The Economist has this very interesting article about a study into human evolution conducted in strip clubs. Basically:

• In humans, women have evolved the ability to hide when they are ovulating (and therefore fertile) from men
• In evolutionary terms this is beneficial to the women because the men never know when they are fertile and must therefore hang around all month
• The theory is that this would trigger an evolutionary arms race and that men would evovle heightened sensitivity to women’s fertility

To test this, the researcher enlisted the help of lap-dancers. His idea was that attractiveness would translate into earnings:

• Lap-dancers who were ovulating (and therefore fertile) earned an average of $335 per shift compared to just $185 for those who were menstruating.
• Those on the pill (and therefore never fertile) earned a flat average of $185 per shift.

So it seems that men have evolved a sensitivity to femail fertility and that fertile women seem more attractive to us.