I’ve mentioned my award-winning undergraduate research in sex differences in cognition several times, and I don’t mind saying I’m pretty proud of it. It is my first paper published in a science journal, and it will always be special to me for that reason. I also was very fortunate to have brilliant, highly distinguished co-authors which were just fantastic to work with and from whom I learned a great deal: Justin Rhodes of the Beckman Institute at the University of Illinois; Elliott Sober, the Hans Reichenbach Professor and William F. Vilas Research Professor in the Philosophy Department at the University of Wisconsin—Madison; and Theodore Garland, Jr. ,Professor of Biology at the University of California, Riverside. I am ever grateful.
Although the paper was published in the December (current) edition of the Quarterly Review of Biology, the University of Illinois has just issued the press release complete with its own YouTube video. Since it is now making the rounds in the media, I thought this would be an ideal time to talk a bit about the paper and perhaps answer some questions.
Male Superiority in Spatial Navigation: Adaptation or Side Effect?(pp. 289-313) Edward K. Clint, Elliott Sober, Theodore GarlandJr. and Justin S. Rhodes
Here is the video. The cartoons are a bit cutesy, but all-in-all, I am pretty impressed with it. The basic details are covered in a simple, easy-to-follow manner.
Media coverage round-up:
The Smithsonian Magazine | Men Are Better Navigators Than Women, But Not Because of Evolution
National Geographic (Carl Zimmer) | Of Men, Navigation, and Zits
Richard Dawkins Foundation for Reason and Science | Males’ superior spatial ability likely is not an evolutionary adaptation
Up until now, the prevailing explanation about why there are sex differences in humans with regard to performance at various spatial abilities has been developed mainly by Irwin Silverman and Marion Eals. First I must say they do not argue that men are better than women, but than men and women are each somewhat specialized at different kinds of spatial tasks. Men are better at way-finding, but women have been shown to be superior in measures of object location memory (Silverman et al., 2007). Like most scientific research, the studies conducted by Silverman & Eals (and others) were designed to evaluate specific hypotheses, such as questions as to what differences exist and how they may be characterized. In most scientific domains, comparatively few papers aim to test the explanatory model or research program itself, but that was part of our purpose here.
Is testosterone the culprit?
Our study does not provide direct evidence that testosterone spillover is responsible for the sex differences. We assessed existing evidence suggesting that it may be, and further found that such an explanation is consistent with our findings. Some of the more fascinating evidence in this regard includes the observations that women who have received a dose of testosterone perform better on the typically male-advantaged spatial ability tests and that castrated male mice perform almost exactly like female mice, while untreated males substantially outperform females.
Why just 11 species?
Even though our results turned out to be statistically significant, we would surely have liked more species and I hope that other researchers add to the data here. In order to include a species in the study, I had to have 4 pieces of data: male & female home range size and male & female performance at a relevant navigation-related task. Although things like home range size are often studied by behaviorial ecologists, rarely is the information broken down by sex. Spatial ability performance is also very commonly measured in animals, but again, it is not generally studied as a factor of sex. I found many studies focusing on a given bird species’s home range and spatial ability, for example, that I could not include because all individuals were reported as a single group. This was very frustrating for me. Finding data for 11 species meant sifting through thousands of papers and reading hundreds.
Scientific skepticism & evolutionary psychology
As a skeptic, I believe in tough love. That is, I believe that the more you like a particular idea, the more effort you need apply in trying to stress and rend it— to find its weaknesses and determine its strength. It is the only sure way that one can, as an individual, stay honest. That’s one reason I really enjoyed producing this study. My co-authors are much more critical of the general scientific output of evolutionary psychology than I am. As both a skeptic and an evolutionary psychologist-in-training, it was incumbent upon me to listen carefully to what these men, some of the sharpest of critics, had to say. On a more meta level, I saw first hand that people could have very substantial disagreements and still all be passionate, honest, and smart. Better still was that we could work together and do some good science, a shared goal.
On the other hand, this paper also happens to highlight the poverty of the criticism of some of evolutionary psychology’s most confused and least informed opponents. Let us consider some popular critical refrains.
Evolutionary psychologists assume everything is an adaptation / EP hypotheses are untestable.
Nope; We tested an adaptationist explanation and found it unsupported by the data. Not every trait is an adaptation.
Evolutionary psychology doesn’t have “real” or legitimate methods, like biology does.
Two of four authors are trained biologists, another a philosopher of biology; We used methods taken straight from biology: cross-species comparative analysis as well as a reconstructed phylogeny in order to conduct a phylogenetic independent contrasts test to correct for species relatedness. Methodologies from biology and other disciplines are critical to and commonly used by evolutionary psychologists.
Evolutionary psychologists are sexists who want to justify the status quo.
Our proposed explanation for sex differences is that they are a quasi-random spandrel, with no particular moral or sociopolitical significance. The reigning adaptationist explanation articulated by Silverman & Eals also explicitly notes that neither sex performs better across the board, and that it depends on which measure you happen to be looking at. Also, some excellent work in this area has been done by prominent women (Ecuyer-Dab & Robert 2004a, 2004b; Linn & Petersen, 1985).
Evolutionary psychologists only study white American undergraduates and ignore cultural differences.
Setting aside the fact that humans are only one data point because this is a cross-species analysis, all assertions made about humans in this paper include strong cross-cultural support. For example, the sex differences in human navigation have been found in dozens of cultures, with no culture ever studied bucking the trend— none. Data on human home range included westerners and hunter-gatherers. We also discussed the possible effects of cultural differences in the paper. The evolutionary psychologists who reviewed the paper during peer-review had plenty of questions about culture.
This paper is sure to be controversial and is, in some respects, atypical of published literature in evolutionary psychology. Nonetheless, it is an ev psych study and I have the fondest hope that it contributes positively to the understanding of how our minds evolved. I am an evolutionary psychologist.
Ecuyer-Dab I., Robert M. 2004a. Have sex differences in spatial ability evolved from male competition for mating and female concern for survival? Cognition 91:221–257.
Ecuyer-Dab I., Robert M. 2004b. Spatial ability and home-range size: examining the relationships in western men and women (Homo sapiens). Journal of Comparative Psychology 118:217–231.
Linn M. C., Petersen A. C. 1985. Emergence and characterization of sex differences in spatial ability: a meta-analysis. Child Development 56:1479–1498.
Silverman I., Choi J., Peters M. 2007. The hunter-gatherer theory of sex differences in spatial abilities: data from 40 countries. Archives of Sexual Behavior 36:261–268.