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The psychologists therefore designed a test focused on the ability to discern and later recall the location of objects in a complex, random pattern. In series of tests, student volunteers were given a minute to study a drawing that contained such unrelated objects as an elephant, a guitar and a cat. Then Silverman and Eals presented their subjects with a second drawing containing additional objects and told them to cross out those items that had been added and circle any that had moved. Sure enough, the women consistently surpassed the men in giving correct answers.
What made the psychologists really sit up and take notice, however, was the fact that the women scored much better on the mental-rotation test while they were menstruating. Specifically, they improved their scores by 50% to 100% whenever their estrogen levels were at their lowest. It is not clear why this should be. However, Silverman and Eals are trying to find out if women exhibit a similar hormonal effect for any other visual tasks.
Oddly enough, men may possess a similar hormonal response, according to new research reported in November by Doreen Kimura, a psychologist at the University of Western Ontario. In her study of 138 adults, Kimura found that males perform better on mental-rotation tests in the spring, when their testosterone levels are low, rather than in the fall, when they are higher. Men are also subject to a daily cycle, with testosterone levels lowest around 8 p.m. and peaking around 4 a.m. Thus, says June Reinisch of the Kinsey Institute: "When people say women can't be trusted because they cycle every month, my response is that men cycle every day, so they should only be allowed to negotiate peace treaties in the evening."
Far from strengthening stereotypes about who women and men truly are or how they should behave, research into innate sexual differences only underscores humanity's awesome adaptability. "Gender is really a complex business," says Reinisch. "There's no question that hormones have an effect. But what does that have to do with the fact that I like to wear pink ribbons and you like to wear baseball gloves? Probably something, but we don't know what."
Even the concept of what an innate difference represents is changing. The physical and chemical differences between the brains of the two sexes may be malleable and subject to change by experience: certainly an event or act of learning can directly affect the brain's biochemistry and physiology. And so, in the final analysis, it may be impossible to say where nature ends and nurture begins because the two are so intimately linked.