The first purpose of this book was to provide a state-of-the-art review of the research on play in children and other animals. So what is the state of the art? Let us return to the unanswered questions raised in chapter 1 to see if we are any closer to some answers.
Throughout the book, numerous attempts were made to differentiate between play and other forms of behavior. In an attempt to differentiate play, exploration, and tool use in chapter 3, it was necessary to propose a category halfway between "pure" exploration and play that corresponded to mastery behavior: functional exploration/practice play. In chapters 4 and 5, considerable evidence was found for the distinction between play- and serious fighting in young animals and children (e.g., developmental course, signals, contexts, intensity, duration, tactics, targets, consequences), but research suggested that play-fighting often becomes rougher with age and presumably more difficult to distinguish from serious fighting. The distinction between locomotor play and serious flight seems less problematic, but differentiating between play and other affiliative interactions was difficult in social object play, especially for infants and toddlers (see chap. 8). The same difficulty arose in chapter 9, when considering play between parents and infants.
Another question is whether the diverse forms of play considered here (i.e., solitary object play, play-fighting, locomotor play, and the various forms of social object and social pretend play) are under the operation of the same or different motivational systems. That is, when play is activated, are the various play forms interchangeable, with the specific play behaviors a function of the materials or partners present? Or are each separate and