Twin Studies and Beyond

Brian Palmer has a very weak article in Slate trying to make the case that “Twin studies are pretty much useless.” The article is supposed to be about the problem of twin studies as a method but it begins by raising the specter of eugenics. As if that were not enough guilt by association, Palmer then argues that twin studies threaten democracy or at least they would if they were true. (The argument is unclear but seems to rest on the false assumption that if genetics matters then nothing else does. Need I quote the tiresome point that poor eyesight has high heritability but that doesn’t make eyeglasses useless etc.)

After having muddied the waters, the author’s primary argument is this:

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.

On the first point, the fundamental assumption is not that MZ twins are identical but that they are more identical than fraternal twins. The math is a bit easier if you assume that MZ twins share all of their genes and fraternal twins share 50% on average but this is not necessary. In fact, if you take into account that MZ twins differ genetically this raises the variation that you should ascribe to genetics. If twin one smokes and twin two does not and you assume that they share 100% of their genes then you must conclude that smoking does not vary with genes. If the twins share only 99.99% of their genes then smoking may vary with genes.

On the second point (the equal-environments assumption), Palmer writes as if comparing MZ and DZ twins was the only source of heritability estimates. In fact, heritability estimates are found by looking at twins raised together and twins raised apart, siblings and siblings raised apart, parents and child correlations and so forth and the results from these studies are broadly similar.

Even more important, for an article that goes on about “modern genetics” the author seems completely unaware that it is now possible to do a whole-genome analysis. That is, instead of assuming that siblings share 50% of their genes on average it is possible to estimate, sibling-pair by sibling-pair, how many genes siblings share and then correlate that with various characteristics. Obviously, it takes a lot more data to do a study like this but it has been done. Visscher et al., for example, use data from 3,375 sibling pairs to estimate the heritability of height. Interestingly, they find a heritability of 0.8, very close to that found in traditional studies.

Using whole-genome methods it is not necessary to assume equal environments for MZ and DZ twins. In fact, using these methods you can do genetic studies across unrelated individuals. For example, in Genome-wide association studies establish that human intelligence is highly heritable and polygenic, the authors note:

Data from twin and family studies are consistent with a high heritability of intelligence, but this inference has been controversial. We conducted a genome-wide analysis of 3511 unrelated adults with data on 549 692 single nucleotide polymorphisms (SNPs) and detailed phenotypes on cognitive traits. We estimate that 40% of the variation in crystallized-type intelligence and 51% of the variation in fluid-type intelligence between individuals is accounted for by linkage disequilibrium between genotyped common SNP markers and unknown causal variants. These estimates provide lower bounds for the narrow-sense heritability of the traits.

Twin studies have their problems, just like any method. The thrust of recent advances–advances which have been made to analyze and surmount the kinds of objections that Palmer raises–however, is that the results from twin studies are robust.

Ok, here is a final and telling point.  Palmer argues that “Mutations and environmental factors cause measurable changes to the genome as life progresses.” Now that is true but you can judge how eager Palmer is to discredit twin studies regardless of the science by how he quickly concludes from this something which is truly laughable:

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.


The man's a fool.

There is also the womb, which interestingly the fetuses both the same and differently and we don't know how.

I keep reading this sentence, trying and failing to discern its meaning.

Twins in the womb encounter various phenomena like common exposure to hormones and chemicals that tend to make them alike, and TTTS and other placental abnormalities that tend to make them different. These things aren't genetic, but they have an effect on twin outcomes that's hard to control for. The precise net effect on twin studies is not obvious.

Thanks, that makes a lot of sense.

add an "affects" after "interestingly"

begin sarcasm

He must be one of those ant-science Republicans.

Wow; I'd seen this discussed elsewhere and figured the article traded in a lot of these fallacies, but this is eye-opening. It's incredible how easy it is to dupe lay audiences with seemingly convincing scientific evidence that's completely fabricated. Seeing fairly intelligent folks passing along this article makes it clear why it's so simple for interested parties to spread nonsense on climate change, evolution, etc.

There are many problems with abuse of statistics in science based on faulty assumptions, but modern genetics is one of the least problematic fields specifically because of the history of eugenics and the ease of tricking yourself into thinking you've found something important when you've got an entire genome to look at.

Modern liberalism and HBD are in fundamental philosophical conflict. Modern liberalism takes the idea that people and subsets of people must be virtually identical. This is taken as essentially a religious given, no different from "the bible says so." Unfortunately, science proves otherwise. There are huge differences between the genetic traits of different people and subsets of people, and its not just limited to things like physical strength. Liberal denial of HBD is far worse then global warming denial from a science hatred metric.

Apparently, TenthRing has studied at the University of Michelle Bachman. Modern liberalism? Liberal denial?

Some of us liberals have three digit IQs, y'know.

Cool story, bro.

1) Guilt by association with a person I never mentioned.
2) Then two terms with question marks...
3) I'm not dumb! "rebuttal"

though not too many!

You know being 'anti-science' is probably not where you want to be attacking liberals, given that 52% of scientists self identify as 'liberal' and only 9% as 'conservative' (with 35% being moderate). And when it comes to HBD, since the DNA evidence is incompatible with the creation myth(we incontrovertibly evolved from more than 2 people), you might really want to stay away from this one. Just saying.

1) Irrelevant statistics that have nothing to do with the argument.
2) Creationism reference. Really? Pretty far out of left field.

I'm seriously starting to wonder if the people replying here have "three digit IQs".

It depends on where you put the decimal point.

Yeah. I would bet anyone 10$ that more of the authors of those twin study papers voted for Obama than for McCain. Twin Studies are not repressed science, it is mainstream science.

Slate isn't liberal. It is just peddles in pointless contrarianism.

