Genetic diversity and economic development: Ashraf and Galor respond

As I noted in a postscript to my last post, Quamrul Ashraf and Oded Galor have prepared a response [Update: the response is no longer online] to the Harvard academic critique of their paper on genetic diversity and economic development (I recommend having a look through the comments on that post, where Jade d’Alpoim Guedes, Nick Patterson (both authors of the critique), Henry Harpending and others continue the debate).

Apart from the broader question of whether this work should even be undertaken, the Harvard critique focused on two issues: causation and the statistical foundations of the work. Ashraf and Galor are quick to dismiss the statistical critique:

[O]ur critics have falsely suggested that we treat socioeconomic and genetic data as if populations are independent of one another. On the contrary, our empirical analysis accounts for the possibility of spatial dependence across observations, including analytical methods that correct for spatial autocorrelation in “error terms” and bootstrapping. This criticism of our work thus reflects either a misunderstanding of the techniques that we employ or a superficial reading of our work.

The response on causation is more detailed, and one of Ashraf and Galor’s arguments is one that I did not expect to see. They write:

The key is that the measure of intra-population genetic diversity that we employ should be interpreted as a proxy (i.e., a correlated summary measure) for diversity amongst individuals in a myriad of observable and unobservable personal traits that may be physiological, behavioral, socially-constructed, or otherwise. …

A careful reading of our research should make it apparent that our use of the measure of genetic diversity from the field of population genetics does not imply that our hypothesis is one of biological determinism, nor does it imply that DNA material is directly important for economic outcomes or that some genes are more important than others for economic success. The fact that the measure of genetic diversity we use is based on variation across individuals in non-protein- coding regions of the genome (and, thus, in genomic characteristics that are not necessarily phenotypically expressed so as to be subject to the forces of natural selection) is clear reason why our findings should be interpreted through the lens of our measure serving as a proxy for diversity more broadly defined.

The more relevant question to ask therefore is to what extent the measure we use can reasonably be considered a proxy for diversity in unobserved phenotypic or socially-constructed characteristics. There is indeed an emerging body of scientific evidence that establishes remarkable correlations in this regard.

Ashraf and Galor are also quoted running this line in a Nature News piece on their paper:

 Galor and Ashraf told Nature that, far from claiming that genetic diversity directly influences economic development, they are using it as a proxy for immeasurable cultural, historical and biological factors that influence economies.

After reading this, I went back to the paper to confirm my previous understanding of it, and if Ashraf and Galor intended to use genetic diversity as a proxy, it is not clear. The paper appears to finger genetic diversity and the phenotypic expression of that diversity as the relevant causal factors, with no suggestion it is a proxy. For example, they write:

The hypothesized channels through which genetic diversity affects aggregate productivity follow naturally from separate well-established mechanisms in the field of evolutionary biology and from experimental evidence from scientific studies on organisms that display a relatively high degree of social behavior in nature (e.g., living in task-directed hierarchical societies and engaging in cooperative rearing of offspring). The benefits of genetic diversity, for instance, are highlighted in the Darwinian theory of evolution by natural selection, according to which diversity, by permitting the forces of natural selection to operate over a wider spectrum of traits, increases the adaptability and, hence, the survivability of a population to changing environmental conditions. On the other hand, to the extent that genetic diversity is associated with a lower average degree of relatedness amongst individuals in a population, kin selection theory, which emphasizes that cooperation amongst 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.

I would like to see a more direct defence of their argument about the causal mechanisms. However, Ashraf and Galor do suggest in their response that further research on the causal mechanisms is required.

The timing of this debate has highlighted the extent of continued disciplinary divides. Ashraf and Galor released the working paper a couple of years ago, and they have since presented it in a raft of conferences and seminars. It was then accepted for publication in the American Economic Review, but the current debate was only triggered when the paper was mentioned in Science (gated). The pre-publication of working papers so prevalent in economics, and which is starting to gain traction in other fields, still relies on the working paper getting in front of people who might be interested in commenting. The reality is, however, that publication in a reputable journal remains the point at which a paper comes to others’ attention – or becomes “important” enough that it deserves a response. (On that note, I always welcome critique of my working papers).

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?
  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?

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

5 comments

  1. One hilarious criticism that is central to the letter of the Anthropologists is omitted from the discussion above. I find the response of Asharf and Galor fantastic, uncovering the shallowness of the entire letter.

    “Second, on the conceptual front, our critics have raised several concerns. They challenge our findings that diversity can be beneficial for innovative activity, stating in their letter that our hypothesis must be fundamentally flawed because it “implies that the Maya and Aztecs should not have been able to achieve high population densities because of their low genetic diversity.” Such an inference is based on a misunderstanding of basic empirical methodology. In simple terms, it is equivalent to suggesting that if we were to observe a 100-year-old person who smokes then research that concludes that smoking is harmful must be flawed.

