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I'm going to ask you a question and I want you to give to me straight.

Why do you workout?

Why does any guy workout?

I'm going to go out on a limb here, and say you train because you want to look great clothed and even better naked. You train so you can hook up with girls.

So let's embrace our vanity for a moment while I propose an idea:

While most things concerning women seem to be a complete mystery -- why they need to have their mouths open to apply mascara, for example -- what attracts the average woman is fairly easy to decipher. You just have to know where to look. Or more important, where she's looking.

But first, a quick evolutionary biology lesson.

Before Rolex, breast implants, and Match.com, men and women needed a way to decipher which potential mate had good genes and could provide and protect, and which one had bad genes and would likely be sickly and cowardly.

So primitive people were left with one of two possible situations:

Mate with someone with better fitness variance and their child would grow to be a healthy, capable specimen who'll have a long life and pair-up to create their own healthy child.

Or mate with one who has bad genes and get stuck with a Cheetoh-eating, wannabe-thug who doesn't move out of your cave until he's 35.

Not much has changed in our brain's hardwire since Prehistoric times. Women are still looking (although perhaps subconsciously) for guys who show better fitness variance. Therefore, by using weight-training, good eating, and proper supplementation, you can display your "good genes."

Luckily for us, we've got a few tricks up our sleeves: our body posture, movements, and gestures.

A recent study in the Archives of Sexual Behavior reveals that women are most attracted to mesomorphic, muscular men who have a hip-to-waist ratio of 0.9 and shoulders that measure 1.6 times the size of their waist.

And another study of thousands of women by psychologist David Buss confirms that women are universally attracted to men who offer physical protection. So the question becomes: what body parts do women find attractive that also offer the idea of physical protection?

Watch this one-minute video of me interviewing random girls at the Las Vegas airport for the answer!

Nate Green, Built for Show, Avery, Penguin Books