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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.