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So he checks out other girls, and he's always in the mood, but does that old "slave to his brain" excuse hold up? Perchance.

Dr. Louann Brizendine, author of "The Female Brain," just ka-bammed the world with her new book "The Male Brain," a look at the junk in men's upper bunks. The truth? Sometimes it's really not you, it's him. But he can't help it. For discussion, five of the book's biggest OMG moments:

He IS Always Thinking About It
All that stuff about fellas needing to "spread their seed around"? True. Guys think about sex "day and night," Brizendine says, and their sex drive even takes up more brain space than women's.

Men Are Better Liars
They're not all big fibbers, but studies have shown that men's brains let them feel way better about hustling than a woman's does. So the next time his poker face sends your alarm bells ringing, it might be for good reason.

Spooning Is for Girls
If he gives you the cold, snoring shoulder after sex, relax: It's simply his brain's bedtime. Oxytocin, the same brain chemical that makes women into spoonmongers turns the male brain into Ambien-laced mush. You say potato, he says zzz. And it's natural.

Yes, He Checks Out Other Women
And his brain isn't giving him any other choice. As guys run into attractive women, they literally can't help going googly-eyed. "This is an autopilot behavior," says Brizendine. Just like excessive drooling in dogs.

His Lack of Wood Might Be a Compliment
"Many men have told me that their 'stage fright' is proportional to how hot the woman is and how much they want to impress her," says the good doc. So if your hot-and-heavy moment suddenly goes Limp Bizkit, smile: His brain's just working hard to keep you around, even if the one between his legs is having performance issues.