For example, she said, a 2008 Stanford University study found that on-the-fence voters were unconsciously swayed by candidates who looked more like them. The study morphed photos of the participants and political candidates and, while the test subjects didn't consciously detect the blended images, they consistently favored the ones that most resembled themselves.
In romantic relationships, Johnson said, research has shown that pairs tend to be of similar attractiveness.
"There's a long-standing pattern where a person's own level of attractiveness is matched in their partner," she said.
Being of equal attractiveness doesn't necessarily mean that they have similar facial features, but it could lead to common facial characteristics, such as facial symmetry and youthful qualities, Johnson said.
"Across dimensions, people who are similar tend to be attracted to each other," she said. "'Birds of a feather flock together' characterizes most aspects of interpersonal attraction."
Still, Andrew Trees, author of "Decoding Love: Why It Takes Twelve Frogs to Find a Prince, and Other Revelations from the Science of Attraction," said he was dubious about a website that claimed to match look-alikes.
While it may be true that many couples resemble each other, it's not necessarily the case that they were initially attracted to their doppelgangers. Overtime, they may mirror each others' expressions and share habits that contribute to appearance, Trees said.
"One researcher did discover that as couples are together for a long time, their faces do start to look more alike," he said.
Trees also said researchers have found that people are drawn to those that look like them because the faces look familiar. For example, one study found that if you flash the same face to someone several times, that person will find the face to be increasingly attractive.
"If you see a face that's like your own, that's obviously going to be very familiar and there's something appealing about that," he said. "It's not that I question there might be some attraction there, I just don't know if there's an underlying scientific basis to say those people are compatible."