Since Thomas and Chess popularized the concept of temperament in childhood (e.g., 1977), many studies devoted to temperamental differences among children have been published, and the nine-dimensional structure devised by Thomas, Chess, and their collaborators has attained textbook status. Applied initially to the study of infants, but later to older children as well, this approach categorizes temperamental differences based on clinical experience into nine more or less independent traits: Activity Level, Rhythmicity, Approach-Withdrawal, Adaptability, Threshold of Responsiveness, Intensity of Reaction, Quality of Mood, Distractibility, and Persistence. Beyond these first-order constructs, second-order constructs of three clusters-easy, difficult, and slow to warm up-have been proposed based on factor analyses of the nine scales.