"Informal sanctions are a major determinant of a society’s social capital because they are key to the enforcement of implicit agreements and social norms. Yet, little is known about the driving forces behind informal sanctions. We systematically examine the determinants of informal sanctions by a large number of experiments. Our experiments allow us to identify the relative importance of three major potential factors: (i) strategic sanctioning for selfish reasons, (ii) non-strategic sanctions driven by spitefulness, and (iii) non-strategic sanctions that are driven by the violation of fairness principles. In addition, the observed sanctioning patterns provide insights into the relevance of different fairness principles.nOur findings show that the violation of fairness principles is the most important driving force of sanctions but, in addition, a non-negligible part of the sanctions is driven by spitefulness. We find surprisingly little evidence for strategic sanctions that are imposed to create future material benefits. While non-strategic sanctions are of major importance in our experiments, strategic sanctions seem to play a negligible role. Within the class of fairnessdrivennsanctions the motive to harm those who committed unfair actions seems mostnimportant."