It's a misconception that women get super connected to the people they sleep with but that men never do that "needy" emotional thing. In truth, sex releases bonding chemicals oxytocin and vasopressin into female and male brains, and it's vasopressin that helps a man bond with you. For an animal-kingdom example, consider the usually monogamous male prairie vole, a cute little mouselike creature. Larry and his colleagues discovered that without the vasopressin effect, the vole would turn into a promiscuous cad. No vasopressin effect, no monogamy. When a human male is under the influence of vasopressin, as all are during sex, he forms a bond with you that's kind of like an animal claiming a home; your scent, your eye color, even your apartment all become cues that make him crave you. Another animal example: If you give a male hamster a shot of vasopressin to the brain, he'll run around peeing like crazy to mark territory—that's his place, nobody else's. Release a guy's vasopressin by having sex with him, and he'll unconsciously start to view you as the territory he's bonded to. You don't have to like it, but this is where much of that famous male possessiveness comes from.

Your brain thinks your boyfriend is your baby.

I know that headline sounds nuts, but hear me out! When a mother gives birth, the cervical and vaginal stimulation immediately releases oxytocin in her brain, which contributes to feelings of reward and motivation. This phenomenon is one of the things that helps a new mother see her baby—whom others might regard as a pooping snot factory—as the most precious thing ever. But get this: When a man and woman have sex, the stimulation activates much of the same circuitry. Just as a new mother associates her pleasurable emotions with her baby's face, this circuit prompts a woman having sex to look at the man, register his face as trustworthy, and associate that specific face with the pleasure she's feeling. In fact, studies have shown that when women look at pictures of their romantic partners and pictures of their children, the brain patterns actually overlap. We believe this is why women tend to nurture their lovers while men tend to protect theirs.

Size really does matter.

Scientists used to think—and some still do—that the only job a penis had was to deliver sperm. But why, then, has it evolved to be so much longer than necessary? (The average erect non-porn-star penis is about five inches, but it only takes a two-and-a-half-inch one to get you pregnant.) We believe it's because the human penis is also built to trigger that release of oxytocin in a woman through cervical stimulation. Does that mean that average (and larger-than-average) guys could have more luck getting women to fall in love with them faster? Well, a large penis isn't required in order to have a strong connection with someone—but yes, we think it could help!

Love is an addiction (but sex isn't).

No doubt you've heard a lot about sex addiction lately, thanks to guys like Russell Brand and David Duchovny, but I believe that phenomenon is more likely a form of obsessive-compulsive disorder (which is a whole different story!). Love, on the other hand, is an addiction. Passion fades over time because the brain's reward system physically changes in the same way it does for a heroin addict: Its receptors for dopamine, a neurochemical that triggers euphoria, are altered, and in place of the rush, you feel more of a need. You must have your partner just to avoid feeling miserable. Even though being with him or her produces less pleasure than at the beginning, being without him or her produces a feeling of loss. Sigh.