Twin delusions

Fascination with identical twins goes back a long way: think of the founders of Rome, Romulus and Remus, and the astrological symbol Gemini. Our modern understanding of genetics has given us a scientific perspective on identical twins, and yet it too is accompanied by a mythology.

By virtue of being genetically identical, identical twins would seem to provide an ideal source of evidence concerning the heritability of traits. Since identical twins share physical characteristics, is it not conceivable that they also share traits such as intelligence and personality? There is, however, a fly in the ointment: the influence of the environment in which twins are raised. While eye colour has a purely genetic basis, couldn't intelligence depend on how twins are jointly brought up? There are two ways around this obstacle.

Twins: Take 1

The first depends on a rather unlikely occurrence: identical twins separated at birth. Which brings us, my dear Watson (and Crick) to the curious case of Sir Cyril Burt (1883–1971), an English educational psychologist. In "The Mismeasure of Man", Stephen Jay Gould writes that Burt:

... published several papers that butressed the [claim that IQ is inherited] by citing very high correlation between IQ scores of identical twins raised apart. Burt's study stood out among all others because he had found fifty-three pairs, more than twice the total of any previous attempt.

But perhaps this was too good to be true:

Princeton psychologist Leon Kamin first noted that, while Burt had increased his sample of twins from fewer than twenty to more than fifty in a series of publications, the average correlation between pairs for IQ remained unchanged to the third decimal place—a statistical situation so unlikely that it matches our vernacular definition of impossible.

Gould goes on to review further evidence that Burt faked many of his results. But by the time this came to light, the damage was already done: Burt's studies had influenced British educational policy for decades.

Twins: Take 2

A second way around the impact of environment is to compare identical twins with same-sex fraternal twins. If a trait is inherited, we would expect identical twins (known as monozygotes, MZ) to be more similar than fraternal twins (dizygotes, DZ), irrespective of environmental influences. A particularly disturbing example of this argument is in a 2005 paper titled "Evidence for substantial genetic risk for psychopathy in 7-year-olds" (Viding et al. J. Child Psychol. Psychiatry 46:6, pp 592–597). Measures of psychopathy were reported by teachers for each member of nearly four thousand twin pairs. A child with a high score (the "proband") was then compared with his or her co-twin. The figure below is a hypothetical example, but it qualitatively represents the study findings:
As depicted in the figure, identical co-twins (MZ) were closer to the proband than fraternal twins (DZ). In fact, the authors studied two traits: callous-unemotional (CU) and antisocial behaviour (AB). The reported "remarkably high heritability for CU, and for AB children with CU".

Megan McArdle cited this study (via a summary article) in a recent blog post:

Some years ago, I remember reading Jonathan Kellerman's Savage Spawn, a book on sociopathic children, and how nearly impossible it is to treat them. [...] It's truly heartbreaking: a child who doesn't seem capable of loving its parents, or anyone else. It seems to be mostly genetic, and nearly completely immune to any current treaments.

While I can't comment on the claim that it's "nearly impossible to treat" (though I wonder what the substantive basis for such a claim would be), I do question the claim that it's "mostly genetic". Do identical twins really grow up in the same environment as same-sex fraternal twins? I doubt it. As Lea Winerman wrote in A Second Look at Twin Studies in the April 2004 issue of the American Psychological Association Monitor: "... some research suggests that parents, teachers, peers and others may treat identical twins more similarly than fraternal twins." Winerman provides a balanced review of the continuing controversy about twin studies, raising a number of other problematic assumptions.

Time to move on ...

Back in April I expressed my skepticism about genetic determinism in the context of sex differences. I don't doubt that genetic factors can play an important role in a variety of traits. But the more complex the trait the trickier it is to sort out the relative contributions of genetics and environment. And the more likely it is that our own prejudices will swamp the evidence. We've been down this road before.

Nature vs. not sure

The perennial nature-vs-nurture debate just won't go away. This is particularly true with regards to gender differences, a subject of broad interest.

I'll acknowledge my biases up front. I have long been skeptical about biological determinism. This is partly because of its historical association with racism, sexism, classism, and the eugenics movement. But it's also because, particularly in recent years, there has been a tendency to overstate the importance of genetics in explaining human behaviour. Part of the explanation for this "genohype" may be the dramatic achievements of the Human Genome Project together with the rise of the biotechnology sector. Just as the success of Darwin's theory of natural selection led to Social Darwinism, today's molecular genetics revolution has put a new wind in the sails of biological determinism.

In the scientific world, the nature-vs-nurture debate is generally accepted to be an ill-posed problem. Because the environment affects the expression of genes, it is not a question of nature versus nurture, but of nature vis-à-vis nurture. Nevertheless, the ways in which and the extent to which nature and nurture influence human behaviour remain controversial. And beliefs about this can have profound consequences.

But one thing's for certain, and that's uncertainty. Despite the way results from studies of gender differences are often portrayed, we're usually left with more questions than answers. Here I want to comment briefly on two considerations that should be borne in mind.

Does the difference matter?

It's common to read reports stating that, for example, "women perform task X better than men". What this really means is "on average women perform task X better than men, and this effect was found to be statistically significant". The magnitude of the effect may be small or large. The degree of overlap between women and men may be small or large. (And of course the study may be flawed.)

To what can the difference be attributed?

Assuming the difference is real and meaningful, we're still left with the question of whether it represents an innate biological difference or an environmental (cultural) difference. For some reason it seems that people quickly jump to the conclusion that gender differences are innate. But in most cases it is extremely difficult to sort this out. Cultural effects can be extremely subtle. As has been pointed out (by ?), the concept of "wet" wouldn't mean much to a fish.

Grist for the mill

Here are three interesting articles that touch on some of these issues. First, a review by Viv Groskop of "The Sexual Paradox: Troubled Boys, Gifted Girls and the Real Difference Between the Sexes" by Susan Pinker. Next, an interview with professor of language and communication Deborah Cameron about her book "The Myth Of Mars And Venus". Finally, a New York Times article by Elizabeth Weil about the movement for single-sex public education based on gender differences.

I've really only scratched the surface of this issue (not to mention related ones), and there's lots of stuff out there (a Google search of "gender differences" gives 2,450,000 results). Comments?

Update 09Apr2008: It seems there's an almost unlimited number of links that could be added. Here's another review of Susan Pinker's book, from the New York Times. Here's an entertaining retort to an argument about gender differences based on evolutionary psychology. And here's a piece that argues: "Nowhere do scientific findings get more mangled than when they’re about the differences between men and women." Finally, here's a conservative view on gender differences.

Update 11Apr2008: Here's a response to some of the arguments about single-sex schooling.