Is anxiety all in the genes?
Alice, a young scientist working in a laboratory, likes her work, and her supervisor values her conscientiousness and perfectionism. But when her colleagues take a break she usually sits on the periphery, fingering her hair and blinking rapidly if someone asks her a question.
She dreads when it is her turn to tell the staff about experiments she is working on. She usually doesn't sleep well beforehand. Her mouth becomes dry and her palms sweaty when she stands up to give her report.
Between the two World Wars, when Sigmund Freud's ideas were dominant, most psychiatrists and psychologists would have blamed childhood experiences. Perhaps Alice's mother was hypercritical and she felt guilty at anger toward a mother who had punished her occasional disobedience severely.
I had accepted such accounts as a truth while a graduate student at Yale University from 1950 to 1954. But I began to question the influence of early experience in 1962, when Howard Moss and I reflected on evidence from a group of normal adults, born in the 1930s, members of a study at the Fels Research Institute in Ohio. About 15pc of these adults, who had been timid, fearful and shy during their first three years, resembled Alice: Shy, unsure and reluctant to take on difficult challenges.
When Howard and I wrote the book Birth to Maturity summarising the project, we suggested the children inherited a constitutional disposition. Almost 20 years later, Richard Kearsley, Philip Zelazo and I examined evidence we had gathered on the effects of day care on Chinese-American and Caucasian infants who attended a daycare centre or were raised only at home from age three to 29 months.
We were surprised that the Chinese-American infants, whether at the centre or raised only at home, were more timid, suggesting the groups began life with distinctive temperaments.
We have been following a large group of Caucasian children born healthy to intact, middle-class families in the Boston metropolitan area in 1989 or 1990, from four months of age until 2007-08, and have arrived at several conclusions.
First, about 20pc, as infants, were unusually responsive to unexpected events. When colourful toys they had never seen before were moved slowly in front of their faces, the four-month-olds thrashed their limbs, arched their backs, and cried, as if this was too arousing. The infants, whom we call high-reactive, tended to be shy when they met unfamiliar people and afraid on entering unfamiliar rooms or encountering unfamiliar objects in their second year.
Most were shy when interacting with unfamiliar adults or children at four, and, at seven, many required a light while sleeping, were afraid to spend a night at a friend's home, and quiet in school. As adolescents, many worried excessively about the future and were anxious at events most adolescents regarded as unrealistic.
The majority have not seen a psychiatrist and probably will not develop any psychiatric disorder. However, about 20pc have been unable to cope with their temperament and were diagnosed with social anxiety disorder, depression, or both. Only one of our original group of 90 committed suicide. Their parents may wonder if this temperamental bias is a serious disadvantage in contemporary society, but fortunately many occupations require individuals with this temperament.
These adults like to work alone in settings in which they have control, jobs like computer programmer, writer, scientist or historian, and make important contributions to society. The biography of poet TS Eliot and the memoir of biologist Rita Levi-Montalcini, who both won Nobel prizes, suggest they had Alice's temperament.
The power of an infant's temperament rests with its ability to limit what a child might become, rather than determining a personality.
Only 20pc of our high-reactives were unusually shy, introverted 18-year-olds, but over 90pc were not consistently worry-free youths. Knowledge of a person's temperament lets one predict confidently what they will not become, but is less able to predict the persona they will develop.
Dr Jerome Kagan is a professor of psychology at Harvard University