    In following this line of argumentation, our critics have fallen prey to the trap of conducting thought experiments without holding everything else constant. Indeed, Amerindian populations in general have the lowest degree of diversity worldwide but, as we establish empirically, additional factors have contributed to the prosperity of some of these populations and the stagnation of others. Our research does not suggest that diversity is the only determinant of development. In fact, we consider variations in numerous other observed factors across societies that contribute to their economic development. Thus, in order to use the example of the Maya (or the Aztecs) to falsify our hypothesis, the right thought experiment is to ask: if we were to take another society that is identical to the Maya (or the Aztecs) with respect to all factors other than in the extent of intra-population diversity, how would the level of development in that society differ from that of the Maya (or the Aztecs)? Framed differently, given that actual societies differ from one another in many respects (geographical, institutional, cultural, etc.) and not just in their levels of diversity, once all these other differences have been used to explain not only their comparative development but also the differences in their levels of diversity, is the unexplained variation in development related to the unexplained variation in diversity? Indeed, this is precisely the question that is answered by a regression analysis of the type that we conduct, one that not only controls for differences across populations in observed factors other than diversity but also addresses the issue of correlation vs. causality that arises when differences in unobserved factors (as captured by variation in the “error terms” in a regression model) are statistically correlated with differences in diversity.”

  2. Try running a reality check on Ashraf and Galor’s assertion: “the most homogeneous country, Bolivia, placed at 0.63 and the most diverse country, Ethiopia, at 0.77.”

    Ethiopia and Bolivia, rather than being polar opposites on all but the most contrived measures of genetic diversity, have a surprising number of things in common, including fertile highlands and populations that have some Caucasian ancestry.

  3. I quite approve of Dr. Enrico Spolaore’s use of different levels of diversity in neutral genes as a clever way to measure the effectiveness of pre-Columbian barriers to diffusion of technology, culture, and people. Unfortunately, Ashraf and Galor and many of their supporters and critics are using Spolaore’s concept more carelessly.

    Consider the example from Ashraf and Galor’s abstract about how Bolivia is the most genetically homogeneous country and Ethiopia the most genetically diverse.

    The germ of a true idea in this is that in 1491, the population of what is now Bolivia were descendants of people who had gone through a series of genetic bottlenecks as humans expanded Out of Africa. So the indigenous population of the Altiplano had less diversity of neutral or junkish genes that aren’t strongly acted upon by natural selection. (These are the genes that population geneticists such as Cavalli-Sforza focus upon since they don’t do all that much, so they tend to be passed on by predictable rules of randomness. In contrast, functional genes can prove to be favorable or unfavorable mutations and can spread wildly or die out quickly.) This does _not_ mean that South American indigenes necessarily had less diversity of functional genes.

    In contrast, Ethiopia is back near where the modern human race presumably got started, so it’s population didn’t have to go through various bottlenecks such as Out-of-Africa and Into-America.

    In reality, this is an overly stylized notion about Ethiopia since even in 1491 some of the Abyssinian highlands were populated by a clearly mixed race black and white population reflecting movements back and forth across the Red Sea, with the highlands of Ethiopia and the highlands of Yemen sharing a not wholly dissimilar and not wholly unconnected culture for thousands of years. The Abyssinians themselves claim to be the descendants of the son of the Queen of Sheba, who was presumably from the Arabian peninsula. (Arabs are mostly descended from people who went through the Out of Africa bottleneck, thus reducing their neutral gene diversity.) Moreover, Abyssinians were in occasional contact with other Old World civilizations — in 1306, for example, Emperor Wedem Ar’ad of Ethiopia sent a diplomatic mission that called upon Pope Clement V at his palace in Avignon seeking an alliance of Christians against Muslims.

    On the other hand, population geneticists don’t really like to sample in cosmopolitan cities like Addis Ababa, even though that’s where most of the population is today. They prefer to get their samples in isolated tribal settings, of which Ethiopia has an abundance in some of its more remote regions, especially in the western lowlands. These tend to display a lot of neutral gene diversity since their ancestors have mostly been there for a very, very long time.

    In any case, the basic idea is that Ethiopia is back near where the modern human Out of Africa expansion began and Bolivia near the far end. So, that implies that it was particularly difficult before intercontinental sea travel became reliable for valuable innovations to diffuse from Ethiopia to Bolivia or vice-versa. For example, the Bolivian potato is a highly useful crop, but before Columbus it was unlikely to get to Ethiopia because it’s an awful long walk.

    Now, you could argue that we pretty much already knew that and therefore we don’t need Spolaore’s genetic distance measures derived from Cavalli-Sforza. But, it’s an elegant way to quantify a whole lot of otherwise murky prehistory.

    Where it goes wrong is that people always forget that population geneticists, who are trying to figure out the genealogies of racial groups, prefer to look at mutations in the most neutral, least important genes available. So, when everybody today passes on the urban folklore about how there is the most genetic diversity in Africa, yes, that’s true about genes that don’t do much of anything. But, if you are interested in diversity of genes that do matter, well, then you’ve got a very methodologically tricky issue on your hands.

    For example, consider Bolivia, which was indeed relatively homogeneous in 1491 in neutral genes. Yet, there already existed economically important genetic diversity in functional genes. The Amerindians of the Bolivian highlands often possessed a favorable mutation adapting them for living and reproducing at high altitude which the Amerindians of the Bolivian lowlands did not possess. To this day, Altiplano Indians find the Amazon region physically uncomfortable and Amazonian Indians find the Highlands uncomfortable.

    This mutation for thriving at high altitude in the Andes has been identified (interestingly, it differs from the Tibetan equivalent). Presumably, Ethiopian highlanders also have some kind of genetic traits that let them do well in thin air too (as their Olympic distance running success suggests), but last I checked the precise genes haven’t been identified yet.

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