To which extent do freelancers experience a causal effect on their income due to a voluntary membership in a professional organization? This question can be answered by estimating fixed-effects-models for men and women in the framework of a microeconometric evaluation with data of the German Socio-Economic Panel (GSOEP) from 1985 until 2009. Estimations show positive causal effects of a voluntary membership: for men in the short term 1.84 %, in the medium term 2.72 % and in the long term 4.12 %; for women the effects attain in the middle term 5.18 %. Thus the social capital that is generated due to a voluntary membership in a professional association causes an increasing income.