According to Mancur Olson, organization of large groups providing collective goods would not be hampered in spite of free-rider problem if membership is compulsory and/or selective incentives are offered. As usually closed shop is illegal, it is a key question for economists why employees should join a trade union despite the costly membership fees, if they can benefit from public goods (such as higher wages, better working conditions, etc.) regardless of his or her
membership. What factors can justify trade unions’ attractiveness today?