If the new horror movie "Us" and the many evil-twin plotlines in soap operas are to be believed, there's a doppelganger lurking around out there just waiting to take over your life with your partner, boss and best friends none the wiser. ("Doppelganger" is a German word meaning "double-goer," a person who looks exactly like you but is no relation.) Fortunately for those of us who scare easily, the math says that it's just about impossible.
"If we are talking about measurements of the face there is a 1 in a trillion chance that 2 or more people will match one another on 8 measurements of the face," emails Dr. Teghan Lucas, forensic anthropologist at The University of New South Wales and Flinders University, both in Australia. She published a study on the subject of doppelgangers in 2015 where she compared people for eight facial and eight body measurements to see how alike they really were. Some of the fastidiously precise facial and head measurements she refers to include ear length, head circumference and the distance between the centers of the right and left pupils.
When she looked at the rest of the body, the odds of finding a doppelganger became bleaker. "If we look at measurements of the body, the chance is even lower at 1 in a quintillion based on 8 measurements. This is because these measurements are larger and thus have a larger range which means there is less chance for people to match each other."
So, the more measurements you consider, the less likely it is that anyone's going to 100 percent stack up against another person in true doppelganger fashion. "Two people may look very similar to the naked eye but when you start measuring they will not match each other," Lucas says.
Despite these odds, many of us have had the experience of being approached by a stranger and asked if you were someone you weren't, or related to someone they could not possibly be. So it's tempting to believe in the potential for doppelgangers.
"A lot of the people we see as doppelgangers are people we don't know very well," explains Dr. Michael Sheehan, a neurobiologist at Cornell University. "You're a lot more likely to see a doppelganger of an acquaintance than your mom. You know your mom really well. If someone looks similar you can appreciate it," he says, adding that you'd nonetheless be likelier to quickly spot the differences, as opposed to someone you don't know as well.
"There's only so much variation out there," he says. "Some people will happen to look similar."
Just because doppelgangers aren't really a thing doesn't mean that researchers will stop their investigation into body characteristics any time soon. In fact, the subtle differences in human features are becoming more important in criminal investigations, as found by Lucas and co-researchers in the International Journal of Legal Medicine study. "We looked at whether the face really was the best for identification and we found that it wasn't; measurements of the body are more unique than those of the face," Lucas explains. "This is extremely useful as many criminals who are caught on CCTV systems (such as those robbing a bank) cover their face, so we needed to find out if we can use the body in the same way as the face is used and we can!"
Even if you do run across someone who looks eerily similar, it's likely that time, hairstyle and other factors will distort any doppelganger potential. "Growing up one of my younger brothers was like the spitting image of me when I was 3," Cornell's Sheehan says. But time has done a number on that. "We look similar-ish now, but not the same."