A perceptive look at changing ideas about temperament, plus some strong opinions about the implications of current concepts. A research-oriented psychologist, Kagan (Harvard; Unstable Ideas, 1989) begins with the view of Galen, a second-century physician, that temperament is linked to an excess of one of the four bodily humors--black bile, blood, yellow bile, and phlegm. That temperament had a biological basis was a popular concept until the end of the 19th century, when it came into conflict with the egalitarian notion that all human beings are equipped with essentially similar psychological qualities. In the second half of the 20th century, research with young children led to new ideas about the influence of biology on temperament. Kagan looks at some of this research and questions its reliance on parents' responses to questions about their children's emotions and behaviors. He describes in considerable detail his own 15 years of research into the physiology of inhibited and uninhibited children: In studying hundreds of children, he found that two- thirds of infants who at four months were highly reactive to various auditory, visual, and olfactory stimuli developed into shy, timid toddlers, and two-thirds of minimally reactive infants became sociable and outgoing. In his final chapters, Kagan ponders the implications of his findings, arguing strongly that the existence of inborn temperamental biases does not excuse asocial behavior. Free will, he insists, is not undermined by temperament, and though we may not be able to control our emotions, we can control our actions. Written for the general, but not the casual, reader, this work's extensive chapter notes on Kagan's methodology make it especially valuable to psychologists and psychiatrists.