The first time he saw the beautiful young woman across the crowded room, their eyes met. One week later he told he loved her. Three months later, they were married.

Does this really ever happen outside of the movies?

Scientists say we are genetically wired for the possibility of love at first sight, but why it happens to some people and not others is largely a matter of timing and self-assurance.

If you are lucky enough to fall in love immediately and the feeling is mutual, it still isn’t possible to know if it will last. A dinner date that starts on a love-struck note could turn sour before the check arrives, yet another “lightning just struck us” couple will go on to have a life-long relationship. Because love is hard to replicate in a lab, there is little research on when and why—and for whom—love at first sight works out.

According to an annual “Singles in America” survey of more than 5,000 singles ages 21 to 70-plus, sponsored by the dating site Match.com, 59 percent of men and 49 percent of women in 2014 said they believe in love at first sight, and 41 percent of men and 29 percent of women say they have experienced it.

The survey and numerous psychological studies have found men fall in love faster than women, says Helen Fisher, a biological anthropologist and New York City-based senior research fellow at the Kinsey Institute for Research in Sex, Gender and Reproduction. “Men are so visual,” she says. “They see a woman who appeals to them physically, and it will trigger the romantic love system faster.” She adds, “Women are custodians of the egg, so they are more careful romantically.”

Romantic love is one of three systems—along with the sex drive and feelings of deep attachment—that humans developed for mating. Romantic love’s intense desire for connection with the other person typically lasts 18 months to three years, experts say. Its evolutionary purpose is to help people pick one partner and bond in order to raise a child.

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