It's easiest to spot around the jawline, you can see it in the shape of the chin, but often it will be around the eyes and in the shape of the forehead too. Sometimes it will be in the colouring, it's more obvious in two blondes or two dark people, but don't be fooled if one is fair and one is dark, the similarities can be just as strong. It's not just a figment of my imagination, there's some science to back up the theory, attraction between people who look alike is a real thing.
The most recent example is a pretty good one too. George Clooney is a gorgeous looking man so if half the world fancies him it makes sense that he'd see the appeal himself and sure enough his new bride, Amal Alamuddin, looks very like him.
They're not just both dark and brown-eyed, their face shape is almost identical, they both have eyes that slant down on the outside, broad foreheads and square chins.
George's VBF Brad Pitt is another serial mirror dater. He and Gwyneth Paltrow even had the same haircuts at one point, which highlighted how alike they looked. His ex-wife Jennifer Aniston has the same angled jaw as Pitt as does his current wife Angelina Jolie, whose entire face shape is like her husband's, most noticeable when someone does a freaky Photoshop and removes their hair.
It's not just Slebriddies who do it either, we are all prone to a bit of genetic narcissism it seems. The theory is that we are, on some level, programmed to advance the cause of our own genes by not diluting them with anything too dissimilar.
A study was conducted where heterosexuals were asked to rate faces according to attractiveness. In amongst all the random faces was their own tweaked to look like the opposite gender. It was, without fail, the face people rated as most attractive.
It might also go some way to explaining the cases of Genetic Sexual Attraction where relatives, separated at a young age and reunited as adults, find themselves attracted to each other.
Studies in Britain of people reunited after adoption found that in up to half of reunions there is some kind of sexual attraction, from mild to obsessive.
It happens because family members who have been separated at a crucial age do not experience the Westermark Effect whereby people who live together in close domestic proximity during their early lives are desensitised to any sexual attraction to each other. It is believed that people who experience GSA miss out on that.
There have also been cases where siblings who did not know they were related fell in love as adults. Most recently, just last August in Brazil a woman called Adriana called into a radio show which specialised in reuniting long lost family members.
She had been searching for her mother, Maria, who had given her up for adoption when she was a baby. The show reunited them and when Maria explained that she also had a son, Leandro, and explained the details, Adriana said "Leandro is my husband." They have been married for six years and have a daughter and had no idea that they were related.
Choosing a mate based on physical similarity is no guarantee of longevity, Miley Cyrus and Liam Hemsworth looked remarkably alike, as did Kate Winslet and Sam Mendes and Tom Cruise and Katie Holmes but none of those relationships have lasted.
However it does seem to improve the odds of long lasting romance to such an extent that there's a dating site based on the principle. Find Your Facemate, a dating site which matches people according to how similar they look, operates off the very principle that couples have a better chance of staying together if they look like each other.
They use an algorithm that assesses 66 different facial points and then matches them with the closest they can get to their doppelganger.
It is why people can seem to have a type, a woman whose boyfriends all look similar for instance, but perhaps it's not so much that the exes look like each other but that they look like the person who picks them.