Book excerpt: Drawing on the latest findings from neuroscience, as well as cultural observation and her own unique laboratory research, psychologist Kelly Lambert puts forth a provocative new theory about the cause and treatment of depression. Today's young adults are up to ten times more likely to experience depression than their grandparents were. Could it be that in our increasingly automated world, the reduced physical effort needed to accomplish anything may somehow interfere with our level of happiness and subsequent responses to stress? Neuroscientist Kelly Lambert finds compelling evidence that having to work hard for rewards significantly improves mood and prevents depression. Beginning with her innovative research on rats - she compared ''trust-fund rats'' (whose rewards came with no effort on their part) to hard-working ''trained-to-succeed'' rodents - Lambert offers hope of treatment for people without debilitating (and often ineffective) drugs. Drawing on a wealth of information from the fields of anthropology, neuroscience, and evolutionary psychology, Lambert develops a unique theory suggesting that physical effort directed toward tangible outcomes activates particular regions of the brain and builds resilience against the emotional emptiness and negative thinking associated with depression. Whereas most therapies emphasize the importance of mental activity, Lambert reminds us of the importance of physical activity in establishing control in a fast-paced culture that is focused more on the prospect of immediate gratification than savoring the fruits of our labor.