Helen Fisher

Your sweetheart calls you by another's name. His eyes linger too long on your best friend. He talks with excitement about a girl at work. And the fire catches. Jealousy - that sickening combination of possessiveness, suspicion, rage, and humiliation - can overtake your mind and threaten your very core as you contemplate your rival.

If we remained perpetually infatuated, we couldn't eat, sleep or work.

In courtship, who wins and who loses will determine who passes on their DNA to tomorrow.

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.

Throughout evolution, ostracism was death indeed.