Over the period during which IQ's have been measured, the average score has drifted slowly upwards. This is not my field, but I gather from what I have read that no really satisfactory explanation has been found for this phenomenon. The purpose of this note is to suggest one possible explanation which, so far as I know, has not been discussed in the relevant literature. Our species faces a serious problem, inherent in our basic design. Human beings have large brains, presumably because they contribute to higher intelligence and thus greater reproductive success.
Large brains require large skulls.
One possible solution is to widen the female pelvis. But a wider pelvis results in a skeleton less well designed for running, hence a greater risk of being eaten by predators. Another possible solution is for the child to be born sooner, hence smaller. But the earlier the child is born, the harder it is to keep it alive and the longer the period when it is helpless and so requires extensive adult assistance. The solution, as anyone familiar with the mathematics of constrained maximization would expect, is a compromise, pushing on all margins.