What two explanations are offered for why unions have declined?

What will be an ideal response?

There has been a sharp decline in union membership since the mid-1950s when about 25% of the work force was unionized. Two complementary hypotheses are given to explain the decline. First, the structural-change hypothesis is that changes in the structure of the economy and of the labor force have limited the expansion of union membership. Second, the managerial-opposition hypothesis is that the opposition of management to unions increased because union firms are thought to be less profitable than nonunion firms. The policies employed by management against unions decreased union membership.