When chimps threaten, they open their mouth and show their teeth. It's a little like waving a knife in front of you. It's very primitive, and therefore bizarre.

Every time you cuddle with your children, you're likely to be driving down your testosterone.

Real competition can drive up testosterone, which boosts libido.

The Great Depression of the 1930s saw more American unmarried women working from nine to five, mostly in repetitive, boring, subordinate, dead-end jobs. But the number of working women doubled between 1870 and 1940. During World War II it doubled once again.

People compose poetry, novels, sitcoms - for love.