Brian Palmer attempts a takedown of twin studies that shows the opposite of what he suggests

Pause for a moment to examine that astonishing claim—that Americans’ stubborn insistence on disagreeing over hot-button national issues is the result neither of two parties adjusting their views to appeal to a shifting political center nor of the fact that the issues on which we have learned to agree simply fall out of the political debate. (How many slavery proponents do you know? I hear there used to be a few of them in the U.S. Senate.) Nope. It has to be the series of base pairs in our cellular nuclei.


Twin studies rest on two fundamental assumptions: 1) Monozygotic twins are genetically identical, and 2) the world treats monozygotic and dizygotic twins equivalently (the so-called "equal environments assumption"). The first is demonstrably and absolutely untrue, while the second has never been proven.

So not to get too much into the first paragraph but the suggestion here is not that “abolitionism is in your genes” so much as abolitionism isn’t about abolitionism per se. Abolitionism is about – say – the ability to empathize with “the other” and that is in your genes.

Indeed, if you believed that the issues were really about the issues then a deeper puzzle would be why people disagree at all. People who cared only about the issue itself should tend to converge on compromise views if for no other reason then that the very fact that some other rational person holds conflicting views should cause you doubt the veracity of your own convictions.

No, people are likely biased towards things that “feel right” and what feels right is probably heavily influenced by genes.

Now on to the twin studies. Palmer points out correctly that identical twins are not, in fact, genetically identical. However, this implies that twins studies underestimate the influence of genes.

That is the way twin studies work is that one assumes that any difference between identical twins must be due to something other than genes because they have the same genes. However, if they don’t actually have the same genes then this causes you to attribute things which are really genetic in source to non-genetic causes.

Especially, the more we think the brain of identical twins are different, the “No Two Alike” puzzle melts away. That puzzle is how it is possible that conjoined twins can be so different. They have the same genes because they are monozygotic and they have the same largely the same experiences because they are literally joined at the hip (or head or chest.)

How then do we explain the sometimes large disagreements between conjoined twins. Can being “on the left” really be that much of a life changer?

Now what about the similarities between mono and dizygotic twins. Maybe being treated as an identical twin makes you different. That’s could be true but is beside the point of genetic research. What you really want to say is being treated as a twin makes you more similar.

Perhaps. This sounds plausible. But, it sounds equally plausible that identical twins might want to distinguish themselves. That constantly being confused with one another would cause them to invent differences. We really don’t know.

Importantly, however, concern about this issue is undercut by another one of Brian’s points

There have been numerous studies showing that dizygotic twins who look similar have more personality traits in common than those who are easily distinguishable.

Yet, doesn’t this say that these personality traits are genetic in origin? Its simply that the genetic influence is mediated by the fact that people respond to you based on the way you look.

We know that taller men make more money. Maybe that’s the result of the higher marginal product stemming from height. More likely its because tall men are treated differently. However, that shows that height and hence genetics is a determinant of life outcomes.

I think the twin research is still compelling evidence that gives us a deep insight not just into the effect of genes but on “how genetic” a given trait may be.

And as a last note, I would say that I sense that some of the push back against genetic determinism is push back against determinism more broadly. However, unless you want to reject the block universe then rejecting twin research and genetic determinism doesn’t get you out of the determinist box.

A tabula rasa world is just as deterministic if you think that life experiences make the man.