Final answer:
The decline in U.S. union membership is chiefly attributed to the shift to service industries, globalization, and a perceived redundancy with workplace protection laws. Additionally, lower unionization in the U.S. relative to other developed countries also boils down to cultural differences and differences in union coverage scopes.
Step-by-step explanation:
The main causes for the recent trends in union membership rates in the United States include a number of economic and social factors. One significant factor is the shift from manufacturing to service industries, where unionization rates are traditionally lower. Another contributing factor is globalization, which has increased competition from foreign producers and incentivized businesses to cut costs, often at the labor's expense. Moreover, there is a general reduced desire for unions due to a belief that workplace protection laws now in place redundantly cover the protections that unions would typically negotiate for workers.
Comparatively, union rates are lower in the United States than in many developed countries due to various reasons, including cultural differences towards unions, the degree of government social benefits and workplace regulations, and the fact that in many countries union negotiations cover even non-union members, which is not the case in the U.S.