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.

When you fight, anger drives up testosterone in both men and women.

Men have a psychological need to show off their courage and strength. When he sees you talking to another guy, that instinct kicks in and he jumps to protect you and prove he's worthy of your love.

When you're in the throes of this romantic love, it's overwhelming - you're out of control, you're irrational, you're going to the gym at 6 A.M. every day - Why? Because she's there.

I don’t think we’re an animal that was built to be happy; we are an animal that was built to reproduce.

The brain was not built to walk into a bar, where you know nobody, and start a conversation. That's not the way humanity has courted.

We evolved in a tropical climate where the smells of plants and flowers were all around us. We spent a lot of time in the trees with a lot of sunlight and no clothes.