How Identical Twins Develop Different Personalities

By Lisa Raffensperger | May 9, 2013 1:13 pm

Why are you who you are? At a simple level, biology says you’re a product of your genes and your environment—or, nature and nurture. But the explanation is trickier when you consider identical twins raised in the same home: they have the same genes, and grew up in the same environment, but, if you’ve ever met a pair you’d probably agree, they’re different people.

A study on mice has tried to get to the bottom of this puzzle—and it finds that the ways identical mice interact with their environment differs, changing both their brain structure and their subsequent behavior. Mice that begin indistinguishable thus grow increasingly different, by virtue of how they interact with their surroundings.

Researcher Julia Freund and colleagues at various German universities began with 40 genetically-identical young female mice. These mice were housed in a custom-built mouse paradise: a cage with five levels, tubes to climb through, boxes to hide in and toys throughout.

To monitor how the mice interacted with their playground environment, scientists implanted RFID tags (like the ones that are plastered on CDs to prevent shoplifiting) under the mice’s skin and tracked their movements with small antennas throughout the cage. They gave the mice unlimited food and water and sat back and waited.

After three months, all the mice had grown more active and adventurous. But though the mice at the start all demonstrated a similar level of wanderlust, by the end of the experiment their travel patterns were decidedly different. While some mice hung around a home area and occasionally ventured outside their comfort zones, others spent equal parts of time in all the cage’s corners. What’s more, the mice’s wanderlust at the end of the experiment was correlated with how many new neurons they’d added in their hippocampus, one of two known areas where new neurons are born in adult mammals’ brains.

It’s been known that physical activity promotes adult neuron-growth (called neurogenesis), but in this case the researchers found it didn’t fully explain the differences. Mice that were very active but in a limited range showed less neuron-creation than mice that wandered over a greater area. The researchers thus concluded that the mice’s divergent experiences of their environment were driving their brain changes. In the end, about one-fifth of the differences in neurogenesis between the mice was attributable to how far the mice wandered, the researchers report in Science.

Thus identical twins, though they start with the same genes, likely develop different personalities in the same environment partially based on how they interact with their environment. This lived experience, in turn, probably changes their genes: Previous research has found that human identical twins accumulate epigenetic changes as they age, making them more dissimilar over time. In this way small initial personality differences could snowball—changing behavior, which changes brain—and result in our colorful, unique selves.

Image by Kati Neudert / Shutterstock

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MORE ABOUT: brain, personality
  • carolannie

    Which of course partially explains why poor people on average can have lower IQs than rich people, which seems to never change even when poor babies are adopted into rich families, if only because of epigenetic changes even in the womb

    • Gary Roby

      Where are you getting the information that poor people have lower average IQ scores than rich people? And how does this article possibly support that claim?

      • mikejb

        I think IQ is more based on education than intelligence at lot of the time. On average rich are better educated than poor. After it costs a lot of money to get a good education.

        • annglover

          There are many different kinds of intelligent; not just the classic Stanford Binet type.

    • Bharati_shahida

      FWIW One of India’s presidents, Dr. Abdul Kalan (a Muslim) and (the low caste) Dr. Ambedkar who was chosen to write India’s constitution, were extremely intelligent. The difference was local rich folk saw their ability and gave these 2 poor lads scholarships to get educated. Later, the Indian govt recognized their intelligence and ability. So poverty may not imply low IQ, simply a lack of resources. Also in India, and all over the world, most hugely rich folks come from very poor backgrounds rather than from the middle or rich class. These now successful folks often shape the world for the rest of us!

  • Curt Nelson

    Yes, but why do they interact differently with their environment?

    • Joseph Carboni

      Perhaps mutual exclusivity in space? When one mouse is in one place in one room playing with one toy, the other mouse cannot be in the former’s place, being forced to do something else for everything the twin does. Thus, the internal states of each mouse are changed due to the presence of the other one and diverge. Just a thought.

      • Curt Nelson

        Maybe, if the mice were housed together, but I would have thought that each mouse would have its own environment to exclude such factors to do with them interacting.

        • J1984

          Very true.

          The experiment should be repeated to compare 40 mice in the same cage with 40 mice in individual cages, with replicated environments.

          The biggest environmental factor is the mice reacting against each other, it would seem.

    • Nikita Kotlarov

      Twins raised apart are more similar to each other than twins raised together. We have a drive that helps us differentiate, so that when we lived under more difficult circumstances, and in small communities, all roles could be filled. There is no use to have 3 butchers and no builder in the same tribe.

  • J1984

    This article is very meaningful to me personally, because I am an identical twin who has a very different personality than my brother.

    I have struggled a lot trying to understand why I am so introverted and shy, while he grew to be more extroverted and socially active.

    Even though the article suggests change is much easier early in development, the article gives me hope that I can change, and the sooner the better.

    I do accept myself for who I am in the present, but am resolved to give myself a better life in the future.

    • garedawg

      Nothing wrong with being introverted. You might be the lucky one.

    • Peji

      I have identical twin sisters and one is outgoing and the other more introverted. The introvert changed dramatically when she went away to college and they live in different cities. It is much harder to be “compared” to a twin than to a sibling – I personally am glad to have been the little sister than a twin and yet am jealous – there is something magical about their relationship. I think this is a stunning discovery because random chance could so influence the outcome – e.g. one has a minor accident and avoids stairs after that, or got smacked by a playmate and reacted by being fearful…etc.

    • Tammie Metcalfe

      How are you an identical twin if your different sex? um go back to school

      • Amanda Topper

        Who r u talking to just curious cause I didbt see anyone posting about bein dif sex from their twin

      • annglover

        It was identical twin sisterS. Read it again.

  • rameshraghuvanshi

    I request you send this article to Pinker great evolutionary psychologist who boastfully wrote book Blank Slate on this subject and try to imposed on readers that nature is important and identical twins have same personality..He boastfully wrote let people call him raciest.

  • Dave Lenowitz

    Or could it be that our genetic code has a meaningful amount of randomness in it, such that even for genetically identical individuals, the response to identical experiences may not be the same. Once you start down a very wide tree with even a small amount of randomness at the beginning, you can see widely divergent outcomes…..Butterfly flapping its wings so to speak.
    Just sayin’

  • Leticia

    which volume/issue is this in Science?

    • Lisa Raffensperger

      That link has now been added, Leticia.

  • Leticia

    Thanks a lot Lisa!

  • Jack Gray

    I guess what I’m wondering is why the mice had different ways of reacting to that (same) environment in the first place. Where did that difference in behavior come from, simple chance?

  • Amanda Topper

    My identical twin boys had very diffrent personalities from day one. Charlee was quiet an calm an laid back an Otis was awake an alert an curious

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