Sex!

Hogamous, higamous, man is polygamous;
Higamous, hogamous, woman's monogamous.

Or so runs the traditional doggerel.

"Bullshit", says psychologist Christopher Ryan, co-author of "Sex at Dawn: The Prehistoric Origins of Modern Sexuality".

This is a problem because there is no reason to believe monogamy comes naturally to human beings.  In fact, for millions of years, evolutionary forces have cultivated human libido to the point where ours is arguably the most sexual species on Earth.