The possibility of two people seemingly unrelated look exactly like each other. Is it very common or a rare occurrence? Is there a scientific explanation to that if so?
In: Biology
I can’t name a study, but I would hazard that it’s uncommon, but not rare. My reasoning: There are about seven billion people, most of them have all their facial features and limbs, plenty of them are within the same height (4-6) age (10-100) and weight (90-200) range, most of them wear clothes and haircuts that are shared by other humans of their appearance.
If you lined up everybody in the world and put them adjacent to the two people who looked the most similar to them you would not have difficulty finding practically identical partners with any average, unremarkable looking person. With very distinctive people, who possess unique combinations of features, nonstandard bodyplans and faces, and/or bizarre fashion sense, it would be drastically harder to place them, but you would still have a reliable continuity from one person to another.
This is related to the birthday paradox.
The birthday paradox is about the probability of two people in a group having the same birthday. For example, suppose you’re at a party. What is the probability that someone else at the party has the same birthday as you? Obviously it depends on the number of people there, but it’s not a very high probability. If there are only two of you there, the probability that the other person has the same birthday as you is 1 in 365 (not accounting for leap days). If there are 10 other guests, the probability that at least one of them shares your birthday is about 1 in 37. In fact, there have to be at least 253 other guests there before the probability of someone sharing your birthday is larger than 1/2.
This is all quite intuitive. There are 365 days in a year, so every person at the party is like a lottery draw from 365 numbers. The probably of winning that lottery isn’t that high, and you need to play a lot before your odds of winning it become substantial.
But now, let’s ask instead: what is the probability that *any two people* at the party have the same birthday. It turns out that this probability grows much more quickly with the number of guests. In fact, if there are 23 people at the party, there is already more than a 50% chance that two of them will have the same birthday.
How can this be? The most intuitive explanation I’ve heard is this: the first situation we talked about was you pairing up with every other guest at the party, and comparing birthdays. If there are N guests at the party (including you), the number of pairs you can make with other people is N-1. Every new person arriving at the party just adds 1 to this number. However, the number of *all possible pairings* among guests at the party grows much more quickly. With 3 guests, there are only 3 possible pairings. With 4 guests, there are 6. With 5 guests, there are 10. 10 guests: 45 pairings. 23 guests: 253 pairings.
Hang on… 253? Where have we seen that number before? That’s right: that was the number of other guests there needed to be to make it more than 50% likely that one of them had your birthday. That number created 253 pairings with you as a member. But we no longer care just about you now. We’re satisfied if *any* two people share a birthday. So, now we just need to have enough guests so that there are 253 unique pairings among them (most of which don’t involve you). Turns out: you only need 23 guests for that.
Do you have any idea how many pairings there are among 7 billion people (the approximate population of Earth)? It’s a lot. A whole lot. More than 10^(19), which is a 1 followed by 19 zeros, or 10 quintillion (10,000,000 trillion). So no matter how small the chance that *you* specifically would encounter someone who looks exactly like you, the number of possible encounters will offset that small chance to make it a certainty that some pair of doppelgangers is going to meet.
There is also the question of how similar two people have to look for us to be impressed and say “you look exactly alike!”. Many people, for instance, are surprised to find out that the Olsen twins are not identical twins but paternal twins. That is, their genetic similarity is (statistically) no closer than “regular” sisters who didn’t share a womb. If you examined their anatomy carefully (don’t, you’ll be arrested) you’d find plenty of differences that reflect their non-identical DNA (which statistically has only 50% overlap). And yet they still look similar enough that many people assume they are identical twins. This is all just to point out that human genetics may be more variable than human *appearance* – at least to the human eye.
You may also remember those pictures of the two bearded ginger dudes on a plane who were seated next to each other on a plane and discovered they were doppelgangers. Their resemblance was deemed impressive enough that it was reported on worldwide. And yet if you look more closely, you’ll see their facial features are quite different. If you had dyed their hair and shaved off their beard & ‘stache, it’s doubtful that they would have given each other a second look after boarding the plane. In other words: sometimes it’s just a few features that need to match for us to be impressed by the resemblance, and the chance of that happening is quite high.
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