The Romance System. This produces the cocaine rush you get from beginning love. And cocaine is more than an idle metaphor. The reptilian brain — one of the nervous system's most ancient parts — floods you with dopamine, just as it does after you snort a line of blow. The dopamine gives you the same high, lack of sleep, delusional optimism, and obsessive thoughts. The great poet Robert Palmer was right: You can be addicted to love.

Research shows that the female brain naturally releases oxytocin after a 20-second hug. The embrace bonds the huggers and triggers the brain's trust circuits. So Brizendine advises, don't let a guy hug you unless you plan to trust him.

(Creepy.)