One is Not Like the Other: Differences in Identical Twins

This week, a mom posted a question on my Facebook page: “If twins are identical why do they have different issues? I thought they share the same genes.”

Yup, monozygotic (MZ) or identical twins do share the same genes. 100 percent of their DNA, in fact. Whereas dizygotic (DZ) or fraternal twins are merely two siblings born on the same day and share only 50 percent of their genetic make up at best, identical twins are genetic clones of one another. That’s why identical twins look and sound remarkably alike, score within points of each other on IQ tests, and even have similar likes and dislikes.

Still, if you’ve ever met identical twins or have them in your own family, you’d know that that’s not always the case and in fact, many identical twins are very different. From subtle nuances in their personalities to taste in food and music, identical twins often show preferences that are markedly different from their cotwins.

Many of these variations in character can be explained by outside influences or the environment (the “nurture” side of nature-versus-nurture theory). Peers have a strong influence on all kids including twins and often set the tone from what’s cool to wear to what sports to play. Plus, once they reach the school years many twins have a need to be seen as “different” from their cotwins and will go to great lengths to show the world that they are indeed an individual. (French psychologist Rene Zazzo called this twin phenomenon “The Couple Effect.”)

Two identical blonde twins dressed in red shirts.

But that explains personality differences. What about genetic differences? Yes, I know that I just wrote that identical twins have the exact same DNA, and they do, but believe it or not, this identical DNA acts differently. How do we know? Well, some identical twins are not susceptible to the same diseases as their cotwins. For instance, if one twin is diagnosed with Hodgkin’s disease, a highly inheritable cancer, it’s actually very rare for both identical twins to develop it. Furthermore, identical twins have a very low rate of concordance (presence of the same trait) for many psychiatric diseases such as schizophrenia and bipolar disorder. And what about developmental disorders like autism and even attention deficit disorder (ADD)? Again, while the rate of concordance is higher for identical twins than it is for fraternal twins, it’s by no means a slam dunk–just because one identical twin is diagnosed with the disorder, the other may never develop it. By why is that?

Scientists believe that although twins have the same genome (or DNA), they have different epigenome (the way that their DNA is “organized” in a cell). Throughout their lives, something changes or modifies the epigenome causing one twin’s genes to react differently than her cotwin’s. Although researchers aren’t quite sure what triggers the changes to the epigenome, many speculate it has something to do with the environment. And research has shown that while the epigenome of very young twins is nearly identical, the older they get, the greater their epigenome differs! As they grow up, identical twins move on with their own lives and come into contact with different people and places. They attend different colleges, start their own careers, get married, move to different parts of the country and so forth. As a result, they come into contact with vastly different environmental factors. And this, it seems, affects their epigenome.

So the next time you hear someone say, “Wow, they’re just so identical!” You can just smile knowing that that’s rarely the case!

16 thoughts on “One is Not Like the Other: Differences in Identical Twins

  1. Ironic Mom

    I find this fascinating. Even though my twins are fraternal, I have close friends who are identical twins. It’s amazing how different they are when you get to know them…but then they do two things similarly which has you shaking your head!

  2. Don Mashburn

    I am an identical twin and at 53 years old I can honestly from experience say my brother and I act, talk and think like one person sometimes. While visiting my twin, I needed something out of his toolbox. I laughed when I opened the toolbox and discovered that his toolbox was 75% like mine…same brand and type of tools!! My twin even had a glue stick in the upper right hand tray….EXACTLY like mine. Only identical twins share this uncommon ability.

  3. Nanner

    This is a really fascinating article. I concur that though my identical twins are very much alike with similar tendencies, these are certainly expressed in different ways. For instance, my first-born twins were (and still are!) very intense, demanding and ‘spirited’ souls. Yet, in one it came across as being loudly demanding and controlling. While in the other twin, that same trait was expressed as being anxious and clingy while holding inside lots of anxiety and not expressing it openly – kind of a ‘clingy withdrawal’. It is funny that my husband observed that he and I differ in the same way! It is also interesting to note that one is a righty and the other, a lefty and to see how this affects their personalities and abilities! I like to think of my house as a big, living, breathing science project minus the creepy nodes all over their heads!

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  5. Cheryl Leppo-Roution

    I have identical twin boys who are mirror opposites of each other which has always intrigued me since they share the same DNA and were raised in the same way eating the same foods and sharing the same experiences until they started school. I did this not to make them the same but to be fair, Once my father and I were discussing this and he felt their differences were attributed to their different souls. In any case, I am unclear how your genome changes with an environment, could you explain.
    Also I read a book in college about whether identical twin coincidences are any more random then coincidences between non-twins. I wish I could remember the name of the book but it was very statistically relevant to the twin-share concept. How much do they really share only because they are twins.

    1. Christina Baglivi Tinglof

      Scientists aren’t completely sure what changes the epigenome and are continuing to study this but they say it can be something as simple as one twin putting his fingers in his mouth more often than the other!

    1. Jenna

      My identical twins are 8 yrs old. One has been diagnosed with ADD ,and the other meets none of the criteria for the diagnosis. They have had largely different likes, dislikes, interests and approaches to life from a VERY young age. It is interesting to think that the same materials organized in different ways can produce such beautifully different results.

  6. Nancy S. Mure

    I am an identical twin and grew up grateful for having a partner in life – someone to ride the bus with, someone to experience the first day of school with. I was never alone but sometimes I found I wanted to be. You see was the twin that wanted an identity. I was the one who wanted to be unique. The question then is can identical twins be unique and identical. Would that make them unidentical?

  7. Tasha

    Nancy- Identical twins are Unique!
    My husband is an identical twin. He was diagnosed with ulcerative colitis, which has to do with an intestine malfunction. He is now cancer free for 8 years, and his twin never has had a problem. Same DNA but different… Puzzling, but this article has opened my eyes on differences.

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