Recently, I read of genetic mutation that causes slower response times in a large variety of organisms. It was noted that this mutation was quickly cleared from the gene pool of all organisms except humans. Also, it was found that in people who have this mutation, the average IQ was lowered, although no effect on the IQ of an individual could be predicted. So why don't humans clear this mutation from our gene pool? I would guess that there are multiple reasons.
I would say that one obvious reason has to be that, at least in modern times, men prefer women who are less intelligence than themselves.
Another likely factor is that more intelligent people of both genders tend to have fewer children.
Also, it is probably desirable for complex human societies to have people of a variety of capabilities, because different jobs require different skills. In fact, one study found that people of average intelligence make the best drivers. Those with lower IQs had slower response times, causing them to be less able to avoid accidents. Those with higher IQs got bored, and were not able to maintain a focus on driving.
Maybe people with this mutation are less impulsive, and therefore less likely to get into fights, etc.
Maybe this mutation has a positive effect of some kind when paired with other genes.
I hope more is discovered about this during my life-time, as it really arouses my curiosity.