Beginning in 1956, the New York Longitudinal Study of Child Temperament collected enormous amounts of data on childrearing practices and behaviors among 138 middle-class white children and 95 lower-socioeconomic-class Puerto Rican children, from infancy to 7 or 8 years of age. Included were results of psychiatric interviews and special sensory, neurological, psychological, and IQ testing. Analysis yielded nine categories of "behavioral style": activity level, rhythmicity (regularity), approach versus withdrawal, adaptability, threshold of responsiveness, intensity of reaction, quality of mood, distractibility, and attention span and perspective. Cluster analysis indicated three clear temperament constellations, evident in about two-thirds of the sample. Easy children, 40% of the total, showed regularity, positive responses to new stimuli, and high adaptability to change. They related well to strangers and new situations and accepted frustration well. Difficult children, about 10%, showed irregularity in biological function, withdrawal from new stimuli, nonadaptability to change, irregular sleep and feeding schedules, and prolonged adjustment to new situations. Frustration produced tantrums. Slow-to-warm-up children, about 15%, combined mildly negative responses to new stimuli with slow adaptability after repeated contact. In contrast to difficult children, they showed mild intensity of reactions, whether positive or negative, and less irregularity of biological functions. If allowed to reexperience new situations over time and without pressure, the slow-to-warm-up child gradually came to show positive interest and involvement. The remaining 35% of the children showed varied clusters of behavioral styles, all within normal limits.