Tech billionaire Elon Musk once again sounded the alarm on what he sees as humanity’s greatest existential threat: declining birth rates. In a post on X (formerly Twitter), the Tesla and SpaceX CEO responded to a user who cited a Fortune report stating that women need to have an average of 2.7 children to maintain population stability.

“If things continue as they are, humans have their days numbered,” Elon Musk warned. He reshared the post and added his own calculation, noting, “People who have kids do need to have 3 kids to make up for those who have 0 or 1 kid or population will collapse.”

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Elon Musk, 53, is the world’s richest man and a father to 14 children with five different women. His personal decision to have several children is part of his effort to combat global underpopulation, he has earlier said.

“I’m doing my best to help the underpopulation crisis,” Elon Musk said in 2022, adding, “Good for big families. I want to have as many children as I can, to spend time and be a good father.”