A classic study of the parasite problem is presented in Mancur Olson`s 1965 work, The Logic of Collective Action.  In labour relations, the problem with parasites is that the costs of organizing a union and negotiating a contract with the employer can be very high and employers find it too expensive to introduce multiple salary and benefit scales. Some or all non-union members may find that the contract is also beneficial to them.  Trade union safety agreements are explicitly mentioned in the labour laws of many countries. They are heavily regulated in the United States and, to a lesser extent, in the United Kingdom, by laws and court orders. .