Some confounds in gender differences in cognition: handedness, sexual preference, hormones

Keeping to my week long theme of gender differences in cognition (here and here), here is an article by Diane Halpern in eSkeptic. It not only summarizes a lot of what is known about gender differences (even though it is reprinted from 1993) but also goes into confounding factors like prenatal hormones, sexual preference and most importantly handedness -- all of which affect verbal and mathematical ability. I found this passage about the confounds of handedness and homosexuality particularly interesting:

The idea that the brain is a sex-typed organ has generated a great deal of interest. There is a large and growing body of literature that suggests that cognitive abilities vary both as a function of one's sex and preferred hand, that is, whether you are more or less left or right sided. Some of the most recent research is showing that prenatal hormones, the ones that direct and reflect the sexual differentiation of the fetus, are the same ones that determine handedness. Consider, for example, a large study in which the researchers reported sex by handedness interaction on cognitive tests. They used three large samples in different geographical areas of the country, so they had built-in two replication samples. They used multiple measures of spatial and verbal ability, and they found that while, overall, males performed better than females on 14 out of 15 of the different spatial tasks, across three geographically distinct samples, they found that left-handed males performed poorer than right-handed males on all 15 of these tests across all three samples. On the other hand, left-handed females performed better than right-handed females on 12 of these tests. Reverse results were found with verbal abilities with right-handed females out performing left-handed females, and left-handed males out performing right-handed males. It is not important that you keep the specific sex by handedness interactions straight; what is important is with replications and large numbers of tests, many psychologists are finding differences that depend on one's sex and one's laterality. Sex by handedness interactions have been noted by numerous other investigators, although they are not all easy to interpret.

These results are particularly important because we have no reason to believe that sex role pressures, learning environments, or any other psychosocial variable differs as a function of laterality. That is, there is no environmental hypothesis that we have that can explain these results. We do not socialize left-handed girls differently from right-handed girls, or left-handed boys differently from right-handed boys.

There are several theories that have been designed to explain some of these sex by laterality differences. The most popular one is by Geschwind and Galaburda, who proposed a biological theory of cognitive sex differences. They believe that prenatal hormones are important determinants of brain development. By itself, that is not a very controversial position. Geschwind and Galaburda also found strong positive relationships among left-handedness, high levels of prenatal testosterone, both chemically induced (people taking drugs) and secondary to maternal stress, and allergies such as asthma, hay fever, and other immune disorders, particularly those involving the thyroid. We also know that there is a greater proportion of males than females who are left-handed, which would be predicted by this theory, because males are exposed to greater levels of prenatal testosterone. The next plausible question is whether there are any data that might support this relationship among sex, handedness, prenatal hormones, and cognition. The answer is yes.

...

It is very difficult to explain associations among a set of variables as diverse as these without some sort of unifying theory that would help to tie them to some common origin or common influence. Unfortunately this gets even more complicated. Other evidence in support of Geschwind and Galaburda's hypothesis were provided by Sanders and Ross-Field. I believe they were among the first researchers who reasoned that male homosexuality might also be determined by the same prenatal variables that are involved in cognitive sex differences. This possibility lead to the prediction that male homosexuals, as a group, would resemble females in their cognitive abilities more than they would resemble heterosexual males. Using several different tests of spatial ability, they found that their samples of male homosexuals demonstrated spatial abilities similar to that of the female samples. Both male homosexuals and females were significantly lower in their visual spatial abilities than the heterosexual males. They replicated this finding in three different experiments, and it has now been replicated by several other investigators. It seems that many people are supporting the same finding. There are also several reports in the literature showing that male homosexuals and male and female transexuals, are more likely to be left-handed than any other groups. This suggests again that these variables are related in ways that are not easy to unravel.

It is a great article and definitely worth reading in its entirety even though it is rather long. I particularly liked this personal note:

When I went into cognitive psychology I did not plan to conduct controversial research. It started when I was teaching courses in cognitive psychology and the psychology of women, and the same question about the relationship between sex (or if you prefer, gender) and cognitive abilities came up in both classes. It seems that almost everyone is interested in this topic, which is probably why it has received so much press coverage in the last several years.

In order to answer the question of how women and men differ in their thinking, I began to review and synthesize the research literature on sex differences in cognitive abilities. If you ever try a computerized search in this body of literature you will be overwhelmed with the number of citations on these topics. About 14 years ago when I was conducting my research for the first edition my book Sex Differences in Cognitive Abilities, I had planned to show the weakness of the evidence in support of biological bases for any claimed cognitive differences between the sexes. If there were any differences, it seemed to me that they must be small and insignificant. Instead I found that the differences are sometimes large and that some of the biological data used to explain the differences were too strong and too consistent to ignore. I also found that the effects are not simple, and that other variables influence the findings. We usually talk about laterality as left or right handedness, but it is really a continuous variable that extends over many different indices of right or left sidedness. Laterality interacts with sex so that the kind of answers we get to questions about sex differences in cognitive abilities depends upon what I call sex bilaterality interactions. That is, some of the results seem to depend on both one's sex and one's preferred hand. (Emphasis mine.)

Here is her take home:

Finally, we must not fall prey to the dangers of self-fulfilling prophecies. The data presented here represents average differences, based on large samples of males and females. No single individual is average. Group average data have little to do with individual performance. There is considerable between-sex overlap in all of the cognitive abilities, with large numbers of males demonstrating high verbal abilities, and large number of females demonstrating high visual-spatial quantitative abilities. The literature concerning cognitive sex differences has been proliferating in recent years because the questions are of profound human interest. But the most important issue is not how women and men differ on the average. We should keep in mind the words of the 18th-century British writer who was once asked, "Which is smarter, men or women?" He replied: "Which man, which woman?" (Emphasis mine.)

All the evidence suggests that the wealth of human variety in any group -- left-handed straight women, etc. -- overwhelms the differences between groups. Human beings are many and various and defy simple description.

UPDATE: Also here is a debate between Steven Pinker and Elizabeth Spelke on the topic. Also long but definitely provides both sides of the issue. The talks are available in audio form with Power Point slides.