Last week I presented a summary of the method and findings of Ashraf and Galor’s American Economic Review paper The ‘Out of Africa’ Hypothesis, Human Genetic Diversity, and Comparative Economic Development (for the latest ungated version, go here). As discussed in that post, one limb of Ashraf and Galor’s argument is that genetic diversity provides a greater range of traits for the development and implementation of new technologies (which I’ll call innovation). In this post, I look at that claim in more detail.
The measure of genetic diversity used by Ashraf and Galor is based on non-protein coding regions of the genome. This is common in population studies to prevent selection from distorting attempts to track evolutionary history. However, Ashraf and Galor note that the genetic diversity shaped by the founder effect as populations moved out of Africa likely included other phenotypically expressed genetic diversity. For example, they note one study in which craniometric (head shape) diversity declines with distance from Africa.
At a population level, increased genetic diversity can have evolutionary benefits. Increased diversity provides a broader suite of traits on which natural selection can act. Where the environment changes, it is more likely that traits favourable in that environment will be present, and if the environment is particularly unstable, genetic variation will provide a basis for some of the population to be viable through the range of conditions.
Ashraf and Galor refer to a couple of studies in support of their hypothesis, but their examples do not build a strong case for their specific argument linking genetic diversity and innovation. In one case, they reference a study in which it was found that less diverse, inbred populations of fruit fly became extinct at lower concentrations of salt. However, the authors of that study were unable to differentiate whether inbreeding or lack of diversity drove the outcome. Other studies that isolated the effects of low diversity did not always show the same result (for example, in this study of flies).
Another more relevant study found that more diverse honeybee colonies have higher workforce efficiency (I can’t give a a link to this study, but a summary of work in the area is here). This finding provides evidence that diversity has an effect on production outcomes. Ashraf and Galor expand their discussion of the effect of diversity on bee colonies in the Web Appendix, where they discuss an experiment published in Science. In that study, colonies with various levels of diversity were released, with more diverse colonies founding their colonies faster and accumulating food stores more quickly. The authors of the study proposed that greater response variation to changing conditions may be behind the higher fitness of diverse colonies. If there is a trade-off between response thresholds and activity efficiency, or if response thresholds result in some behaviours missing from a worker’s repertoire, higher genetic diversity will provide a basis for the full suite of required traits.
One question overhanging these bee studies is the direction of the effect. As the Ashraf and Galor hypothesis has a countervailing force whereby diversity decreases cooperation, diversity could also be expected to harm colony production. Whether diversity is beneficial or not would depend on the relative effect of the two forces. Some of the bee studies note the potential for intra-colonial conflict and ask whether factors such as recombination rates may reduce conflict. But when comparing to the hump shaped relationship between human genetic diversity and economic development, it is not easy to place the bee colonies on the curve. Any effect of diversity on bee colony success could be justified after the fact as being at a certain point on the curve.
Towards the end of the paper Ashraf and Galor examine the hypothesis for human populations by regressing the number of scientific papers published per year per person against genetic diversity and a range of controls including social infrastructure, years of schooling, risk of malaria and distance to waterways. Continent fixed effects are used, which eliminates comparisons across continents, as well as other controls for sub-Saharan Africa and OPEC. The result of the regression is that genetic diversity is a significant factor in scientific output, with a one per cent increase in diversity linked to an increase in scientific articles per person per year of 0.02.
Despite this statistical evidence, I am not convinced. Partly, I suspect missing variables - that is, the qualitative traits that have been under selection since the Out of Africa event. The inclusion of controls such as social infrastructure also makes the analysis difficult, as it is closely related to potentially relevant factors such as IQ and levels of trust (the data is available on the AER website, so I should test my musings).
Still, if I were to favour a hypothesis of diversity affecting innovation, I can see two hypothetical pathways. First diversity may increase innovative activity as it allows for gains to trade. If there are different kinds of intelligence or other innovative traits, a broader basket of traits may provide more opportunity for technological innovation. The bee studies seem to fall into this camp.
Second, diversity may provide a higher probability of a favourable innovative trait being present in a new environment. Diversity would lead to a useful trait being present, but that trait would then spread and genetic diversity related to that trait would be eliminated by selection.
I lean toward the second pathway. Different forms of intelligence are highly correlated (hence the search for the g factor), and we would expect that a group with higher absolute intelligence would outperform a group with more diverse intelligence levels (unless, of course, those at the top end are exceptional).
However, having favoured this second pathway, I am not convinced that either is the case. I prefer a hypothesis that selection pressures pushed traits in a certain direction, with diversity a secondary factor (if a factor at all). Partly this comes from the nature of innovative activity, which is affected by diversity but relies heavily on the innovative capacity of those involved. Innovative capacity is also only a subset of the traits that affect evolutionary success. If the hypothesis was one of evolutionary success, I would give more weight to a diversity hypothesis. This is also one of the many reasons that extrapolating bee or other studies to humans can be problematic. For the bees, we are measuring proxies for evolutionary success, while innovative activity is only one of many factors that may affect evolutionary success for humans.
Given the above, further research is required to give the diversity-innovation hypothesis standing. But what approach should be used to tease out this question? Cross-species comparison provides one potential avenue, but how would levels of diversity across species be compared or levels of innovation measured? The low genetic diversity in humans relative to other species may complicate the analysis. I also suspect that a cross-species approach might be more useful in assessing the effect of diversity on levels of cooperation (the subject of a forthcoming post) as it may be too difficult to develop useful indexes of innovative behaviour in other species. Alternatively, further work on isolated human populations could be useful.
My preferred direction of research would be to directly analyse the selection that has occurred on the various populations, genetic evidence permitting. I would also examine what other controls are useful in the analysis. IQ is one option, although a Flynn-like hypothesis of economic development and IQ ratcheting up together would make that difficult.
As an end note, some of the debate about the paper raises 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). I have deliberately skirted that issue for this post, but as you can see below, my thoughts are forthcoming.
My posts on Ashraf and Galor’s paper on genetic diversity and economic growth are as follows:
Does genetic diversity increase innovation? (this post)