Piaget's work on infancy is based on the diligent observation of and experimentation with his own three children, Lucienne (born 1925), Jacqueline (born 1927), and Laurent (born 1931) and comprises three volumes, The Origins of Intelligence in Children (OI; 1936/1952), The Construction of Reality in the Child (CR; 1937/1954), and Play, Dreams and Imitation in Children (PDI; 1945/1962). These volumes, which have been characterized as “three of the most remarkable and original documents in psychology” (Russell, 1978, p. 92), can justly be said to have revolutionized the way in which developmental psychologists think about and study infants (Vonèche & Vidal, 1985). Even though there was certainly research on and theorizing about infant development before these volumes were published (e.g., Baldwin, 1894/1906; Bühler, 1918; Koffka, 1924; Stern, 1914/1930), Piaget's work on infant development was unparalleled in terms of its originality, scope, and systematicity, and, as I will argue, it still is today.

The work on infancy followed an already impressive line of research that Piaget had conducted on cognitive and moral development in preschool and school-age children (see Chapter 16, this volume). According to Piaget (1952, 1954/1973), his infancy research changed the way he approached development. In his early work, he focused on verbal exchanges in order to understand the logical, rational thought of the child.