Are Bugs Messing With Your Mind?

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Two biologists propose that the presence or absence of parasites in our environment exerts a strong top-down influence on the extent to which we have historically developed family ties and even religion. Surely it’s not the whole picture — I sure hope we’re more complicated than that — but could it be part of the picture?

One of two particularly fascinating abstracts of forthcoming papers which came across my desk in the last month is Parasite-Stress Promotes In-Group Assortative Sociality: The Cases of Strong Family Ties and Heightened Religiosity. Corey L. Fincher and Randy Thornhill, both from the Department of Biology at the University of New Mexico, link the development of family ties and heightened religiosity to the presence of environmental stressors in the form of parasites.

And when I say ‘link’, there’s more to it that makes this paper especially interesting. While Fincher and Thornhill’s data are ultimately correlations — which, as we’ve commented here many times before, do not by themselves make for causation — I can’t help but wonder whether their proposed explanation of the data really does carry some logical plausibility.

Here’s the abstract for the forthcoming paper:

Abstract: Throughout the world people differ in the magnitude that they value strong family ties or heightened religiosity. We propose that this cross-cultural variation is a result of contingent psychological adaptation that facilitates in-group assortative sociality in the face of high levels of parasite-stress while devaluing in-group assortative sociality in areas with low levels of parasite-stress. This is because in-group assortative sociality is more important for the avoidance of infection with novel parasites and for the management of infection in regions with high levels of parasite-stress compared to regions of low infectious disease stress. We examined this hypothesis by testing the predictions that there would be a positive association between parasite-stress and strength of family ties or religiosity. We conducted this study by comparing between nations and between states in the United States of America. We found for both the international and the interstate analyses that in-group assortative sociality was positively associated with parasite-stress. This was true when controlling potentially confounding factors such as human freedom and economic development. The findings support the parasite-stress theory of sociality, the proposal that parasite-stress is central to the evolution of social life in humans and other animals.

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As I noted above, the underlying data are just correlations. The suggestion that the authors “examined this hypothesis by testing the predictions that there would be a positive association…controlling potentially confounding factors” is apt to make any self-respecting philosopher of science cough up their breakfast. These are ‘predictions’ and ‘controls’ of the weakest possible kind. Methodologically speaking, calling this kind of ex post facto data collection ’empirical testing’ is, in my view, stretching the meaning of the words too far. (See pages 28-32 of the manuscript itself for more on ‘controlling’ for confounds; statistical control of confounds is not the same as experimental control of variables.) This is nothing whatsoever like predicting the effect of manipulating a variable and then going on to manipulate that variable and measure the effects. Now it’s not quite as bad as I’m about to make out, but really it is more akin to proposing a theory that its being Tuesday morning causes the sun to rise and then coming along on Wednesday, ‘testing’ your prediction by checking whether the sun rose yesterday morning, and concluding — yay! — that you’ve empirically tested your theory about the Tuesday morning sunrise impetus and found it to be well-supported.


But… On the other hand, doesn’t it seem to make some sense, for example, that where there are relatively more disease-carrying parasites, we might be a bit more hostile toward strangers who might be playing host to them, and where there are relatively fewer disease-carrying parasites, we might be a bit more open toward strangers? Could this type of differential play a part in developing family ties…even influencing belief in God?

Intellectually, I find myself recoiling at the authors’ use of phrases like “empirical testing” and “predictions”, yet instinctively I find it hard to object to the idea that parasite-stress might play the type of role that they suggest in understanding how we humans have developed.

What do you make of it? Let us know in the comments!

(This paper has been selected as a target article for open peer commentary at the prestigious journal Behavioral and Brain Sciences, one of the foremost print publications in its field. If you’d like to have a look at the full text of the paper, the pre-print is available here: Fincher and Thornhill Preprint: “Parasite-Stress Promotes In-Group Assortative Sociality: The Cases of Strong Family Ties and Heightened Religiosity”. If you’re interested in writing a commentary on the article directly for the journal, have a look at their instructions.)

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