In his 1963 paper on the aims and methods of ethology, Tinbergen credits Julian Huxley for identifying causation, survival value, and evolution as the three major problems in biology. To this list of three, Tinbergen added the problem of ontogeny. The distinction that Tinbergen described among causal, functional, developmental, and evolutionary questions is one of the most lasting legacies of ethology. The idea that each of these questions can be asked of any behavior, and that each requires its own distinctive answer, continues to shape the way we think about behavior. Tinbergen did not believe these questions stood in isolation and never interacted. He pointed out, for example, that we can ask about the cause, function and evolution of development (Tinbergen, 1963). Contemporary behavioral ecologists often make the point that the cause of behavior is easier to analyze and understand and if its function is known (Krebs and Davies, 1997; Stephens and Krebs, 1986). This latter idea has been criticized, however, for confusing the essential difference between causal and functional explanations of behavior (Bolhuis, 2005, this volume; Bolhuis and Macphail, 2001; Macphail and Bolhuis, 2001).

The reasons for drawing a distinction between causal and functional explanations of behavior are so clear and so familiar that it is surprising the two are sometimes confused by students and, occasionally, by professional researchers. The difference between causal and functional explanations can be illustrated with a non-behavioral example. The function of a hammer is to drive nails.