There is an insightful and data-rich blog called hbd* chick that is well worth our reading. A recent post discusses Steven Pinker’s new book about the decline of violence. I haven’t read the book, but the discussion on hbd* chick’s blog suggests that Pinker does not give much weight to the possibility of genetic change in European history causing the long term declines that we see, for example from Eisner’s careful tabulation of homicide rates in Europe from Medieval times.

In the Western European countries for which Eisner found data on homicide rates there was a bumpy but steady decline from Medieval times through the twentieth century. Here, for example, is his chart showing homicide rates in England: note the log scale on the Y axis. The rates are homicides per 100,000 population.

The data from other European countries are similar but some, especially Scandinavia, seem to have undergone the change later by a few centuries.

Homicide rates (these are local rates and do not include war casualties) declined dramatically or so it seems. There is a near hundred-fold difference between English homicide rates in 1300 and 2000. What if, as Gregory Clark would suggest, these declines reflect genetic, i.e. evolutionary changes, in the populations. How much selection would be required to cause such a decline in violence? An interesting recent development in human genetics is the resurgence of classical quantitative genetics as an older faith that we would quickly find “genes for” one thing and another mostly failed. It seems that RA Fisher got it essentially right in 1918. A good entree to current literature is this remarkable paper showing that cryptic distant genetic kinship predicts IQ correlations just fine.

In this tradition our model is that there is some underlying partially heritable trait that we might call “propensity to violence”. It doesn’t matter what we call it since it is just some direction in a high dimensional space of characters. This trait has a Normal distribution in the population, and individuals with a high enough value of the trait commit homicide. In other words homicide is in our model a classical threshold trait. We assume that selection on this trait is mild so that directional selection acts to shift the whole distribution up or down.

In 1300 the homicide rate was about 50 per 100,000 people, or 0.5 per thousand. Homicide must have caused on the order of 1 to 2 percent of all deaths and a much higher proportion of deaths of young adult males. Our assumption of a Normal distribution of the underlying trait immediately implies that the threshold was 3.3 standard deviations greater than the mean (from any table of the Normal distribution). Natural selection, social selection we would say in this case, disfavors homicide and the distribution is shifted so that the homicide threshold is surpassed by only 1 in 100,000 people rather than 1 in 2,000. By the year 2000 the homicide threshold is at 4.3 standard deviation from the population mean. In other words selection has moved the distribution 1 standard deviation in 700 years or 28 generations.

This amount of change, 1 standard deviation, would correspond to a change in IQ of 15 points or a change in mean male stature of about 2.5 to 3 inches. What strength of natural selection would have been required to cause this amount of genetic change?

The workhorse of quantitative genetics is the “breeder’s equation” which says that the response to selection is the product of the additive heritability and the selective differential, which is the difference between the population mean of a trait and the mean among parents. It is usually written as

In the present case we need a response of 1/28 of a standard deviation per generation. Assuming an additive heritability of 0.5 (the true value is probably 0.8 or so from literature on the heritability of aggressive behavior in children) the selective differential must be about 1/14 or .07 standard deviations per generation. In terms of IQ this would correspond to a one point IQ advantage of parents over the population average and in terms of stature parents with a mean stature 0.2 inches greater than the population average. This would occur if the most homicidal 1.5% of the population were to fail to reproduce each generation.

Justice was famously brutal and harsh in Medieval and Renaissance England so this may not be an entirely meaningless exercise. In this excellent essay Peter Frost suggests that the nearly the same selection against violence occurred in the several centuries before the fall of the Roman Empire, and he provides grisly details of Roman treatment of criminals.