Human activities often correspond to collective actions in which the contributions of each member are combined into a single group product. Hence the amount of effort exerted by each participant is difficult to evaluate. This is typically the case in collective problem-solving tasks, as in a wealth of other tasks pertaining to various types of organizations (administration, education, industry, etc.). The complete list would be impossible to spell out here, but it certainly matches the widespread belief that working in groups is much better than working in isolation. To reason, to plan, to solve problems as a group: not only are these activities perceived as powerful means of socialization, and hence of social integration (in school, in the company, etc.), they are also thought to enhance each participant's effort and productivity to the benefit of all. Unfortunately, things don't always turn out as well as expected.