Monozygotic twins are not genetically identical? I understand that there can be mutations once in a while, but I thought they generally were identical? Did I miss something?

How far do you want to push "identical"? Among the billions of cells in the human body, each twin probably have some cells that have mutated. How often does this matter? Probably never.

(Also, don't overlook the bacteria component. A lot of your body's functions (think digestion) happen via non-human bacteria. This is significantly genetic since you tend to have the same bacteria as your mother, but it isn't written anywhere in your genetic code.)

Did I miss something?

Not really. Palmer is in the squid ink business. Due to the internet's routing around the mass media, the truths surrounding HBD are now sufficiently painful to progressives that they need to start active denials of the science.

Palmer does point out a mechanism -- copy number variations -- that differentiates individuals after conception. He claims that they are very frequent and suggest they have a lot of effect, claims I am inclined to strongly doubt.

I'm not sure why the last line should be considered laughable. Please bear with me: I'm just an aficionado.
What I think the author means is that as people grow older they are more and more influenced by epigenetic modification, that tends to muddle the waters when comparing genetic similarities. I'd really appreciate if someone could clarify this for me.

If the last line were true then if you met twins in their middle age you wouldn't even guess they were related, laughable.

"it’s very difficult to make any assumptions whatsoever about the similarity of their genes"
The problem is with the word assumption here. Why do you need to make assumptions? Why can't you examine the similarity of genes of middle-aged twins?

For those who read the editorial in the post, perhaps they should read the support in the article:

"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.

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."

I would be interested in a discussion of the siginificance of this and what geneticists, not economists, say about it.

Well, as Tabarrok says, the more seriously you take that argument the more important genes are in explaining what people become. If identical twins are truly only a little more similar than fraternal twins, we can take all the percentage heredity estimates and increase them.

That may be true. But, Tabarrok said you didn't have to even do that.

Be an empiricist. No one is saying that you wouldn't be able to IMPROVE your argument by showing that the dissimilarity between identical twins, for example, is confined to a segment of a person's genes, for example, and from that you learn that this segment is the key segment, or you learn that a segment in combination with another segment variant leads to a different result.

In other words, differences may show more than similarities, but, if you don't look, you don't know or learn.

There are two other points that could be made from this post as well:

1. Genome wide studies, across populations, "estimate that 40% of the variation in crystallized-type intelligence and 51% of the variation in fluid-type intelligence between individuals is accounted for by linkage disequilibrium between genotyped common SNP markers and unknown causal variants." 40 to 50% heritable. What accounts for the other difference?

2. The idea of imutable heritability may change if we find out what parts of the genome are responsible for this 40 to 50% heritability component.. If, for example, a certain segment or segments of the genome create proteins which are in turn responsible for higher intelligence, then that knowledge could lead to medicines or a pill that makes you smarter. In other words, your genes may not dictate your future as they did in the past. Heritability may not signal destiny, but only something that requires a pill in the morning before exams.

But, if we can make people smarter and we do not address other elements of a persons environment that, say, facilitate gang behaviour or criminal behaviour, all we may have done is create a class of smarter crooks.

As someone observed on another site, Palmer's piece is so bad that even the Slate commenters called him out on it.

"even Slate commenters" ;)

"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."
This is especially funny if you keep in mind the stylized fact that heritability INCREASES with age.

Maybe we should first understadn what heritability means before mocking people's claim? May I suggest Cosma Shalizi's post on the subject:

Scientific consensus to the contrary notwithstanding, most people approach scientific claims according to their own biases. Conservatives cannot believe that anthropogenic global warming is real because if it were, they fear, it would constitute an almost ironclad case for government intervention. Likewise, liberals cannot stomach claims for a strong genetic component underlying human behavior because they fear that its existence would render much intervention futile. There are some notable exceptions, but for the most part if you let me read one or two of a commentator's political essays, I claim I can predict their position on any scientific question.

Or, conservatives see that AGW proponents are not even proposing the level of economic reduction necessary to prevent the climate change in their models; and liberals are exceedingly frightened of anything approaching Nazis and racism.

And since both have killed millions of people, they are both right.

Reminds me of the South Park episode with the ''Chewbacca defense'' ... But seriously, there is a new asst prof economics at NYU who does interesting work on the weaknesses of twin studies. He might be onto something big. Yet the bottom line is that until someone makes a big discovery, twin studies are a brilliant empirical tool.

All the twin study results are confirmed by more rigorous methods... so it seems they produce useful results.

Is anyone else reminded of typical left-wing arguments against free market economics.

"Well your model makes too many simplifying assumptions. Consider X, Y and Z, which all contradict the assumptions of model A that you use to make conclusion B. Therefore the only possible conclusion we must reach is completely unrelated (leftist) conclusion Q."

When in fact X is a trivial change to the model, Y actually raises the support for conclusion B, and Z could possibly have an effect but basic common sense dictates that it's not important.

While there's no information given about the author's scientific credentials, it's apparent that at least in this case, three generations of imbeciles were not enough.

Steven Pinker addresses the equal-environment assumption in The Blank Slate: "What about the expectations of parents, friends, and peers? A neat test is provided by identical twins who are mistakenly thought to be fraternal until a genetic test proves otherwise. If it is expectations that make identical twins alike, these twins should not be alike; if it is the genes, the should be. In fact the twins are as alike as when the parents know they are identical."

He cites Scarr & Carter-Saltzmann, 1979.

What, no one brings up epigenetics?

I recently submitted a paper which evaluates the equal environments assumption empirically using a more diverse range of controls for environmental similarity, and a larger group of outcomes than have previous studies. As is usually the case with research, the evidence is distributed more evenly than are the opinions of the researchers.

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