An absolutely fascinating new study has shown that women prefer more stereotypical-looking masculine men (strong jawbones and prominent eyebrows, both of which correlate with higher testosterone levels) for short-term flings, and less masculine-looking men for long-term relationships (via Boing Boing). I had wondered where the evolutionary pressure came from for more feminine male faces (i.e. why all men don't look like Neanderthals).
Of course, evolutionary psychology hypothesises that women choose resource-rich men as partners to help bring up their children, but then have flings with younger, more masculine men to provide better genes to their offspring. Recent studies have indeed found that ovulating women prefer more masculine-looking men, and that men are more wary of masculine-looking men when their partners are ovulating.