Labor unions are a device for limiting competition, in this instance competition among workers. The union bargains on behalf of all members, presumably resulting in a higher wage for workers and higher costs for employers. Use this fact, combined with the fact that international trade has played an increasing role in the American economy over the last fifty years, to explain the observation that union membership has declined as a share of private sector employment over this period in the United States. Your explanation should be consistent with the fact that union membership has not declined as a share of public sector employment over this period.