A final group of assumptions deals with the role of institutional change. First, a large number of studies point to reforms of national legislation governing collective bargaining as important determinants of density decline. Some policy reforms have made it more difficult for unions to recruit members. In Sweden, for example, policy-imposed contribution increases have reduced membership (Kjellberg, 2011). In some countries, the shift to collective bargaining at the enterprise level has contributed to reducing the bargaining power of trade unions and thus their relevance and attractiveness to workers (Australian Bureau of Statistics, 2000; Legree, Schirle and Skuterud, 2014). Second, the degradation or erosion of institutions that have historically fostered unionization may explain some of the decline in density. .