A Thought About Dual Inheritance Theory

I recently had reason to re-read Eric Alden Smith’s chapter on Three Styles in the Evolutionary Analysis of Human Behaviour. This chapter was published in 2000 and discussed the relative merits of Evolutionary Psychology, Human Behavioural Ecology and Dual Inheritance Theory (DIT). Some might say that this chapter is now out of date, and that the boundaries between these three sub-disciplines, or styles, has begun to erode with scientists now pursuing particular kinds of question with specific methodologies. Moreover there is some evidence, folk might say, that researchers recognize the need to discuss population level behavioural patterns, in order to proscribe parameters for the more proximate musings of, for example, evolutionary psychologists.

Given the rhetorical nature of the preceding paragraph one might assume that I think the boundaries are eroding. I certainly think that conceptually they ought not to have been there for evolutionary psychology and human behavioural ecology, and I suspect a deep sociological theory will reveal why such issues existed, perhaps revealing an abiding problem of funding and departmental pecking orders. I do, however, think that DIT ought to be seen as a stand-alone activity.

On page 32 Smith outlines the core of DIT. DIT claims that culture exhibits heritability, variation and fitness effects, but that cultural inheritance is different from genetic inheritance. So, neo-Darwinian modelling can be applied to cultural change but with modifications to account for the difference in inheritance. Furthermore, given this distinction between inheritance systems, cultural evolution can lead to genetically maladapted forms. Smith then moves on to claim that DIT can sustain particular forms of group selection:

“Since collective action often involves systems of widespread and indirect reciprocity (e.g. serving as a soldier on behalf of one’s society is reciprocated with various kinds of rewards to the soldier or his kin), it qualifies as a form of social exchange. Boyd and Richerson… have constructed models of cultural group selection of such group-beneficial behaviours. These models show that cultural inheritance plus conformity transmission (‘when in Rome, do as the Romans do’) can in principle create and maintain significant between-group differences despite reasonable rates of migration between these groups, thus avoiding a major obstacle facing classical forms of genetic group selection.

“The mechanisms of group selection most commonly proposed in this regard is group dissolution due to warfare, with refugees from defeated groups being absorbed by allied groups; if culturally transmitted traits that favour socially altruistic traits (e.g. contributing to the collective good of military defense and offense) decrease the probability of group defeat and dissolution, then these could spread by cultural group selection…” (Smith, 2000: 33)

Putting to one side the technical problems of group selection, another way of stating this effect (if it indeed qualifies as an effect) is that a defeated person can be housed within the population that defeated him or her and learn that contributing in a particular way to the group will sustain him or her. We can hypothesize that humans have good learning mechanisms that enable them to conform at the appropriate point.

More critically, if one removes the possibility of changing strategic response for absorbed refugees, the victorious groups will change the ratio of committed soldiers to uncommitted members both through absorption and the natural attrition of soldiers as a consequence of what they do. As these proportions change so will the population dynamics within the group. This is likely to erode between group differences and increase within group variation thus undermining the claimed possibility of group selection.

So, either absorbed refugees can change and conform, in which case the explanatory action is at the psychological level and we are back to normal evolutionary concerns with regard to the selection of such learning mechanism, or they cannot and the constitution of the groups will change and prevent sufficient between group differences for the kind of selection wished for.

My charitable suspicion is that DIT folk heuristically think of groups as organisms with phenotypes, and see them competing in a given ecology with other such individuals. However, once they have modelled some dynamic interaction between such individuals, perhaps in a similarly approximate way that gene-level selectionists might look at individuals, they remember that they are groups, that the inheritance is within and between groups, that it is different from genetic inheritance in many other ways (not least its low fidelity), and so they try and force the group selection point. Indeed, they call it cultural group selection in an attempt to move away from contentious issues in biology.

To conclude, I think that this use of (cultural) group selection by DIT theorists shirks explanatory responsibility. What is interesting is how humans ever begin cooperating in the first instance, how human psychology facilitates the emergence of stable equilibria in a variety of social games. Evolutionary psychology will tell you that evolutionary game theory will predict the emergence of certain strategies that are the product of gene-level dynamics. By establishing these strategies we can then begin to look at the psychological mechanisms that are, by extension, part of this strategic response, and then map the variation in these phenotypes. Such activities are entwined with behavioural ecological concerns about facultative responding to different ecological facts, in order to (try and) maximize fitness. I recognize this is a rather a skimming treatment of the situation, but it serves to emphasize what DIT is not delivering. DIT at best might highlight something to look at – the operation of conformity or some such – but by forcing what is at best an analogy with genetic inheritance, and thereby biological evolution, the discipline misses the intrinsic puzzle of the phenomenon, a phenomenon that evolutionary social psychology can take care of. To this end, DIT is in no sense a style of evolutionary analysis, as Smith has claimed.

(If you want to see how to think about all of the underlying biology properly then you should read West et al. (in preparation). This paper is the best I have read on the topic of cooperation, and deals with the claims from group selectionists decisively and fairly. It deserves to be widely cited upon publication.)


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