Biology in the News Explained

The problems with twins

A paper discussed in an earlier post (Alford et al., 2005) relies for much of its justification on a series of papers by Thomas J. Bouchard and coauthors. An expansive claim for genetic heritability of all sorts of behavior and attitudes is found in Bouchard and McGue (2003). Frankly, the arguments made are even more disturbing in this paper.

Why disturbing? What is truly the harm in scientifically separating out the genetic and environmental influences on everything from social attitudes to “vocational interests”? The harm is that such papers serve no function other than to fan the flames of bigotry.

We are likely not yet at the crest of the pro-genetics wave. There are more and more studies being published which claim genetic bases for all sorts of traits (e.g. politics, aversion to new foods, obesity). One walks a precarious path interpreting the actual significance of this work for the general public, who is under the mistaken impression that scientists know far more about human genetics than they actually do. The mainstream media certainly tends to work more as a blunt instrument than as a nuanced filter of published scientific studies, happily being used by authors and journals who have a strong self-promotion agenda.

The straw man constructed by the pro-genetics crowd is that obviously genetic variation exists or we would all be the same. But of course it is biological significance of genetic traits that matters, not whether or not the variation actually exists. Even the most rigorous study that shows valid statistical significance for variation in a trait does not necessarily demonstrate biological significance – in fact, the huge sample sizes often used in these studies, which has the intended effect of increasing the chance of reaching statistical significance, undermine arguments for biological significance (see figs. 1 and 2). If it takes a survey of 10,000 people to detect a difference, then there is clearly so much overlap in the groups being compared that the authors should be hard-pressed to convince anyone that it matters. A slight fluctuation about the mean is much more likely to be amplified into statistical significance, although the direction it is significant is determined randomly. This was brilliantly demonstrated in a pair of papers about birth order and IQ.

Where do all the heritability estimates come from? For example, Bouchard and McGue report “heritability of IQ is about 50%.” This of course is based on twin studies. But the logic used by all of these authors (none of whom I have encountered so far are actual geneticists) to produce such an exact value of genetic heritability is flawed. They make the assumption that if you compare differences in scores on surveys between fraternal twins with the differences between identical twins, that difference is the genetic component of the trait. This assumes that the environment in which fraternal twins are reared is as similar as the environment for identical twins. This assumption has naturally been both challenged and defended, but for the moment, let us concede it as valid. That leaves us with the corollary assumption that the difference between fraternal differences and identical differences is therefore entirely genetic. This is where the problems with these studies lie (discounting the obvious problems with using tests or surveys – which are biased by authors, affected by mood of the taker, etc. – to make sweeping statements about genetics).

Identical twins not only share a genotype, but also a phenotype – they look the same. As explained in the previous post on this topic, how you look is going to affect your social attitudes, not to mention self image, mostly because of your interactions with other people, who clearly behave differently to people who look different. Until a study is conducted in which one half of 30 identical twin pairs has a dramatically altered appearance (e.g. is in a wheelchair or has had major facial reconstructive surgery), all the twin studies (even those in which the identical twins were reared apart, long a mainstay of the pro-genetics camp) declaring the percent genetic contribution for any subjective phenotypic trait will be meaningless.

These papers harken back to the dark days of phrenology and craniometry – the methods employed are no more scientific, because we know no more about how genetics affects these traits now (more than mere speculation) than we did back then about how the brain functioned. But, the results are used by those who are racist or sexist to defend their views. All traits are a unique combination of usually complex genetics and environment. There is no way to establish that a person has a particular “genotype” for intelligence or social attitudes, and even if there were, the expression of that trait will be dependent on the environment in complex ways that are not easily measured. Most important, because of the huge overlap in any trait associated with the brain across all types of people, for any given individual, there is no way to determine what part of their intelligence, personality, or skills are based on their appearance, even if there actually is a true statistical difference in these traits for different races or sexes (Fig 2.). But studies such as these are used by people who wish to have their stereotypes confirmed “scientifically,” and frankly one has to wonder if the authors are not such people themselves.


Alford, J.R., C.L. Funk, and J.R. Hibbing, 2005. Are political orientations genetically transmitted? American Political Science Review, 99:153-167.

Bouchard, T.J., and M. McGue 2003. Genetic and environmental influences on human psychological differences. Journal of Neurobiology 54: 4–45.


2 Responses to “The problems with twins”

  1. Jacob says:

    Excellent points. There are certainly some features of all-too-common statistical analysis that people rarely question and simply take for granted.

    I read the report of a study this morning on curvy women producing smarter children; it seems like some of these researchers are really reaching in their correlative attempts.

  2. Brian says:

    Is there any evidence that people of similar appearance necessarily have similar traits (other than twin studies?). This appearance argument you’re making seems quite weak.

    In any case, more recent studies are showing differences in gray matter.

    It is clear that intelligence is at least partly genetically determined. This was supported by the discovery in 2001 that the volume of the brain’s grey matter, made up of “processor” cells, is heritable and correlates with certain elements of IQ (Nature Neuroscience, DOI: 10.1038/nn758). The amount of white matter, which provides the connections between these processors, has since been shown to be heritable too (Journal of Neuroscience, vol 26, p 10235).

    Now it seems that the quality of these connections, which is governed by the integrity of the protective myelin sheath that encases them, is also largely genetic, and correlates with IQ….

    By comparing brain maps of identical twins, which share the same genes, with fraternal twins, which share about half their genes, the team calculate that myelin integrity is genetically determined in many brain areas important for intelligence. This includes the corpus callosum, which integrates signals from the left and right sides of the body, and the parietal lobes, responsible for visual and spatial reasoning and logic (see above). Myelin quality in these areas was also correlated with scores on tests of abstract reasoning and overall intelligence (The Journal of Neuroscience, vol 29, p 2212).

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