Liberal professions and membership in professional associations - Income and membership effects

Among the reasons for voluntarily belonging to a professional association is the expectation of a higher income for freelancers. Can this expectation be confirmed? Is it financially worthwhile for freelancers to belong to a professional association? In addition to individual relevance, membership in professional associations is also of importance to society as a whole in the context of deregulation. The above questions are answered using newer microeconometric evaluation methods that determine causal effects using non-experimental observations - in this case of the Socio-Economic Panel.