The basic idea is this: Whenever parents have genetic traits they can pass on to their children that are more valuable for boys than for girls, then they have more sons than daughters. Conversely, whenever parents have genetic traits they can pass on to their children that are more valuable for girls than for boys, then they have more daughters than sons. Physical attractiveness — being beautiful — is good for both boys and girls, but it’s much more advantageous for girls. Physical attractiveness of a woman is one of the most important considerations for men when they select both long-term and short-term mates, but a man’s physical attractiveness is important for women only when she’s looking for short-term mates. Women like to have affairs with good-looking men, but they don’t necessarily want to marry them, unless of course they are also rich and powerful.
That’s from this worthwhile interview with Satoshi Kanazawa, author of Why Beautiful People Have More Daughters: From Dating, Shopping, and Praying to Going to War and Becoming a Billionaire– Two Evolutionary Psychologists Explain Why We Do What We Do.
(hat tip: Ryan Holiday)