You, the reader, are probably a guy, and you are probably interested in dating girls, who are probably girls. At the risk of generalization, this feature will deal with ways that guys can attract and hold onto girls, using facts that come from scientists.
Science Fact: Men are so much worse than women at reading body language that they often assume it means the opposite of what it does.
Explanation: Girls can use body language on girls, and guys can use body language on guys. Both of these work fine. But when one gender tries to use it on the other, something baffling happens. It gets all mixed up and ceases to communicate what it's meant to, like a Happy Birthday e-mail from an old person that turns into an unintelligible rant halfway through ("SO WATCH OUT 2014... OBAMA BUILD SPACE MOSQUE ON MOON... LOVE GRANDMA"). In particular, guys do not speak whatever dialect of body language girls do. For example:
Girl's Perspective: A girl sees a guy she finds attractive. She tries to catch his attention from across the cafeteria with an interested glance, but the guy blanches and looks away. Undaunted, she primps in his direction and casts him her best flirtatious look. He responds by cramming a handful of chili cheese fries in his mouth. Finally, in desperation, she makes direct eye contact and licks her lips, a gesture which certainly cannot be misunderstood. He sits there robotically eating his chili fries. Clearly he does not share her attraction, so the girl sighs and gives up.
Your Perspective: You make eye contact with an attractive girl, but then she immediately looks down and away, indicating that you are an unlovable mutant, which you already knew. When you look back up, she raises her eyebrows, surprised you have the audacity to look in her direction when you are such a smelly failure. At least you still have your chili fries. You vaguely notice that the girl is having some kind of tongue problem, and you wonder if she is wearing cupcake-flavored chapstick, or one of those other delicious flavors girls get to wear. Boy, you could go for some cupcake-flavored chapstick right now.
The Science: A 2008 study collected 280 pictures of women who were trying to appear either flirty, friendly, sad, or rejecting. Women looked at the pictures and read them just fine, but when guys looked at them, big question marks floated up over their dumb faces. "This girl wants to be my friend," they thought, looking at a picture of dangling her underpants at them. "And this one is in love with me!" they concluded, looking at a picture of a girl menacingly wielding a can of mace.
For whatever reason, we men mistake non-romantic gestures for romantic ones, and romantic gestures for friendly ones. If a woman kisses us on the mouth, we are likely to conclude that she seeks instruction in CPR. If a woman hits us with a golf club, we think she is asking us on a date. One study confirmed this by videotaping women at a fake job interview where the interviewer periodically asked inappropriate questions. ("How many words per minute can you type? How naked am I imagining that you are right now? All the way naked.") The women deflected these questions with clearly uncomfortable smiles, but men who watched the videos on mute thought the women were trying to look flirty. In short, no matter what a woman is doing, it's safe to assume we have no idea what it means.
So What Should I Do About It?
For starters, realize that the more attractive you find a girl, the more likely you are to misread her friendly signals as flirtatious ones; lust makes us worse at reading body language than we already are. Furthermore, remember that flirting is more likely to actually be flirting in "mate-relevant" settings (e.g. milling around at a school dance, as opposed to trolling for hot chicks at a funeral).
But if there's any one point to take away from all this, it's that flirtation isn't a single behavior; it's a series of flirtatious things (fluttering her eyelashes, pointing her body toward you, repeatedly offering sustained eye contact). Learning what all the flavors of flirtiness look like is probably the best way to overcome your natural analytical handicap of not being female. One psychologist spent 2,000 hours observing flirtation, and she came up with 52 kinds of flirty body language. Her list includes subtle behaviors like nodding at you and casually glancing around the room, as well as overt ones like smiling at you while raising her eyebrows. It also includes stuff like skirt-hiking, but if you need to be told that a girl who is repeatedly hiking up her skirt at you is flirting, then you are probably beyond help.
Have you ever misread a signal?