In another experiment, evolutionary psychologist Geoffrey Miller quantified how sexually attractive a woman is to a man by recording the earnings of lap dancers in a strip club. And he tracked how this changed over their monthly menstruation cycle. As it turned out, men gave twice as much in tips when the dancer was ovulating (fertile) as when she was menstruating (not fertile). But the strange part is that the men weren’t consciously aware of the biological changes that attend the monthly cycle – that when she is ovulating, a surge of the hormone estrogen changes her appearance subtly, making her features more symmetrical, her skin softer, and her waist narrower. But they detected these fertility cues nonetheless.