It's more than just a saying people use when someone falls for a person who is completely different than them. There is actually a scientific reason behind this phenomenon.
There are four different symptoms in the brain that carry traits linked to personality: dopamine, serotonin, testosterone and estrogen, Helen Fisher, who is a biological anthropologist, the Chief Scientific Advisor of Match.com and the author of "Why Him? Why Her?" told Yahoo Health.
Dopamine-oriented people are the thrill-seekers, who love novelty, spontaneity and flexibility; serotonin-driven people are generally traditionally-minded and conventional; they seek calmness, rules and set schedules, and are often more religious; testosterone-oriented people are usually analytical, logical, skeptical, a little lacking in empathy, but great at math and science; and estrogen-oriented people are intuitive, emotionally-expressive, diplomatic and imaginative, with great people and verbal skills.
Fisher surveyed Match.com users to learn about their personalities, and then took note of the kinds of people they responded to on the dating site, reported Yahoo Health.
While not all people were found to be chemically attracted to their opposites, there were patterns in those who did. The testosterone systems attracted the estrogen systems (and vice versa), which are opposites of each other.
The other two systems attracted themselves, meaning that some people are also chemically inclined to find a partner who is similar to them.
The survey didn't reveal if opposites are actually a good match, but Fisher pointed out that opposites appeared to attract even in the ancient days.
"For many years, they brought different things to the table that the other desperately needed," Fisher explained to Yahoo Health. "Estrogen needed the technicality, decisiveness and spatial skills of testosterone - someone's got to hit that buffalo in the head - whereas high testosterone needed the ability to read people, the empathy and understanding of estrogen. They have a clear, complementary relationship."