A few years before, Thomson says, he would have interpreted her depression as anger turned inward. But instead he decided that her symptoms might be a way of signaling her unhappiness to people close to her.
He discovered that his client's parents had pressured her to attend the university and major in science, even though her real interest lay in the arts. In the course of therapy, he helped her become more assertive about her goals. When she transferred to another school and changed majors, he says, her depression lifted.
Thomson based his approach on the idea that depression is not simply a disease to be eliminated, but a way of eliciting support from family and friends. It's a concept derived from evolutionary psychology, a burgeoning field that is starting to influence psychotherapy.
Evolutionary psychology sees the mind as a set of evolved mechanisms, or adaptations, that have promoted survival and reproduction. Evolutionary psychopathology -- abnormal psychology through an evolutionary lens -- looks at what has gone wrong.
The discipline is so new that "some people would say it hasn't started yet," jokes Randolph M. Nesse, a professor of psychiatry at the University of Michigan, and one of its pioneers. No one paradigm has won universal acceptance. Evolution-based therapies rely on an eclectic mix of techniques, and their effectiveness is still being tested.
Some evolutionary psychologists emphasize the benefits of what we label as disorders. For example, Edward H. Hagen, a research scientist at Humboldt University in Berlin, with whom Thomson has collaborated, has argued that depression, suicide attempts and deliberate self-harm are rational bargaining tactics to manipulate others into providing support they might otherwise withhold.
Stephen S. Ilardi, an associate professor of psychology at the University of Kansas, suggests that depression results from a "mismatch" between human beings adapted for hunter-gatherer societies and the contemporary world. His therapy -- which he calls "therapeutic lifestyle change" -- emphasizes behavioral remedies, including getting more sleep, consuming more omega-3 fatty acids and increasing social interaction.
A third school of evolutionary thought sees mental disorders as the result of an accumulation of harmful genetic mutations -- flaws in the system.
Many clinical psychologists remain skeptical of all these divergent evolutionary approaches, as well as efforts to devise treatments based on them.
"The idea that evolution is an important determinant of who we are as human beings is unquestionable," says Laurence J. Kirmayer, director of the division of social and transcultural psychiatry at McGill University in Montreal. "The question is, what does our evolutionary history or our theories of evolution tell us specifically about the nature of human problems or about their potential solutions?"
Robert A. Neimeyer, a professor of psychology at the University of Memphis, suggests that evolutionary psychology is better at dealing with typical human behavior than with individual variations. He points out, for example, that while we are "evolutionarily wired for attachment," people grieve losses in ways that vary across cultures and individuals. And treatments must take account of those differences, he says.
The recurrence of mental disorders despite the pressures of natural selection is "really a technical question that none of us have a good answer to," says psychiatry professor Nesse, who has written widely on mood disorders. "We're not at a point where every discovery leads to another discovery. We're at a point where a bunch of people are trying to think hard about it."
In an article in the November issue of the journal Behavioral and Brain Sciences, Matthew C. Keller, a postdoctoral fellow at the Virginia Institute for Psychiatric and Behavioral Genetics, and Geoffrey Miller, assistant professor of psychology at the University of New Mexico, address why diseases such as depression and schizophrenia persist. The answer, they say, is that they reflect the accumulation of harmful mutations.
"There are so many genes that are involved in growing a brain, and each of the genes is vulnerable to mutation in every generation," says Miller, author of "The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature." When too many coincide, illnesses result.
One critic, Joseph Polimeni of the University of Manitoba, in Canada, points out that because so many psychiatric disorders have strong environmental triggers, no single explanation can account for all of them.
Daniel Nettle, a psychology professor at the University of Newcastle, in England, says he finds the mutation theory persuasive for major disorders such as schizophrenia. But he suggests that other problems, such as addictions, may be outgrowths of the changing social environment -- including modern distractions such as bars and casinos. "For our ancestors, it was quite useful to follow impulses strongly and spontaneously," he says, while today, with temptations to indulge at every turn, "suddenly, [these people] have a disorder."
Depression, the most common mental illness, has inspired several theories on its own.