The Hardest Test
Identical (monozygotic) twins come from one fertilised egg and share almost all their DNA. Their faces develop from the same genetic plan, so they are the closest any two different people get. For face recognition, that makes them the ultimate stress test: the smallest possible gap between two identities.
Most face recognition is built to separate people who are genuinely different. Twins sit right at the boundary.
What the Research Found
Some of the best data comes from the Twins Days festival in Twinsburg, Ohio, where thousands of twins gather each August. Researchers photographed hundreds of twin pairs there in 2009 and 2010.
In a 2011 study, P. Jonathon Phillips and colleagues at NIST tested commercial algorithms on those photos. Under ideal conditions (same day, same studio lighting, neutral expressions), the best algorithms could tell many twins apart. When the two photos differed in lighting or expression, accuracy dropped sharply. Photos taken a year apart were harder still.
Humans struggle too. People who don't know the twins do poorly, though friends and family use subtle cues: the way one twin smiles, a mole, a slightly different hairline.
Read more: What Is Facial Embedding? The Technology Behind Celebrity Matching
What About Face ID?
Apple is open about this. Its Face ID security page says the chance a random person could unlock your phone is less than 1 in 1,000,000, but that the chance is higher for twins and siblings who look like you, and for children under 13. Apple recommends a passcode in those cases.
Face ID has an advantage over photo-based matching: it uses a 3D depth map of your face, which can catch differences in shape that a photo flattens out. Even so, there are plenty of videos online of twins unlocking each other's phones.
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Where Twins' Differences Come From
If the DNA is the same, why aren't the faces? Several things push identical twins apart:
| Source of difference | Example |
|---|---|
| Position in the womb | Different pressure and blood supply can affect how the face and skull develop |
| Random developmental noise | Moles, freckles and small asymmetries form differently |
| Epigenetics | Genes switched on and off differently over time |
| Life | Sun exposure, smoking, weight, injuries and dental work |
| Habits | Different habitual expressions leave different lines |
These differences grow with age. Twins who are nearly impossible to tell apart as children often become easier to separate in middle age. A 2009 study by Bahman Guyuron and colleagues of twins with different lifestyles found that the twin who smoked or had more sun exposure looked visibly older.
Read more: The Math Behind Your Face: How Machines Read Bone Structure
Twins and Celebrity Matches
Here's a fun consequence. If identical twins each run a photo through Ollie, they may get different celebrity matches. That surprises people, but it makes sense.
Ollie's model turns each face into 512 numbers, a point in a large space. Twins land very close together. But celebrities are scattered unevenly through that space, so two nearby points can still have different nearest neighbours, much like two houses on either side of a street can be in different school zones. A small difference in face, or just in photo conditions, can tip one twin toward one celebrity and the other twin toward another.
If you're a twin and want a fair comparison:
- Take both photos the same day, same place, same light.
- Use the rear camera from the same distance.
- Both keep a relaxed, neutral expression.
- Compare the full top five lists, not just the first name. You'll probably share several.
It also makes a great way to settle the age-old question of which twin looks more like whom. Try it on Ollie's compare faces tool for a direct similarity score between the two of you.