Politics in your genes?

The story making the rounds now is that your politics may be determined by your genes. I think it may well be true. I've also heard commentary that indicates the commentator either hasn't read the study, or doesn't know how science works.

One person seemed to think the researchers who did this study were claiming that genes determined what people thought – period. In fact, you almost never see a 100% correlation between genes and behavior. What do we see in this case?

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Calculating how often identical twins agree on an issue and subtracting the rate at which fraternal twins agree on the same item provides a rough measure of genes' influence on that attitude. A shared family environment for twins reared together is assumed. On school prayer, for example, the identical twins' opinions correlated at a rate of 0.66, a measure of how often they agreed. The correlation rate for fraternal twins was 0.46. This translated into a 41 percent contribution from inheritance.

Identical twins share 100% of their genes in common. Fraternal twins share (on average) 50% of their genes in common. The difference between 100% and 50% of heredity in common works out to a difference between a correlation of 0.66 to one of 0.46.

I've been playing with the numbers in the article, and the only thing I can figure is that someone subtracted 0.46 from 0.66, and divided by the 50% difference in genes held in common. Thus, 0.66 - 0.46 = 0.2. Divide by 50%, which is the same as multiplying by 2, and we get 0.4, which is very close to 0.41 if the correlations given were rounded off.

Correlation coefficients (R) are not percentages. The proper figure is the coefficient of determination, which is the square of the correlation coefficient (R²). In the case of identical twins, R² = 0.44, meaning that if you know the position of one identical twin on school prayer, you can predict the other's position some 44% more often than random guessing would allow. In the case of fraternal twins, the number drops to 21% better than random guessing.

Another tack I've heard commentators taking is to focus on the specific items reported in the article. No serious researcher believes there's a specific gene for the opinion on school prayer, or property taxes, the Moral Majority, or capitalism. What the researchers are saying is that:

...people's gut-level reaction to issues like the death penalty, taxes and abortion is strongly influenced by genetic inheritance. The new research builds on a series of studies that indicate that people's general approach to social issues – more conservative or more progressive – is influenced by genes.

I find it very interesting that a report like this would be covered in the NY Times. Larry Summers caught all kinds of grief for saying that men and women differ in their tendency to go in to the sciences. One of the debates in modern society is whether men and women are inherently different. The conventional wisdom in the universities has been that all mental and social differences are the result of environment and social conditioning. Folk wisdom, and new research, hold that at least some differences in attitudes are inherent.

The difference between male and female is genetic.