Tyler Cowen found a paper that says unions have no effect on wages. Which, if true, sure wouldn’t explain why business is so scared of them. Ezra’s got a very good response:
There's a sad miscomprehension in the larger political discourse that the primary purpose of labor unions is to demand higher wages and more expansive benefits. This is not true. The enduringly important role of unions -- wage increases or none -- is to give workers a voice in their company, and to imbue that voice with the power to force change. So 90% of what a union does is not bargain for better health care, it's file grievance claims on behalf of its workers. They demand better treatment, safer machinery, family-friendly scheduling, and equitable hiring. And they fight in the other direction as well, giving workers who may otherwise be ignored a channel through which to advocate for process improvements that would otherwise go unheard. Their day-to-day role is to give workers a voice in the workplace, and that remains even if they could never secure another wage increase again.