Research released this month shows that the incomes of the well-off have largely climbed back from the toll of the most recent recession while those of the poor have yet to start recovering.

According to the latest version of “Striking it Richer: The Evolution of Top Incomes in the United States,” by Emmanuel Saez, of the University of California, Berkeley, the income inequality gap has been expanding, rather than narrowing, as the 2007-2009 recession recedes. That trend has been unfolding for more than 30 years, with the highest earners only temporarily set back by the most recent downturn.

The findings, based in part on U.S. income-tax data and census figures, update Mr. Saez’s earlier work with Thomas Picketty, with initial income estimates for 2012. The research will be updated again in January 2014.