I read a lot of science fiction, and I read a story many years ago, but can't remember the author or title. In this story, humans colonized another planet (actually several) and once babies started being born, they "reverted" to a less aggressive form.

The story made an analogy with locusts, which become more aggressive with population pressure, then fly out to other places, and revert back to their less aggressive form.

This story is told from the point of view of one of the medical folks on this planet who is trying to determine why all of the children are not really verbal, seem to be less intelligent, less curious, etc. than expected.

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    The main thing I remember about that story is that I never understood how it could possibly be "programmed into our genes" for every fertile member of the human race to simultaneously start giving birth to a dumber, more primitive form of hominid, just because a few humans had made it to another solar system to colonize a new planet. – Lorendiac Jan 19 '19 at 2:58
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    I can't understand how a small group of humans settling on a distant planet can make Earth less crowded than it was for hundreds of thousands of years when primitive hunter-gatherers sometimes fought tribal wars and were about as violent as modern humans despite having only a tiny fraction of the population density of modern humans. – M. A. Golding Jan 19 '19 at 17:23
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    @M.A.Golding That was kinda my point. Imagine someone living in a village of about 100 people in the middle of the Amazon rain forest. He may not even know that a bunch of colonists have taken off for Tau Ceti. Granting that he knows, how does it affect the local environment in a way that suddenly causes a bunch of previously-inert "junk DNA" in his reproductive organs, and in those of every other human of his acquaintance, to suddenly "wake up" and cause the next generation to be a less-evolved form of hominid? – Lorendiac Jan 22 '19 at 2:46