How likely are you to meet your exact doppelgänger?

With 7.4 billion people on the planet, what are the odds that one of them looks exactly like you?

Not very good, according to a recent study by researchers at the University of Adelaide in Australia. 

To investigate how unique our faces really are, biologist Teghan Lucas compared the faces of nearly 4,000 individuals from the ANSUR database, which contains photos of US military personnel. She analyzed eight key features of each person's face, measuring the distances between features, then calculated the probability that two of the faces would match with exactly the same measurements. 

What she found is that the chances of sharing eight dimensions with someone else are less than one in a trillion. Put in other terms, there's only a one in 135 chance that a single pair of completely identical doppelgängers exists anywhere in the world. 

The findings aren't particularly surprising, considering all the different factors that shape our faces. The Christian Science Monitor's Eoin O'Carroll reports that "for two people to look exactly the same, it would require a mind-bogglingly improbable series of genetic coincidences, followed by an equally unlikely series of environmental events." There are hundreds, or even thousands, of possible gene variations to determine features such as face width, skin pigmentation, and eye color.

Furthermore, he writes, "environmental factors also play a huge role in appearance...For instance, children who are well-fed, particularly during early childhood and puberty, will wind up taller than their less-nourished counterparts."

But even though it's highly unlikely that you have a long-lost "twin" by exact standards, many people have found near-look-alikes. Ultimately, it all depends on how you define "doppelgänger," experts say. 

"For me, it’s when you see someone and you think it’s the other person," says Francois Brunelle, who has photographed more than 100 pairs of unrelated "twins" for his I'm Not a Look-Alike project, to BBC. "It’s the way of being, the sum of the parts." 

This holistic definition makes the label of "doppelgänger" highly subjective, as facial recognition expert Daniele Podini says our perception of other people's faces is "filtered by our own experiences."

Different people "read" different faces in different orders, Dr. Podini explains. For example, if you read a person's face in the order of eyes, mouth, then nose, the size and placement of that person's eyes affect the way you see the rest of his or her face. Another person might read the nose first, leading to a completely different interpretation.

Although our minds process each feature individually, our overall perceptions of a face feel closer to Mr. Brunelle's "sum of the parts" concept. To ensure that we can recognize friends even when they have just gotten a haircut or are wearing makeup, our brains tie all the pieces together, BBC reports. This lets us ignore some of the subtler details, and makes us more generous with the title "doppelgänger."

"For somebody with an 'average' face it’s comparatively easy to find good matches," says Nick Fieller, a statistician involved in The Computer-Aided Facial Recognition Project, to BBC. "I think most people have somebody who is a facial lookalike unless they have a truly exceptional and unusual face."