Billionaire tech leader Elon Musk is once again making headlines, this time about a serious issue—population decline and the future of humanity.

In a number of posts on social media, Musk advised families to think about having at least three children. He explained that this would help balance out the number of people in the world, as many couples now have none or only one child. He warned that if this trend continues, the global population could start to shrink badly.

Musk tweeted, “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.”

He also pointed to history as a warning. He said low birth rates contributed to the downfall of ancient Rome and other strong empires that were doing well but eventually failed. According to him, many historians ignore this fact.

He wrote, “Low birth rate was the primary factor in the fall of Rome and all civilizations that enjoyed an extended period of prosperity with no serious external threats. Shockingly overlooked by most historians.”

This isn’t the first time Musk has talked about this issue. In the past, he has said that a falling population might be a bigger danger to human life than even climate change.