Twins: Nature, nurture, and epigenetics

National Geographic has a really interesting story on what we can learn about human biology and human culture from studying the lives of twins. (Last week, Mark blogged about some of the photos in the story.) The story explains the chance beginnings of the now-massive Minnesota Study of Twins Reared Apart; introduces you to twin girls from China who were adopted by two different Canadian families that now work to keep the girls in each other's lives; and delves into what we know and don't know about why some identical twins are different from each other in very conspicuous ways.

One example of this last bit is the story of Sam and John, identical twin brothers. Both are on the autism spectrum, but they appear to be on entirely different parts of that spectrum, with John experiencing much more severe symptoms that led the boy's parents to enroll him in a special school. Why would identical twins, raised in the same family, have such an obvious difference in the expression of characteristics that are probably mostly inherited? That's where epigenetics comes in.

A study of twins in California last year suggested that experiences in the womb and first year of life can have a major impact. John's parents wonder if that was the case with him. Born with a congenital heart defect, he underwent surgery at three and a half months, then was given powerful drugs to battle an infection. "For the first six months, John's environment was radically different than Sam's," his father says.

Shortly after Sam and John were diagnosed, their parents enrolled them in a study at the Kennedy Krieger Institute in Baltimore. Blood samples from the boys were shared with a team at nearby Johns Hopkins University looking into the connection between autism and epigenetic processes—chemical reactions tied to neither nature nor nurture but representing what researchers have called a "third component." These reactions influence how our genetic code is expressed: how each gene is strengthened or weakened, even turned on or off, to build our bones, brains, and all the other parts of our bodies.

If you think of our DNA as an immense piano keyboard and our genes as keys—each key symbolizing a segment of DNA responsible for a particular note, or trait, and all the keys combining to make us who we are—then epigenetic processes determine when and how each key can be struck, changing the tune being played.

Image: Twins, a Creative Commons Attribution (2.0) image from missbossy's photostream


  1. Missing the far more interesting point that one of the twins had a heart defect that the other did not…

  2. One of the more surreal moments of my bar-going life was spending a night drinking with a pair of identical twins, one who was an extremely stereotypical homosexual male, and the other who was an extremely stereotypical heterosexual male.

  3. Twice now I’ve met twin brothers who are developers (one games, one software).  In both cases, both are programmer/technical types, but each approaches it from a different brain side (right/left).  

    I’ve always known programmers can approach it from either brain side, but it’s been fascinating to see it twice now in otherwise seemingly identical twins.

  4. this Otto & Ewald photo is in one of Mark Sisson’s Primal Blueprint books… Otto was a distance runner, Ewald a shot putter, field games trained guy.

  5. CBC’s Nature of Things recently focused on autism and linked it to the use of antibiotics on infants.  Subjecting John to ‘powerful drugs’ at an early age, no doubt worsened the affects of his autism.  You can watch the doc here:

  6. Twins are never completely ‘identical’.

    “Although identical (monozygotic) twins are likely to contain identical chromosomal DNA sequences at the time the embryo splits into two, they are not truly identical. If they are females, one way in which they will certainly differ is through the random process of X chromosome inactivation. Of course, no matter the gender of identical twins, their cells will also undergo random somatic mutations throughout their lifetime. As they age, their cells are also subjected to epigenetic changes that will certainly differ between twins. Therefore, it is safe to say that no two people could possibly be exactly alike at the molecular level.”

    The key point is that these processes happen in all of us, not just twins.

  7. Twins are never identical. We all have problems with identity and individuality. The dominant majority or group (singletons) construct narratives which the minority are expected to conform to – this works for many groups – gays/women/blacks . When science proves that our prejudices are simply that why are we so surprised?  Scientists themselves do not help in their fascination with twin studies.

    1. What is up with these comments?

      The “fascination” with twins comes from the fact that they offer an opportunity to parse out at least some of the arguments of nature versus nurture.  Those opportunities aren’t perfect, but they are the best we have.

      Understanding nature versus nurture is key to really understanding all kinds of things, like:

      1 What role does DNA really play in our development/thinking/life/body?
      2 What is the role of child-rearing in our development/thinking/life/body?
      3 Are there other factors beyond DNA and child-rearing that affect our development?

      The fact that we get a biological way to study some of these things instead of the unethical method of making 100 test tube babies is a gift.  Some scientists are choosing to pay attention to it.

      But I guess they should have just asked you, because you knew everything already.

      1. Can you look at it from the twins’ point of view? I can. I was suggesting that perhaps the problem is as much psychological as genetic and concerns us all.

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