How, you may be asking yourself, is a good sense of direction like a bad case of acne?
Over many decades, psychologists have measured the minds of men and women, looking for similarities and differences. Reliable results are notoriously hard to come by, because it can be very easy to find differences where none really exist. If you decided in 1970 to look at the fraction of scientific and medical Ph.D. awarded to women–under 5 percent–you might conclude that women’s brains just weren’t suited to the task. Today, that figure has reached about 50 percent. Women’s brains haven’t evolved over the past 40 years. Their social environment has.
Yet a few differences between the sexes do seem to hold up to scrutiny. One is spatial abilities. If men look at an object, for example, they are slightly faster at guessing what it would look like if it were rotated 180 degrees. There are plenty of women who do better than individual men. But overall there’s a statistically significant difference in their average performance. This kind of difference carries over from one culture to another. It’s even detectable in babies.
What accounts for the difference? Some scientists argue that it is an adaptation. Obviously, the evolution of the human race hasn’t hinged on being able to turn a stack of blocks around in our heads. But spatial abilities can have some far more practical benefits. If you can picture a landscape clearly in your head, you are less likely to get lost in it. People who score high on spatial ability tests also tend to do well on navigation tests. And in some studies (but by no means all), men do better at finding their way through a new place–be it a university building or a forest.
But why should men be better at women at navigation? Some researchers argue that we have to look at the different roles of men and women over course of our evolutionary history. Women spent most of their time in small ranges, either caring for children or gathering food. Men, on the other hand, had to rove over much bigger ranges to hunt game. They benefited from a better sense of navigation because they were at a greater risk of getting lost or failing to find game.
Whenever we reflect on human evolution, it pays to compare our species to other animals. And in the case of spatial abilities, the comparison is fascinating. Almost a century ago, the psychologist Helen Hubbet found that male rats could get through a maze faster than females. The difference can also be found in a number of other species.
If the “home range” explanation accounts for the difference, then you’d expect that species with a big difference between male and female home ranges should have a big difference in their spatial abilities. Some studies seem to bear that prediction out. Scientists have long been intrigued by meadow voles and their close relatives, the pine voles. Male meadow voles mate with lots of females, and have to defend a large home range against other males. Pine voles, on the other hand, are monogamous, and the males unsurprisingly have home ranges no bigger than their mates. When scientists look at their spatial abilities, they find that male meadow voles–the ones with big ranges–score much better than females. In pine voles, there’s very little difference.
QED? Not quite.
Two species are not enough to make a meaningful comparison, and so Edward Clint, a psychologist at the University of Illinois, and his colleagues recently made a larger comparison. They gathered data from studies on 11 species, including humans, voles, rats, and even horses and cuttlefish. They compared the differences in home ranges and spatial abilities. In the latest issue of Quarterly Review of Biology, they published their results, which can be summed up with this graph.
If the home range explanation was correct, you’d expect the points to fall along a line going from the lower left corner to the upper right. If you only look at the pine vole and the meadow vole, that indeed is what you find. But if you look at all 11 species, they’re all over the map. There is no correlation whatsoever.
Clint and his colleagues propose a different explanation: male spatial ability is not an adaptation so much as a side effect. Males produce testosterone as they develop, and the hormone has a clear benefit in terms of reproduction, increasing male fertility. But testosterone also happens to produce a lot of side effects, including male pattern baldness and an increased chance of developing acne. It would be absurd to say acne was an adaptation favored by natural selection. The same goes for the male edge in spatial ability, Clint and his colleagues argue. They note that when male rats are castrated, they do worse at navigating a maze; when they are given shots of testosterone, they regain their skill.
There are many sorts of behavior that have been shaped by natural selection. But it’s always important to bear in mind that what looks like an adaptation may be nothing of the sort.
[Image: Reinhard Dietrich, Wikipedia]
Originally published December 28, 2012. Copyright 2012 Carl Zimmer.