[njarboe]

One might see this as a positive feed back loop. One thing a large brain is really valuable for is having ideas about the future you want and then acting in the world to achieve that goal. The farther in the future your goal is the better brain one needs. Thus having to raise a child successfully for a long period of time will help evolve even bigger growing brains and longer childhoods.

On the other hand, maybe humans are now breaking this restriction as more and more births are by C-section. I could see a future where most births are that way (if we don't get artificial wombs first).