It’s Saturday night and the bar is packed. A beautiful brunette is sitting at the bar, sipping a vodka soda and chatting with a friend. Out of the corner of her eye, she catches the glimpse of a man. Their eyes lock.

And they lived happily ever after.

But is it really that easy?

In fact, researchers who study human attraction say, people do tend to form opinions quickly when it comes to romance. Some think it takes just three minutes to decide whether someone’s a potential mate (and that’s before the drinks arrive).

Findings about friends who click immediately might also apply to romantic partnerships. In one study on friendships, people who enjoyed the first few minutes together were likely to develop a close relationship after nine weeks.

But instant attraction isn’t just in our heads: There may be a biological basis to love at first sight.

Studies have found that animals are more likely to mate with partners they’re genetically compatible with. It’s unclear whether this research applies to humans, but some scientists think we might be preprogrammed to spot “the one.” Romantic attraction might serve an evolutionary function: We seek out specific people who will be suitable mates, and we give everyone else the boot.

But meeting that special someone’s gaze and falling in love may be a bit more complicated.

Although about half the American population believes in love at first sight, not everyone falls head over heels right away.

Perhaps surprisingly, women aren’t always the romantics: One study found that men experience love at first sight more often than women. Researchers think that’s because men respond to physical cues more readily than women, and women tend to develop trust more gradually than men.

Also, people might be more inclined to believe in love at first sight when they’re younger. A Gallup poll revealed that folks older than 50 are less likely than younger ones to think people can fall in love immediately (possibly because they’ve had more romantic relationships with different partners).

But it’s unclear how often love at first sight turns into a successful partnership.

One survey in Israel found that only about 10 percent of people say their long-term relationships began that way. While people can be instantly attracted to each other, some scientists say that being in love means really getting to know someone over time.

So don’t throw in the towel if that first date didn’t go so well. Psychologists believe more interactions with someone can make them look more attractive and intelligent. It may be worth giving love at second or third sight a chance, too.

Sometimes one look is all it takes to fall in love. But some experts think real affection develops gradually over time.