Forty years ago, about quarter of American workers belonged to unions, and those unions were a major economic and political force. Now union membership is down to 11.2% of the U.S. workforce, and it’s increasingly concentrated in the public sector — only 6.7% of private-sector workers were union members in 2013.

This isn’t exactly news, and professors and pundits have for years been dissecting the causes of labor’s decline. What doesn’t get talked about so much, though, are the consequences. Income inequality has, for example, become a hot topic. You might think that the dwindling away of an institution that devoted much of its energy to equalizing incomes would be a big part of that discussion. It hasn’t been.

Jake Rosenfeld, an associate professor of sociology at the University of Washington and a past and, one hopes, future contributor to HBR.org, is out to change that. His book What Unions No Longer Do, published earlier this year by Harvard University Press (only the most distant of relations to Harvard Business Review), is an account of Rosenfeld’s attempt to empirically establish (mainly through a lot of regressions of data from the Current Population Survey, the American National Election Studies, and the Federal Mediation and Conciliation Service) the consequences of Big Labor’s decline.

I had heard good things about the book, and for the last few months it’s been sitting near the top of my to-read pile, taunting me. With Labor Day coming up, I figured I should just go ahead and read it. Now I have. It is in fact a good book — careful, wonky, and for the most part not all that hard to read — and an important one for anyone trying to understand the current state of the U.S. economy and politics. You should buy it. But in case you don’t, here, for Labor Day, are the four big things that, according to Rosenfeld, unions in the U.S. no longer do:

Unions no longer equalize incomes. Income inequality (as measured by what the 90th percentile worker makes vs. the 10th percentile worker) remains much lower among unionized workers than nonunionized workers. But remember, only 11% of U.S. workers are now unionized, and Rosenfeld shows that unions’ ability to affect wages for nonunion workers in the same region or industry sector — which used to be significant — is now negligible. Rosenfeld estimates that about a third of the rise in income inequality since the 1970s is due to unions’ decline — the same share that he attributes to economists’ favorite explanation for rising inequality, rising rewards to skilled workers due to technological change.

Unions no longer counteract racial inequality. As Rosenfeld acknowledges, labor unions in the U.S. don’t have the greatest history on race. For a long time many unions wouldn’t let African-Americans join, and some fought hard to keep employers from hiring them. But during World War II this began to change, and by the 1970s black workers were more likely to be in unions than white workers were. Unions shepherded millions of their African-American members into the middle class, and helped bring black and white wages closer together. Since unions fell into sharp decline in the private sector in the 1970s, the private-sector wage gap between blacks and whites has grown. In the much more unionized public sector, the wage gap has narrowed for black men, although black women have lost some ground to white women.

Unions no longer play a big role in assimilating immigrants. Unions also don’t exactly have a stellar history of relations with recent immigrants to the U.S. But in the first half of the 20th century immigrants still found their way in great numbers into unions and even union leadership roles. For the recent great wave of Hispanic immigrants, that hasn’t been the case. Yes, there have been a few noteworthy unionization campaigns among immigrants, like the United Farm Workers in California’s fields and the Service Employees International Union’s efforts among office janitors and hotel workers. But on the whole, Hispanics are less likely to be union members than other workers are.

Unions no longer give lower-income Americans a political voice. The higher your socioeconomic status, the more likely you are to vote and to be listened to by politicians. As political scientists Martin Gilens and Benjamin Page documented in a much-discussed recent study, the policy preferences of organized interest groups and Americans in the 90th income percentile seem to carry a lot more weight in modern political decision-making than those of the 50th percentile. Unions used to be perhaps the most important organized interest group, and Rosenfeld shows that, even now, union members with low education levels are much more likely to vote than non-members with low education levels. But public-sector union members are more educated and more affluent than the population as whole, while private-sector union members are a dwindling and in many ways privileged breed. Not only are unions a much weaker political force than they used to be, they also no longer really represent those at the bottom of the economic ladder.

The decline of unions in the U.S. has often been painted as inevitable, or at least necessary for American businesses to remain internationally competitive. There are definitely industries where this account seems accurate. Globally, though, the link between unionization and competitiveness is actually pretty tenuous. The most heavily unionized countries in the developed world — Denmark, Finland, and Sweden, where more than 65% of the population belongs to unions — also perennially score high on global competiveness rankings. The U.S. does, too. But France, where only 7.9% of workers now belong to unions (yes, France is less unionized than the U.S.), is a perennial competitiveness laggard.

And even if the decline of unions was inevitable or desirable, that still leaves those tasks unions once accomplished — which on the whole seem like things that are good for society, and good for business — unattended to. Who’s going to do them now?

Update: Reihan Salam has a very interesting piece that attempts to explain those strange numbers in my second-to-last paragraph. (In short, France has few union members but lots of people covered by collective-bargaining agreements, while Scandinavian unions don’t act like American unions.)