Teghan Lucas, a comparative anatomy researcher at the University of Adelaide, was fascinated with the idea of doppelgängers, that every person has a look-alike out there in the world. So Teghan analyzed thousands of photos of people, for example measuring the distance between features, to determine the probability that two people would have matching faces. According to Teghan, there's only a one in a trillion chance that you share even eight measurements with someone else. Of course, people can still look very similar even if their eyes and ears aren't separated by precisely the same distance. From the BBC:
"It depends whether we mean ‘lookalike to a human’ or ‘lookalike to facial recognition software’,” says David Aldous, a statistician at U.C. Berkeley...
When you bump into a friend on the street, the brain immediately sets to work recognising their features – such as hairline and skin tone – individually, like recognising Italy by its shape alone. But what if they’ve just had a haircut? Or they’re wearing makeup?
To ensure they can be recognised in any context, the brain employs an area known as the fusiform gyrus to tie all the pieces together. If you compare it to finding a country on a map, this is like checking it has a border with France and a coast. This holistic ‘sum of the parts’ perception is thought to make recognising friends a lot more accurate than it would be if their features were assessed in isolation. Crucially, it also fudges the importance of some of the subtler details.
“Most people concentrate on superficial characteristics such as hair-line, hair style, eyebrows,” says Nick Fieller, a statistician involved in The Computer-Aided Facial Recognition Project. Other research has shown we look to the eyes, mouth and nose, in that order.
Then it’s just a matter of working out the probability that someone else will have all the same versions as you. “There are only so many genes in the world which specify the shape of the face and millions of people, so it’s bound to happen,” says Winrich Freiwald, who studies face perception at Rockefeller University. “For somebody with an ‘average’ face it’s comparatively easy to find good matches,” says Fieller.
Photo from Francois Brunelle's fascinating series "I'm Not A Look-Alike!"