Twin studies and heredity

I’ve been thinking for a while that I should do one or more posts explaining heredity and talking about heredity and weight, and a lot of studies of heredity involve twin studies. Since an article has recently come out–Slate’s Brian Palmer’s “Double Inanity”–that’s critical about twin studies, it’s probably not a bad idea to address some of its criticisms now, even though the article is not specifically about weight. (Palmer also references criticism by scientists, which I haven’t read at this point, that may be more substantial.)

Palmer’s explanation of twin studies goes like this:

Researchers compare some behavior or trait in a set of pairs of monozygotic (identical) twins and a set of pairs of dizygotic (fraternal) twins. In theory, the siblings in each pair have been raised in the same way—i.e., they have “nurture” in common. But their “natures” might be different: Identical twins come from the same sperm and egg and are assumed to share their entire genomes; fraternal twins match up at only about half their genes. So if the pairs of monozygotic twins tend to share a trait more often than the pairs of dizygotic twins—be it the likelihood they will vote, a tendency toward altruism, or a strategy for managing their financial portfolios—the difference can be chalked up to genetics.

This is mostly right, but not quite: most of the twin studies I’ve seen are conducted on adopted twins raised apart, so they don’t share a home environment. (That may not be true in the studies he references in his opening paragraph, though.) This is important for some of his other criticisms.

Palmer goes on to talk about some stuff that’s not really important for our purposes here. We have moral problems with Galton and facts about him may be interesting to include in the article, but for the question of whether twin studies are valid, we’re arguing ideas and facts, not people. Yes, we’ve learned a lot about genes since the first twin studies were conducted, but we haven’t overturned the basic assumptions that those studies are based on. One of the advantages of twin studies is that you don’t have to know the specific genes involved or the way they work–or even that genes exist–to do them; you just have to understand the concepts of heredity and environment (“nature” and “nurture”).

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.

There is something to both of these points, but they only make estimates of heredity less precise; they don’t simply invalidate twin studies.

That identical twins do not, in fact, have identical DNA has been known for some time. The most well-studied difference between monozygotic twins derives from a genetic phenomenon known as copy number variations. Certain, lengthy strands of nucleotides appear more than once in the genome, and the frequency of these repetitions can vary from one twin to another. By some estimates, copy number variations compose nearly 30 percent of a person’s genetic code.

These repeats matter. More than 40 percent of the known copy number variations involve genes that affect human development, and there are strong indications they explain observed differences between monozygotic twins. For example, it’s often the case that one identical twin will end up victimized by a genetically based disease like Parkinson’s while the other does not. This is probably the result of variations in the number of copies of a certain piece of DNA. Copy number variations are also thought to play a role in autism spectrum disorder, schizophrenia, and ADHD, all of which can appear in only one member of a monozygotic twin pair (PDF). If copy number variations can affect discrete and diagnosable disorders, then why shouldn’t they influence far more complex behaviors like your inclination to head to the polls on a Tuesday night in November?

That’s just the beginning of the genetic differences between monozygotic twins. As a result of mutations during development, about one in 10 human brain cells has more or less than the typical two copies of a chromosome. Identical twins also have different mitochondrial DNA, the genetic information stored in the cellar organelle responsible for processing glucose. Research suggests that mitochondrial DNA affects brain size among a host of other neurological traits.

Twin studies also rely on the false assumption that genetics are constant throughout one’s lifetime. Mutations and environmental factors cause measurable changes to the genome as life progresses. Charney cites the example of exercise, which can accelerate the formation of new neurons and potentially increase genetic variation among individual brain cells. By the time a pair of twins reaches middle age, it’s very difficult to make any assumptions whatsoever about the similarity of their genes.

All true. However, these are all things that make identical twins less similar. Meaning that if, say, 75% of weight variation seems to be due to genes in a twin study, but some of the variation even between identical twins is due to different copy variations and such and is chalked up to environmental difference because the study assumes that the identical twins are genetically identical, the study is actually underestimating how much of weight is genetic. (And copy number variants are definitely responsible for some variation in weight, so for weight in particular we are likely underestimating the genetic contribution.) The premise of the article seemed to be that twin studies overestimate heredity, but he’s undermining his own argument without seeming to realize it.

