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The long battle between humans and infectious microbes has left its marks all over us.
It shows up most obviously in the way our bodies are constructed. The thousands of species of bacteria that swarm over us cannot penetrate our multilayered skin. Entry points, such as the eyes and nose, are bathed in moisture to help flush out pathogens. The lining of our lungs releases bacteria-killing compounds. Viruses that manage to infect cells are greeted by proteins that attempt to shred them into genetic confetti. Any pathogen that sneaks past all these defenses then faces an army of immune cells, which can devour and destroy the invaders. Immune cells can also manufacture antibodies, which allow them to launch swift attacks if they encounter the same infection elsewhere in the body.
This elaborate defense system dates back billions of years. Our single-celled ancestors were infected with viruses; when they got bigger they were infected with bacteria; and after they evolved guts, those guts were infected with worms. Any mutation that offered even a little protection against those pathogens had a chance to be favored by natural selection. Over thousands of generations, mutation upon mutation built up our diversity of immune cells, signals, and weapons. There was never a point at which our defenses stopped evolving, because the pathogens were evolving as well. New generations of invaders slipped past our lines of defense, spurring the evolution of immune upgrades.
But it is not just our bodies that have been shaped by this tug-of-war. A number of scientists now argue that the battle against disease has left an indelible imprint on our minds as well.
Over the past few years, Mark Schaller, a psychologist at the University of British Columbia, has been developing an intriguing theory that behavior can be just as effective as microbiology at warding off disease. According to this theory, we have what Schaller calls a “behavioral immune system.” It’s a way of responding to the outside world, and to the people around us, that is so deeply embedded in our minds that we are hardly aware of it.
Schaller and his colleagues have been busily running psychological experiments to test his hypothesis. The results so far are preliminary but provocative. If Schaller is right, this behavioral immune system may prove to have a big influence on our day-to-day lives. It might even influence human nature on a global scale, shaping cultures around the world.
If the familiar, biological immune system were foolproof, it would be pointless to evolve a behavioral immune system too. In reality, however, our defenses are far from perfect. Some pathogens can disguise themselves well enough to go unnoticed, and others breed so fast that our immune systems cannot keep up. Then again, sometimes our immune system succeeds too well, using such overwhelming force against pathogens that it damages our own tissues in the process.
Not getting infected in the first place is a far safer alternative. Scientists have discovered a wide variety of animal species that use behavioral strategies to avoid becoming sick. Some caterpillars blast their droppings like cannons so that parasitic wasps that lay eggs in the droppings won’t be able to follow their scent. Sheep instinctively avoid grazing on grass near their own manure, advantageous because many sheep parasites release their eggs in the animals’ droppings. A female mouse can smell the difference between a healthy male and one infected with intestinal worms. She will avoid the latter and mate with the former.
Our closest living cousins, the chimpanzees, also display behavioral responses to signs of disease. When the primatologist Jane Goodall observed chimps in the 1960s, one of her subjects was a male she called McGregor, who suffered from polio. He dragged himself around by his arms after his legs became paralyzed, and his loose bladder attracted clouds of flies. Before McGregor got sick, he enjoyed hours of grooming from other chimpanzees, who picked out fleas, mites, and other parasites from his fur. But Goodall watched in amazement as the other chimpanzees stayed away from him once he became ill.
