Animal personality is now taken seriously.

In recent years, evidence has begun to show that animals have personalities after all. Chimps, for example, can be conscientious: they think before they act, they plan and they control their impulses, says Samuel Gosling, a Texas-based psychologist. Research has identified similar personality traits in many other species.

The implications of these findings for research on human personality are powerful. Scientists can look to animal studies for insight into humans the same way they now look to animal testing for insight into drugs. Animal research has already begun to shed light on how different types of people respond to medications and treatments—aggressive and passive rats respond differently to antidepressants, for example. The hope is that animals can illuminate the murky interplay of genes and the environment on people's personalities.