The equal environments assumption is similarly questionable. As anyone who’s ever seen a pair of toddler twins in matching sailor suits is surely aware, monozygotic twins do get special treatment. They are more likely than their dizygotic peers to be treated as “two of a kind” by family, friends, and teachers, which must increase their chances of developing similar behaviors. There have been numerous studies showing that dizygotic twins who look similar have more personality traits in common than those who are easily distinguishable.

All true. But as I mentioned above, most twin studies that I’ve seen have been on twins raised apart. No matching sailor suits for those kids. Although, after reading No Two Alike, I’m inclined to think that “two of a kind” treatment would make the twins less similar, not more similar. But again, for twin studies on twins raised apart, that’s neither here nor there.

In addition, genetics and environment aren’t separable elements, so any attempt to discern their independent contributions to an observed trait is foolish. The epigenome, the part of our genetic makeup that determines when genes are transcribed—that is, when they actually become relevant to our existence—is deeply affected by conditions in the womb and the unquantifiable complexity of the external environment.

Well, obviously the environment affects how genes are expressed. If your genes tell you to become 5’5″ and 200 lbs on a Western diet, but you don’t get enough food to even make it to adulthood, those same genes will not result in you getting to be 5’5″ and 200 lbs. And if that same person does get to adulthood but is malnourished, maybe they will be 5’3″ and 100 lbs. But guess what, in a twin study, if the environment is different between the two identical twins and causes one twin’s genes to be expressed differently than another, that effect will be attributed to environment, not to genes.  If you took two identical cars and drove one down a freeway and one on a rutted dirt road, you would attribute the environment’s effects on the car’s performance to the environment, not to differences between the cars.

(Of course, for most twin studies, you will be comparing two households in the same country, not comparing the difference between abundance and malnourishment; “environment” in these studies reflects how much difference in environment there actually was, as well as how much variation in genes there actually was. I will try to talk about this more in a future post.)

Our knowledge of the human genome is far too superficial for anyone to be making percentage estimates of the extent to which our biology sets our destiny. Consider the unfortunate story of the Texas rancher who cloned his beloved, sweet-natured bull only to find that the new model was the polar opposite in temperament. The clone gored his master repeatedly, dislocated his shoulder, ripped open his scrotum, and fractured his spine.

…So, he cloned a bull, and it had a very different temperament than the original? I think we can say that that was mostly environment, with maybe a few copy repeat variations and stuff thrown in. Hey, we just learned from this not-a-controlled-experiment that a genetically identical individual won’t necessarily have  similar temperament! No, we can’t get an exact percentage estimate from this, but all this anecdote proves is that if you think a clone/twin will reliably have the same temperament as its parent/twin, you don’t understand heredity. It supports the author’s main point that genes aren’t destiny, but it doesn’t support the narrower point that this anecdote somehow proves we can’t learn anything from twin studies.

There’s a strong temptation to believe that the same genes that make identical twins look so similar also make them think and act identically. That assumption isn’t just insulting to a twins’ individuality; it displays a reductive attitude toward the incredible complexity of our genetic structure, which scientists are just beginning to understand.

Well, yes, people in general may be tempted to believe that, but scientists doing twin studies are generally smart enough to know that different genes are responsible for looks than are responsible for the way they think and act. They may be subconsciously influenced by this even though they know better, but that’s not the same thing. And he’s wrong about twin studies displaying a reductive attitude; one big advantage of twin studies is that you can treat the genetic structure as a black box that you know damn well you don’t understand. You know that it works somehow, and that it is the same black box–or at least, mostly the same–for identical twins, and a different but related black box for fraternal twins. You don’t have to know how to build a car to know that it will perform differently on the interstate than on a dirt road, and you don’t have to know much about how genes work to know that differences between identical twins–minus those extra repeats and such–are due to difference in environment, not genetics. And that if fraternal twins are more different than identical twins, it’s because they’re more genetically different.

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2 Responses to Twin studies and heredity

  1. G says:

    Thank you for this! I had never heard of copy number variations, fascinating stuff. I guess twin studies are not as bulletproof as they’re made out to be…

  2. Pingback: Genes and BMI Part 2: Evidence for a genetic influence on weight in a post-‘obesity epidemic’ world | closetpuritan

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