Does genetic diversity increase conflict?


Jason Collins


February 22, 2013

Ashraf and Galor’s hypothesis linking genetic diversity to economic growth has two limbs. The first, which I posted about last week, is that genetic diversity pushes out the production possibility frontier through increasing the range of traits in the population for developing and implementing new technologies. The second, the subject of today’s post, is that genetic diversity decreases trust and cooperation between people, increasing social disorder and conflict.

The measure of genetic diversity used by Ashraf and Galor is expected heterozygosity, which is a measure of the probability that two people selected from the population will have the same allele (variety of a gene), averaged across all measured genes. Genetic diversity is often confused with genetic distance, a measure of the time since two populations had a common ancestor. Genetic distance can be calculated using data of the type used by Ashraf and Galor, with that measure being the probability that two alleles from a given genetic locus selected from two different populations will be different. Genetic distance has a resemblance to the genetic diversity measure, but is across populations, not within them. Spolaore and Wacziarg used genetic distance in their 2009 paper where they proposed that genetic distance (or other measures proxied by genetic distance) hindered the transfer of technology, leading to technological and income differences between countries.

Genetic diversity should also be distinguished from relatedness. Relatedness is the genetic similarity of two individuals, relative to average similarity of all individuals in the population. Since relatedness is measured relative to average similarity, relatedness does not increase in a less genetically diverse population. However, the average similarity between population members is higher where diversity is low.

This distinction is important, as Ashraf and Galor’s argument for the causative pathway for genetic diversity and its effect on cooperation is via relatedness. They state:

[T]o the extent that genetic diversity is associated with a lower average degree of relatedness among individuals in a population, kin selection theory, which emphasizes that cooperation among genetically related individuals can indeed be collectively beneficial as it ultimately facilitates the propagation of shared genes to the next generation, is suggestive of the hypothesized mechanism through which diversity confers costs on aggregate productivity.

It may be possible to craft an argument that relatedness is higher in a less genetically diverse population if you considered the relevant population for measuring relatedness to be the global population. Across the global population, two individuals from a less diverse sub-population would have a relatedness marginally above zero. However, this is a stretch. A more feasible argument would be to take Ashraf and Galor’s use of the term relatedness to refer to genetic similarity.

Ashraf and Galor dedicate little time to building the evolutionary basis to their argument in the main paper, but give it some focus in the web appendix. They note a study in which long tailed tits (a bird) provided breeding support to kin, and an analysis of 18 vertebrate species that found a strong correlation between brood rearing assistance and relatedness. They also describe a study in which juvenile spiders cooperate with kin while feeding to increase feeding efficiency.

The disconnect between these studies and Ashraf and Galor’s argument is the degree of relatedness involved. The studies referenced in the web appendix involve relatively close kin, with relatedness a relative measure within the population. For the genetic diversity hypothesis to hold, humans would need a very fine tuned sense of relatedness in the broad sense noted above. Are people more likely to cooperate with those who they are more genetically similar, despite no immediate reference group for comparing that similarity? The problem is that, beyond close kin, there is poor empirical support for kin recognition.

One possible angle in support of Ashraf and Galor’s hypothesis might be to use evidence of the detection of genetic similarity and heterozygosity in mating decisions (such as here, here and here). However, the mating preferences are usually for more dissimilar or heterozygous individuals, suggesting diversity of this nature has a positive effect.

At the close of the article, Ashraf and Galor seek to build their case by using their genetic diversity dataset to examine whether genetic diversity affects trust. Across 58 countries for which a measure of trust can be gleaned from the World Values Survey, they found a significant relationship in the required direction. Increasing genetic diversity by 1 percentage point is associated with a 2 percentage point decrease in the prevalence of trust. In obtaining this result they included controls for geography, OPEC, and sub-Saharan Africa, and used continent fixed effects, meaning that the effect is within continents.

As was the case for the regressions of genetic diversity on measures of scientific output, I find this result unconvincing. The range of controls used and the question of whether there are other relevant variables (such as IQ), combined with the lack of causative pathway, leaves me needing much more evidence.

So, where to from here? As I noted in the discussion of innovation, cross-species comparison is one potential avenue for further research, as is examination of isolated human populations. Inbred human populations might also be an interesting source of evidence, although it can be difficult to separate relatedness, diversity and inbreeding effects. One consideration is that genetic diversity in particularly low in humans relative to other species due to some bottlenecks in our past. Could that low diversity be linked to the generally high levels of cooperative behaviour in humans?

As was the case for my post on the effect of genetic diversity on innovation, I have deliberately avoided the question of whether Ashraf and Galor were directly relating genetic diversity to economic development, or whether genetic diversity is a proxy for phenotypic diversity unrelated to that genetic diversity (such as language). As you can see below, my thoughts on that point are forthcoming.

My posts on Ashraf and Galor’s paper on genetic diversity and economic growth are as follows:

  1. A summary of the paper methodology and findings

  2. Does genetic diversity increase innovation?

  3. Does genetic diversity increase conflict? (this post)

  4. Is genetic diversity a proxy for phenotypic diversity?

  5. Is population density a good measure of technological progress?

  6. What are the policy implications of the effects of genetic diversity on economic development?

  7. Should this paper have been published?

Earlier debate on this paper can also be found hereherehere and here.