Caesarean sections & IQ posits an interesting theory:
As you would expect, there’s a moderate relationship between brain volume and IQ (around 0.4 when the brain is measured by the highly accurate MRI) — after all, human brains got radically larger over the last 5 million years as we got smarter, so there is clearly a connection. One limiting factor, though, is that big-headed babies are more dangerous to birth. Big skulls don’t pass through the birth canal as well. The invention of the Caesarean section has relieved that bottleneck in much of the world. Perhaps some of the Flynn Effect of rising scores on IQ tests stems from more bigger-headed babies being born and fewer women with the genes to give birth to bigger-headed babies dying in childbirth?