Throughout his 44 years of research on child development, Harvard University psychologist Jerome Kagan, PhD, has found that temperamental differences in adults and children are due to both environmental and genetic influences.

His groundbreaking work with Nancy C. Snidman, PhD, documented in "The Long Shadow of Temperament" (Harvard University Press, 2004), for example, shows that very shy adults often begin life as infants who are very reactive to unfamiliar stimuli. They thrash their limbs and cry. Their subsequent tendency to be shy and timid is strengthened if their parents are protective. Yet when parents of shy infants encourage boldness and sociability, the children become adolescents who exhibit less inhibition than their more fearful counterparts.

Work like this has led Kagan to believe that neuroscientists who only study the brain cannot discover all of the keys to the mind's functions. In his forthcoming book "The Three Cultures" (Cambridge University Press, 2009), Kagan examines the vocabulary and contributions of biology, psychology and humanities to our understanding of human nature—and asserts that considerations of each person's past are needed if scientists want to understand human personality and talent.

What can psychological science tell us about human behavior that the other sciences can't?

The physical sciences are making dramatic discoveries about the universe, the cell, the gene, and as a result, biologists, chemists and physicists have become overly confident. There's even a small group of neuroscientists who believe that once we know what's happening in the brain, we will be able to explain all psychological phenomena in the language of biology.

I disagree with that assumption. It is not possible to translate my emotional state at this moment into sentences that only have biological words, like circuits, neurons and neurotransmitters. Psychological phenomena occur at the end of a long cascade of events that began with a brain event, but the end of the cascade—a feeling of guilt, anger or pleasure—has to have its own special vocabulary.

Neuroscientists are confident that one day they will be able to predict what a person is thinking, feeling or planning to do by measuring their brain state. But each brain state permits an envelope of psychological possibilities, not only one outcome, and one has to know more about the history of the person and the situation they're in to predict which member of the envelope is being experienced or will occur. Scientists can't tell which one will be selected by looking only at the brain. A particular brain state can lead to different psychological states in different people acting in distinct situations.

How do history and culture play into the equation?

An important difference between the physical and the social sciences is that many of the questions social scientists ask in the research they pursue are influenced by the historical moment in which they live. For example, if there had not been a Cold War in the '60s, the Pentagon would not have poured millions of dollars into psychological research on game theory. If, after World War II, America had not had a high divorce rate and 50 percent of young mothers working, we would not have become so concerned with the infant's attachment to the mother, and [John] Bowlby's theory of attachment probably wouldn't have attracted the attention and enthusiasm it provoked.

What's a major question that psychologists are uniquely positioned to answer?

We need more research on why social class is so influential on psychological development. Why are crime, delinquency, school failure and drug addiction more frequent among the children of the poor than the children of the more affluent? That robust fact is not a function of genes, but of experiences and identifications with one's class category.

Where else are psychologists needed these days?

When I was a graduate student, research on more accurate diagnoses was an important part of psychology. But when drugs became popular, psychiatrists started to give the same drug to patients with the same symptoms, regardless of the etiology. As a result, psychologists stopped working on diagnosis, and that was an error. We need well-trained psychologists to return to constructing better diagnostic techniques.

For example, psychologist Gerard Bruder, of Columbia University, an EEG expert, discovered that among a group of patients with the same diagnosis of depression, some improve on the major antidepressive drugs and some don't. Bruder measured the alpha power in the left and right frontal lobe and found that those who improved were more active in the left frontal lobe than the right. Those who didn't improve on the drug were more active in the right frontal lobe. You couldn't ask for a better example of a diagnostic marker that tells clinicians whether or not to give a particular drug. We need more psychological and biological evidence for all of the categories in the DSM, and this research is exactly what psychologists are trained to do.