Miscalibrated Minds: Why Don’t We Apply What We Know About Twins to Everybody Else?

Diane Arbus identical twins photo

We’re Inconsistent About How Much Weight We Attribute to Genes
I think our intuition might be miscalibrated when it comes to evaluating how much a person’s genes impact how they turn out physically (which isn’t surprising). What’s a bit strange is that we seem to be closer to the truth when it comes to twins.

Nobody’s surprised when identical twins turn out to have very similar bodies (weight, muscle mass, etc), even into adulthood.

But when it comes to non-twins, people seem to think that “making the right choices” and “willpower” are primary factors in how human bodies turn out, and that we can assign a good amount of personal credit or blame to individuals for good and bad outcomes.

There is a disconnect between these two visions, and I think that it’s the latter that needs to be updated.

After all, even if we put aside the direct ways in which our genes build our bodies (encoding how our tissues grow) and instead look at our abilities to “make the right choices” and exert “willpower”, we find that those are also greatly determined by genetic factors. Identical twins probably turn out very similar in good part because they have almost identical amounts of those qualities of mind.

Wrong by Degrees
This doesn’t mean that all is pre-determined and that if we all stop trying we’ll turn out the same we would have otherwise, but rather that we are playing within certain parameters, and that the part we control is probably smaller than most people think (not non-existent — we still deserve some credit — just more modest).

To be clear, I’m not saying the situation was white and we thought it was black, or even that it’s a black & white thing, but rather that most people’s intuition might be the wrong shade of gray. Otherwise, I would think there would be a bigger variation between identical twins, but they spend their lives making different choices yet most stay very similar to each other (as far as I know — if you know of a study on this, please send it my way).

This article has been cross-posted on LessWrong. There’s more discussion of it in the comments over there.


12 Responses to “Miscalibrated Minds: Why Don’t We Apply What We Know About Twins to Everybody Else?”

  1. James Says:

    But what if willpower and judgement are genetic?

  2. Scott Wiersdorf Says:

    There was an article today that maybe you saw:


    that discusses the lifestyles between identical twins and how it contributes to their differences. The relevant quote from the article for me was this:

    > Most people do not have any way of knowing whether or not they are getting the most out of their genes but if you have a twin sibling then you have an exact comparison.

    which was what came to me when you said “we are playing within certain parameters, and that the part we control is probably smaller than most people think.”

    One of the great joys (and simultaneously—frustrations) of life is discovering what those parameters are and how much they can be pushed. I’m astonished far more at what I *can* learn and *can* do than what I can’t (maybe my expectations are just low!)

  3. Z. M. Davis Says:

    “This doesn’t mean that all is pre-determined”

    But to the extent that something isn’t predetermined by genes, it just ends up getting predetermined by the environment. Draw your boundaries of moral responsibility however you please, but in fact there’s no stepping outside of physics.

  4. don Says:

    I think the analysis of twins should reveal one staggering fact, although they are identical physically, they can be night and day apart in personality. I find that twins show clear evidence for the idea of the mind/soul/i.

    I am married to a twin and live with my sister-in-law, the other twin, and you couldn’t find two people more different. They are genetically identical, but the non-physical (dare I even speak such blasphemy) aspect of their being is completely the opposite of sameness.

    I believe twins present a problem for the material monist assumption of the physical being the only dimension of ontology. I would love to chat more about it (email me if you’d like)

  5. Christos Says:

    Try taking a look at this.

  6. Jerome Says:

    It seems that you could find studies that prove environment is a factor. I just read this yesterday: http://news.bbc.co.uk/2/hi/health/8280431.stm

  7. Michael Graham Richard Says:

    Just to elaborate a bit: I’m not saying that the environment or personal decisions aren’t a factor. Just that we seem to be assigning a bigger % to them than seems to be warranted by the facts.

    Steven Pinker writes about this in The Blank Slate, though he’s mostly referring to psychological aspects while here I’m talking about physical ones (which are related, of course).

    • Brooke Says:

      I agree. I think this discussion could easily sway off topic if we begin analyzing psychological, traumatic, or exuberant conditions or forget we are focusing on identical twins. Obviously different sets of twins will produce variable results, in part due to the possibility of a number epigenetic modifications happening between the twins. But I am intrigued to pursue this further focusing on body composition.

  8. Ismael Says:

    I know a couple of identical twins who are different enough in their body building that at first I didn’t believe they were twins. It’s obvious they are brothers, but not twins.

    One is slightly fatter than the other but both are aprox. the same height (like 1.80 meters, sorry I’m too lazy to convert to feet/inches). Although the slimmer one is like two centimeters taller. My only explanation is that it must have been the environment/education what produced those differences.

  9. Michael Graham Richard Says:


    I suspect that if we looked at a large sample of identical twins, we’d find a kind of bell curve distribution when it comes to how similar they are. A few would be almost identical physically, the vast majority would be very similar, and a few would be pretty different.

    But that’s just a hypothesis. Maybe in reality there’s more difference than I think. It could be because we tend to notice (and remember) twins more when they are very similar than when they are very different.

  10. Brooke Says:

    This blog was forwarded to me last night and I have not been able to stop thinking about it. I found this supporting article http://www.nytimes.com/books/first/w/wright-twins.html and while it is long and detailed, it is worth the read. Having spent a majority of my life involved with health and wellness, this is the first time I have been challenged to think about my own beliefs regarding twins and body mass index outcomes as teens or adults. I have a new perspective.

  11. inhahe Says:

    I personally believe that personality, will, etc. do have a large effect on how appearances turn out. I could be wrong, but it’s the theory I’m going with ATM. It’s not necessarily inconsistent with the physical similarity between identical twins. Take this for example: like Don says, identical twin personalities can be worlds apart. And it’s pretty incontrovertible that if you have bad eating habits you’ll get fat (even if there *are* other factors / genetic predispositions like metabolic rate). So it should follow, then, that in lots of twin cases one should be terribly fat while the other isn’t — say, where one always eats McDonald’s and the other always eats Subway. So if they’re generally identical, what does that tell us?

    I would suspect that splitting from the same zygote, developing together in the womb, and then growing up together might create a strong and lasting bond (be it psychological or metaphysical) between them. How would it be if one were fat and one weren’t? This would create an unhealthy psychological imbalance between them — moreso than it would for dizygotic twins. Their being the same weight would thus be an act of karmic mercy. (Let alone the fact that being borne of the same parents and having the same DNA could very well make their personalities more similar than average, and that may or may not be specifically along the dimensions that, directly or indirectly, determine appearance.)

    For other attributes, it could come down to the fear of self-actualization. That is, if their physical appearances diverged, then it would make it obvious to each of them (and to everyone else) how their respective personalities developed as well. Since it would be plain as day, that makes them take full responsibility and acknowledgement for how they’ve developed, which is probably too much for anybody but an avatar.

    Otherwise it could be in order to be consistent with society’s preconceptions that identical twins should remain identical, although that would (seemingly) be contradicted if identical twins’ appearances diverge more in some other culture(s) in the world, or if not that then at least in pre-scientific times. Come to think of it, even in pre-scientific times people would probably expect identical twins to remain identical — for it not to be so, would require that anybody who notices fully face the reality of personal development.

    I know these ideas are rather outlandish; they’re just presented as food for thought. Obviously I’m far from being a scientistic.

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