The Obstetrical dilemma.

I've been watching the BBC program 'Horizon - What Makes us Human' and there's an interesting segment on the obstetrical dilemma.

The dilemma is 'Why is human childbirth so hard when compares to other primates?'

The conventional answer is that there are two conflicting requirements:

• larger brain are an advantage
• hips that are too wide are a disadvantage

So women's hips cannot evolve to be really wide to make childbirth easy because that disadvantages the mothers too much compared to the advantages of a large brain.

The program shows some research that demonstrates that wide hips don't make walking & running harder for a woman and so there's no walking/running related disadvantage to wide hips. The program goes onto show that as the fetus grows it demands more and more energy from the mother via the placenta. There's a limit to just how much energy the mother can deliver to the fetus via the placenta. It turns out that at 9 months the fetus will need more energy than the mother can supply so birth occurs.

OK.

However, if wide hips are not a disadvantage why don't mothers have wider hips to allow for easier childbirth? What's causing women's hips to not get wider?