The history of twin studies is littered with attempts to discredit them – such as this bit of rubbish. Yet every challenge has been met, with a couple of newish studies knocking off another.
The basic idea of twin studies is that by comparing the similarity of fraternal twins to the similarity of identical twins, you can tease out the influence of their genes. Twin studies tend to find that most behaviours have heritability of at least 0.2 (that is, 20 per cent of the variation is due to variation in genotype), IQ a heritability of over 0.5 and height around 0.8. However, twin studies require an assumption that identical and fraternal twins have equally similar environments, and this is where the critiques begin. If identical twins have a more similar environment, the estimates of heritability may be too high.
The responses, however, are plenty. There are studies of twins reared apart. Adoption studies find similar results. For those who believe that identical twins are treated differently to fraternal twins, there are studies of misidentified twins – where everyone thought they were identical or fraternal, but they were the other. Peter Visscher and friends took advantage of the differences in relatedness between siblings to generate estimates of heritability consistent with twin studies (You are 50% related to your siblings on average, which means you can test how similarity varies with variation in relatedness . For me, that study should have been the final nail in the coffin of any arguments that twin studies hadn’t told us anything).
One critique still floating around is that people who look more similar are treated similarly (although the misidentified twin studies deal with this to a degree). And the New York Times has reported two studies that take on that argument. In the first, Nancy Segal assessed the similarity in personality of 23 pairs of unrelated lookalikes. The similarity – effectively zero. Then in a replication, Segal got a skeptic, Ulrich Ettinger, involved in the project. They found the same result – no resemblance – unlike Ettinger’s expectation that people who looked alike would have similar personalities as people would treat them the same.
As Razib points out, these studies involves a small sample. However, they are yet another piece of evidence pointing in the same direction as all the rest.
I linked to this interview with Robert Sapolsky a couple of weeks ago, but after glancing through it again, I felt it worth highlighting two paragraphs (both for your interest and so I can find them again). First, on the evolutionary purpose of the teenage brain:
What I’ve been thinking might actually be going on is that adolescence is something unavoidable that emerges not because it’s so cool and adaptive, but because the adaptive thing is wait a long, long time before you have fully wired up your frontal cortex. Why might that be the case? Alright, so we’re born with our genome, the combination of your mother and father’s genes, that wind up in that first fertilized egg and that’s it. That’s your genetic legacy. Every cell in your body is destined to have that exact same genome. That turns out not to be true in all sorts of interesting ways, but what that also means is that when you’re thinking about what genes have to do with the brain behavior, by definition critically, if the frontal cortex is the last part of the brain to develop it’s the part of the brain least shaped by genes, and most sculpted by the environment and experience. And I think basically the only way you can have a species that is as complex and socially resilient and socially context dependent and all those amazing things we do, the only way you can pull that off is to have a frontal cortex whose development just bears the imprint of everything you experienced along the way—in effect, that’s been freed from whatever extent the genes are deterministic, which is not very. I think ironically what the evolution of the frontal cortex has been about is genetic evolution to free it as much as possible from the straight jacket of genes.
Second, on reductionism in neurobiology:
[R]eductionism doesn’t actually tell you a whole lot about how this stuff works. I mean reductionism is perfect for like telling you why your clock is broken. What you do is you break it down to its component parts. You find the part that’s got a tooth missing from the gear. I guess there’s not a clock on earth that works this way anymore, but your Renaissance clock. You fix the missing tooth, you put it back, you add the pieces back together and it works. The way to understand a complicated system is to understand its component parts. The way in which that steps away from the ideology is the component parts of the genes and the nerve transmitters and the hormones and the early experience. Okay, so that’s a more sophisticated version of reductionism. You got to be reductive about lots of different domains. But nonetheless, even that more multidisciplinary version of reductionism isn’t going to work because that’s not how complex systems work and humans are a complex system. You got these emergent non-linear chaotic properties. What’s that another way of saying? If you knew every individual’s genome and exactly which gene was active at which point, are you going to be able to predict who’s going to do what next? Absolutely not. If you added in knowing the levels of every hormone in their body at that point, if you added in… it doesn’t work that way. The reductionism breaks down because the reductionism breaks down in the same way that like a cloud that isn’t producing enough rain during a drought or something, the solution isn’t to study half the cloud and then get a research grant to study a quarter of the cloud and smaller, smaller pieces and finally understand the reductive basis of the non-rain and add it up together. That’s not how clouds work when they don’t rain. Humans are more like clouds than they are like clocks. We’re not reductive in that way, which is the case for any complex system.
And if you haven’t read the full interview, do it.