American Scientist

To the Editors:

I found Christine R. Harris's article "The Evolution of Jealousy" interesting (January–February). I appreciate how her comments might encourage better efforts to manage confounding variables in studies of jealousy. At the same time, however, it is a curious coincidence that quite disparate kinds of studies all kept coming to the same conclusion.

Dr. Harris made many comments that suggested she was not fully informed on the literature on adaptations (or exaptations). Most of all, before making the claim that jealousy is a general psychological mechanism, it's important to look closely at the literature on sexual dimorphism. Human males are, on the whole, bigger than females. This phenomenon is apparently an evolved solution to a single conspicuous problem: Males compete with each other for reproductive opportunity. If this is true, then it seems a whole host of hormonal differences (that contribute to things like increased muscle mass and high-risk behavior) are also selected for the same reason. Moreover, it seems something similar could be said of human females—that is, lactation, higher body-fat percentage, the influence of estrogen on oxytocin and so on are also solutions to a single conspicuous problem: reproduction and the costs of reproduction.

If these assumptions are correct, then it would be odd that natural selection would go to all this trouble to create notable physical and hormonal differences in males and females, yet would leave male and female psychology alone. If one accepts Dr. Harris's position, then it seems one must ignore signficant differences in male and female evolutionary environments. A general mechanism for jealousy amounts to being a Cartesian dualist: Evolution works only on the body, not on the brain.

Jordan Burks
University of Dalhousie

Dr. Harris replies:

Dr. Burks confuses the general question of whether there are evolved sex differences in the human brain with the topic of my article, which was the merit of one particular hypothesis about a purported sex difference in jealousy. I never suggested that male and female brains are identical; certainly there are important differences, some presumably reflecting different selection pressures on ancestral men and women. However, this vague a priori expectation does not mean that proposals for the existence of a given sex difference should automatically be accepted without empirical corroboration merely because some reasonable-sounding Darwinian story is advanced. Such stories are altogether too easy to make up, even for diametrically opposed traits. As for Dr. Burks's suggestion that it is a "curious coincidence" that different types of studies initially supported the theory of innate sexually dimorphic jealousy, in fact the article offers several counterexamples. For example, can any test be more straightforward than asking people who suffered infidelity what they focused on? Yet this produced data strikingly at variance with the theory.