One of the most popular books of the year was Sheryl Sandberg’s career guide for women, Lean In (I reviewed it here). Sandberg’s book consists of highly individualistic advice, and there’s no empirical evidence that it works. It’s based on Sandberg’s own experience, but Sandberg had many unique advantages — such as a Harvard education and being personally mentored by one of the most powerful men in the world (Larry Summers) — that vanishingly few other women enjoy.

Moreover, Sandberg neglected to mention one very important thing that every woman seeking to improve her pay, benefits, and workplace status should strongly consider doing: joining a union. As a new study by the Center for Economic and Policy Research (H/T Michelle Chen) shows, for women workers, belonging to a union is associated with significantly higher pay and better benefits, relative to their non-union counterparts. This occurs after controlling for all other relevant factors and it happens for women at every education level — all the way to women with college degrees and beyond.

Consider some of the findings of the study:

— Wages for women in unions are, on average, 12.9 percent higher than it is for women who are not unionized. The union wage premium occurs for women at every education level, even highly educated women. For example, among women whose highest educational attainment is a 4-year college degree, women in unions make 13.0 percent more than their nonunion counterparts. Among women with advanced degrees, unionized women make 9.6 percent more. To look at it another way, being in a union raises a woman’s salary by about as much as a year of college.

— When it comes to benefits, the effect of union membership is even stronger. Unionized women are 36.8 percent more likely to have health insurance than women who lack union representation. The impact union status on health benefits is even more important than the impact of college education. The college premium for health benefits is only 29.3 percent — 7.5 percentage points less than the union premium.

— For women with the least formal education, the union effect on health benefits is even more powerful. Women without high school diplomas are fully 104 percent more likely to have health care if they’re in a union — in other words, unionization doubles their rate of health care coverage.

— The most stunning union effect of all, however, is on retirement benefits. Unionized women are a whopping 53 percent more likely to have workplace retirement benefits. The union advantage dwarfs even the college advantage here. Though a college-educated woman is more likely to have retirement benefits than her non-college educated counterpart, her relative advantage is only 43 percent — 10 percentage points less than the union premium.

Some other notes: presently, about 46 percent of union workers are women, and if current trends continue, women will become the majority of union workers by 2023. You may be surprised to learn that unions increase the earnings of even those women with advanced degrees, but you shouldn’t be. My professor for a grad school collective bargaining class used to say that the typical union member is a woman with a master’s degree. I’m not sure if that still holds true, but it makes sense, given how many teachers and other highly educated public sector workers are in unions these days.

Why do unions tend to boost women’s wages, you may well ask? Research shows that unionized workplaces tend to have two important characteristics. On the one hand, the wage scale is more compressed. There’s not the huge disparity in wages you tend to see in other workplaces. Those at the bottom make more, and those at the top make less. In practice, this helps women, who at all jobs at all professional levels are often at the low end of the wage scale, and rarely at the top.

The other reason why unionized workplaces help women is that raises and promotions tend to be transparent and systematized. Raises, for example, are likely to be equitably distributed among all. And if you want a promotion, there are usually very clear rules about how to go about getting one, specific criteria about how your performance will be judged, etc. When raises and promotions are enacted via unclear systems and criteria, this opens the door for arbitrariness, which in practice, tends to greatly advantage men. Your boss may not be consciously sexist — most bosses and workplaces aren’t. But chances are, if your workplace is not very precise and bureaucratic about these things, there’s room for a lot of bias to creep in. All the individual “leaning in” in the world is not likely to change that — collective action is what tends to change workplace rules and processes.

It’s likely that chirpy self-help books like Sandberg’s will always find a receptive audience in America. But history has shown that, for most groups of workers, individualism will get them only so far. Collective action has long been the most successful, tried-and-true way to significantly empower workers. Not only does it improve the economic status of workers generally, it tends to give even more of a boost to groups of workers which historically have suffered discrimination, such as women and people of color.

Moreover, as Sandberg herself admits, her book is addressed to college-educated, professional class women. But as the CEPR’s report shows, labor unions are an institution that can benefit women of all classes and at all professional levels. As public policy, an outer-directed strategy aimed at strengthening unions is likely to do far more to economically empower women than any number of inner-directed “Lean In”-style self-improvement projects.

Kathleen Geier

Kathleen Geier is a writer and public policy researcher who lives in Chicago. She blogs at Inequality Matters. Find her on Twitter: @Kathy_Gee