The plunge in U.S. worker union members explains the increase in hourly wage inequality among women and men, a Harvard University researcher says. Study author Bruce Western, a professor of sociology at Harvard University in Boston says from 1973 to 2007, wage inequality in the private sector increased by more than 40 percent among men, and by about 50 percent among women. Western and co-author Jake Rosenfeld, a professor of sociology at the University of Washington, tracked the effects of union decline on between-group inequality -- people from different demographics and industries. They also tracked the effects of within-group inequality -- people from the same demographics and industries. Focusing on full-time, private sector workers, Western and Rosenfeld say deunionization -- the decline in the percentage of the labor force that is unionized -- and educational stratification each explain about 33 percent of the rise in within-group wage inequality among men. Among women, deunionization explains about 20 percent of the increase in wage inequality, whereas education explains more than 40 percent, the researchers say. Part of the reason for this gender discrepancy is that men have experienced a much larger decline in private sector union membership -- from 34 percent in 1973 to 8 percent in 2007 -- than women, who went from 16 percent to 6 percent during the same period. \"For generations, unions were the core institution advocating for more equitable wage distribution,\" Rosenfeld says in a statement. \"Today, when unions -- at least in the private sector -- have largely disappeared, that means that this voice for equity has faded dramatically.\"