Identical twins are … identical.

Go figure. But we often assume they are  identical in every way possible!

Their DNA is supposedly the  same. They often sound the same. They sure look similar, to  the point where sometimes even they can't tell each other apart in photos!

I’m speaking from experience. Watching my identical twin  brothers try to identify themselves in baby pictures is honestly painful. One of them even spoke to a  mirror for a brief moment, fully believing it was his brother.

But just because we see a lot  of similarities in identical twins oesn’t mean they’re exactly the same! So let’s look into 5 ways identical  twins aren’t all that identical … including their DNA! [♪ INTRO] You’ve probably heard that no human’s fingerprints are the same as anyone else’s.  They’re like snowflakes, all completely unique and  beautiful in their own way! Now, of course this would be true for strangers.

But surely it can’t be true  for identical twins, right? Wrong! Yes, identical twins tend to  have very similar fingerprints.

And when you look at the different fingerprint pattern types like whorls, arches, and loops, you are likely to find the same  pattern types on identical twins. But again, these are similarities, not  identicalities, if that’s even a word. Their fingerprints still have differences, even if it takes a computer to see them.

A 2012 study collected fingerprints from identical twins at a Beijing Twins Culture Festival. Sounds like a hoot and a half to me! They ran these prints through  fingerprint identification software to better understand the differences  between the prints of twins.

And they found that, while  more similar than non-twins, identical twin fingerprints are indeed different. The software was able to distinguish between them… most of the time. So don’t go committing crimes in  hopes of incriminating your twin.

Your fingerprints are different  enough that it probably wouldn’t work. And also, that would just be mean. A fetus’s fingerprints form in the womb between weeks 13 and 19 of pregnancy,  and a number of factors go into determining what they’ll look like.

Umbilical cord length, position in the womb, blood pressure, nutrition,  the rate of finger growth. You’d think that living in the  same womb for 9 months would mean these factors would be pretty  similar for twins, but differences in some of them are enough to  create all kinds of variations. And those variations can be a lot  more noticeable than fingerprints.

Like height and weight! Not all twins are exactly the same size, even as babies. A 2016 study found that first  born twins were typically larger in weight and height  than their younger twin— even when they were born minutes apart.

The differences weren’t always  huge, but they were there. While researchers in this study  weren’t exactly sure what was driving that difference in size, the placenta  may have something to do with it. Previous studies have suggested  that the weight of the placenta is positively correlated  with birth weights of babies.

Basically, bigger placenta, easier nutrient transfer from mom, bigger baby! So, if the twins do not share a  placenta, the weight of their respective placentas would likely factor  into how much each twin weighs. But even if they do share a placenta, you need to consider if they  grow in the same amniotic sac.

Growing within the same sacs could  affect how the umbilical cord reaches the babies, impacting  their nutrient transfer and blood flow, and therefore, their size. These differences in sizes  often persist into childhood. It can even persist into adulthood.

One of my brothers was born just a tad smaller. Now that they’re full-grown adults, that same brother is still  a bit smaller than his twin. Sometimes those size differences arise after they’ve left the womb, though.

Illness can play a role in growth  when you’re young, and just because two people are twins doesn’t mean  they will get sick at the same time. In identical twins with a height  difference of an inch or more, a study shows that the one that  got sick more as a child is the one that typically ends up being shorter as an adult. So not only did they likely  get teased by their twin for always being sick, now they  gotta put up with being teased for being shorter!

The jokes  never stop when you're a twin. This SciShow video is supported by

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Since nutrition affects growth  as well, let’s talk about the differences in food preferences  and gut microbiomes among twins. Everyone has trillions of  microbes living in their guts. Sounds gross, but really we would have a pretty tough time without them.

They help us break down nutrients from all the wonderful and not so wonderful  foods we eat everyday. Some of them grow best in our bodies when we eat certain foods, like carbs, proteins, and fiber. There’s even evidence that they can  influence our diet and cravings by a few different mechanisms, including  communicating with our brains.

They like what they like and they  aren’t afraid to let it be known. All this is to say, when we crave a certain food, that’s often our symbiotic gut buddies  telling us what they want to eat. But what determines the  makeup of our gut microbiome?

Well, it’s determined partially by our genetics. So you’d imagine identical  twins have similar microbiomes. And that’s true!

In fact, identical  twins have gut microbiomes more similar than even fraternal twins do, suggesting that DNA is playing a part here. But they’re also determined by our environment. And growing up in the same  house doesn’t mean environmental factors don’t create differences  between you and your twin.

So, slightly different gut buddies equals differences in food preferences. For example, one of my brothers loves cornbread and the other can’t stand it. One  likes chocolate, the other isn’t a fan.

Needless to say, putting the right plate with the right food in front of the  right twin can be a challenge. And even if you manage to give  the right twin the right plate, which side of the plate do  you put their utensils on? And no, the answer doesn’t  depend on if they’re eating salad or dessert.

I mean, which hand do they use to eat? That’s right, twins don’t always  have the same dominant hand. One of my brothers is right handed  and the other is left handed.

I always thought that would  make it easier for them to share one of those goofy  two headed holiday sweaters. Now, identical twins do  have the same dominant hand slightly more often than fraternal ones do. This would suggest that genetics  probably influences handedness.

But there are still plenty of differently-handed sets of identical twins out there. In fact, they could make up as much  as 20% of identical twin pairs. So surely environmental factors  into this equation too, right?

Well, one study tested this by looking at twins that were raised in different households. These are called “reared apart twins”. The reared apart twins had  a higher rate of different handedness than twins raised  together, suggesting that there is an environmental aspect to this.

What determines handedness is  still pretty up in the air. But it seems like both environmental  and genetic factors are at play. And identical twins are often  the subjects of genetic studies like this because they have  the same exact DNA … right?

Turns out, no, they don’t!  But it is pretty dang similar. Identical twins arise when one  sperm cell fertilizes one egg cell, creating a single zygote. This is why the medical term for identical twins is “monozygotic” twins.

One zygote. That zygote then splits into  two, creating two different cells that will grow into two different human beings. But if they started from the  exact same egg and sperm cells, how could their DNA possibly differ?

Maybe you’ve already guessed … mutations! Mutations occur in our DNA constantly. When the zygote splits, each  twin has the same genome, but then the cells start dividing and  developing, totally independently.

When replicating as much  genetic material as we have, there are bound to be mistakes. And those mistakes aren't made in the same places in the DNA just because two  people happen to be twins. But unfortunately, it doesn't stop there.

Those differences in DNA caused by mutations can even produce identical twins  with different birth anomalies. While it isn’t common, one  identical twin can be born with a clubfoot or spina bifida and the  other is completely unaffected. Which means, in some identical twin pairs, there can be profound differences.

Of course, identical twins  are similar in a lot of ways. I mean, starting out from the same zygote is going to produce two individuals who look a lot alike. But just because two people  are identical twins does not mean they are the same person.  And science supports that.

For all you identical twins  out there who keep getting lumped together, now you  can tell everyone a few ways in which you and your twin aren’t so identical! I’m still probably gonna  make fun of my brothers for failing to identify themselves  in photos though, sorry! [♪ OUTRO]