggests the proportion of atheists — part of a larger “religiously unaffiliated” demographic — in the United States is currently close to 20 to 35 percent of the population. That’s drastically higher than the 3.1 percent of the population that Pew Research Center has estimated to be atheist. If these numbers are going to keep growing, however, it may have to be through conversions — since births likely won’t be enough.

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