While internationally renowned primatologist Frans B.M. de Waal, PhD, thinks it's inevitable that psychology embrace evolution as a unifying theory for understanding human behavior, he's concerned by the simplistic way many social scientists approach Darwin.

In particular, he said during the "Focus on Science" plenary address at APA's 2001 Annual Convention, they oversimplify the concept of adaptation--the idea that traits evolve to benefit a species.

Biologists are extremely careful about concluding that a trait is adaptive, said the C.H. Candler professor of primate behavior at Emory University's department of psychology and director of the Living Links Center at the Yerkes Regional Primate Research Center in Atlanta. In contrast, he said, many psychologists often assume that if a trait exists, it's adaptive.

He quoted a recently published study that, in its abstract, postulated that "most male facial hair and male pattern baldness are genetically based, suggesting it contributed to fitness." While it's probably true that men's hair patterns are genetically based, said de Waal, it by no means suggests such patterns contribute to fitness.

"Alzheimer's is genetically based," he said. "It doesn't contribute to fitness."

Such assumptions lead to "silly arguments," he says, that attempt to explain why something like baldness increased our ancestors' chances of reproducing.

"Traits come as a package," explained de Waal, "and only the whole leads to survival and reproduction, not only a single trait."

Indeed, some traits are simply maladaptive, leading to costs for individual members of the species.

"Adaptation is really an onerous concept that is hard to apply," said de Waal. "And it's often applied in a simplistic and inappropriate way."

Nevertheless, he added, psychology needs evolutionary theory to bring a unified framework to its myriad theories about human behavior. At the same time, he added, biology needs psychology to help it better understand animal behavior.

"Primates are not little gene machines," he said. "Many of the people in primatology are looking now more at the learning aspect of behavior and cultural traditions--more at the flexibility of behavior than a genetic program."

In the end, he predicted, biology and psychology will be branches of the same science, joined by the concept of evolution by natural selection. The trick for psychologists is to apply the concept to the whole organism rather than small chunks.

Beth Azar is a writer in Portland, Ore.