This book is the product of an intensive cooperation between psych- ogists and sociologists who study solidarity and prosocial behavior, and its fruits are briefly summarized in Chapter 1. The topics of so- darity and prosocial behavior are at the core of both disciplines and thus one might expect that an intensive cooperation like the one that produced this book is not uncommon. Surprisingly however, it is extremely rare that sociologists and psychologists get together to c- bine their knowledge in these fields. Instead, researchers from both disciplines tend to ignore each other’s work quite generally, and the work on prosocial and antisocial behavior is no exception. The conviction that sociology and psychology can benefit from each other’s work led us—a group of sociologists and psychologists at the University of Groningen (The Netherlands)—in 1999 to launch a joint research project on solidarity and prosociality. The aim was to find a common ground on which insights from each discipline could contribute to a broader understanding of solidarity and prosocial behavior. This interdisciplinary research project was called Prosocial Dispositions and Solidary Behavior and it was financed by the University of Groningen as a so-called breedtestrategie program (i.e., a program for broadening disciplinary approaches).