As soon as a guy notices you, he starts going through a checklist to determine if he'll ask you out.

You know that look where his eyes slide past you...then turn back? Wouldn't it be great to know what stopped him? You totally can.

"There are a few things men instantly observe when they meet a woman."

Jackie Black, PhD, author of "Meeting Your Match," says to learn what they are and you can maximize your, um, assets. Then you'll have your pick.

Here are some of the things that they are noticing:

1. How Thick Your Hair Is
This is all about evolution. Since caveman days, dudes have been drawn to lush locks, which signal that a woman is healthy (translation: shaggable).

2. If Your Smile Is Genuine
Sometimes your eyes crinkle a bit when you grin. Most men are good at distinguishing the vibe of this kind of smile, which says you're relaxed and fun.

3. The Size of Your Group
You don't want a big bunch (it'll be hard for him to approach) or just one friend (he'll figure you wouldn't desert her to talk to him). The perfect crew size? Three. Your two buds can keep each other company when you slip away.

4. The Pitch of Your Voice
Studies show that men are drawn to voices in a slightly higher register (think Kristin Bell's), probably because -- yep, evolution -- they're reliable signs of youth and reproductive health.

5. Your Hip-to-Waist Ratio
Yet again, blame eons of human history. Men instinctually look for baby mamas. If your waist is noticeably smaller than your hips, it signifies fertility -- a huge point in your favor. A wide belt will emphasize that hourglass shape.

6. Your Glowiness
Guys associate a radiant complexion with good mental health. That's because stress and anxiety usually cause breakouts and dull skin. So those lotions and bronzers you sweep over your cheeks? Totally worth it.

7. What's Fake About You
Not personality-fake, but artificial-accoutrement- fake: lashes, hair extensions, etc. To guys, these things shout high-maintenance! Not good.

8. Your Eyes
People focus on eyes more than they do on any other body part. And dramatic eye makeup will have him zeroing in on your peepers even more.

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