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See Inside September / October 2009

Don't Know Much Biology: Our Trouble Classifying the Living World

Learning to categorize the life on our planet is surprisingly difficult for the human mind

THINK about what it takes to learn biology. Not textbook biology, the kind you learn in high school with microscopes and dissecting kits. Rather the kind you learn on your own, as a young child encountering the vast and diverse world of living things. How does the human mind link together organisms as varied as hippos and lichen and mosquitoes and rhododendrons? And how do we assemble this diversity into meaningful categories? In short, how do we think about life?

Psychologists are very interested in how the mature mind sorts the living world and where we put ourselves in relation to other life-forms. That is the stuff of philosophy and religion and morality. But how we recognize life—and arrange the living world in our mind—is not as obvious as one would think.

Take something as simple as motion, for example. Many living things move, but so do rivers and clouds and rocket ships. And some living things, such as coral and trees, do not appear to move at all. So it is not just the fact of motion that defines life, but the why and how things move. How does the movement of a bicycle differ from that of a horse? That is a fairly nuanced analysis for an immature mind, and indeed young children find this idea confusing. Kids make a lot of mistakes about what is animated and what is not. Only over time do we outgrow our simple, childish ideas and replace them with a sophis­ticated view of the natural world.

Confused by Motion
Or do we? Do we really discard all our naive thinking as we experience the world and learn about its complexity? University of Pennsylvania psychologists Robert F. Goldberg and Sharon L. Thompson-Schill have been exploring these questions in the laboratory, with intriguing results.

In one recent experiment the researchers showed a group of college students a long list of words, one at a time and very rapidly. Some of the words were the names of plants, others, animals, and still others, nonliving things. The nonliving items were further divided into nonmoving man-made objects such as brooms, nonmoving natural features such as boulders, moving artifacts such as trucks and, finally, moving natural phenomena such as rivers. The idea was to see how quickly and accurately the volunteers used movement and “naturalness” to classify something as living or nonliving. Mistakes and hesitation would be taken as evidence that the primitive ideas of childhood still retain some power.

The scientists were particularly interested in how we think about plants—where our mind tends to put them in the grand scheme of things. Plants are an interesting anomaly because—at least to young children—they do not “do” anything; instead we do things to them, such as climb, water and prune them. If plants move at all, their movement is very subtle, hidden to the casual observer. Not surprisingly, kids often misclassify plants as nonliving.

But how do college students think about plants? Well, it appears that they, too, make mistakes, even with all that formal education. The volunteers in the study were much more hesitant in classifying plants, suggesting that they had to slow down to deliberately overrule their naive taxonomy. They also made more outright errors than they did when classifying animals. In addition, the students were slower to size up moving things in general as well as nonliving natural things—suggesting that movement and naturalness were the features that stymied them.

Stumbling over Plants
To be fair, these student volunteers were not biology majors. And we all know that kids can slip into college without much in the way of rigorous scientific training. But here is the really interesting part. The psychologists subsequently ran basically the same experiment but recruited biology professors—people who make their living teaching university students about the natural world. Indeed, the volunteers in this second study had been teaching college-level biology for a quarter of a century on average—and at two highly prestigious schools, Yale University and Johns Hopkins University.

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