For better or worse, a truism of American politics is that voters vote their pocketbooks. Yet according to a new report on median household incomes by Sentier Research, in 2012 millions of American voters apparently cast ballots contrary to their economic self-interest.

Each month the consultants at Sentier analyze the numbers from the Census Bureau's Current Population Survey and estimate the trend in median annual household income adjusted for inflation. On Aug. 21, Sentier released "Household Income on the Fourth Anniversary of the Economic Recovery: June 2009 to June 2013." The finding that grabbed headlines was that real median household income "has fallen by 4.4 percent since the 'economic recovery' began in June 2009." In dollar terms, median household income fell to $52,098 from $54,478, a loss of $2,380.

What was largely overlooked, however, is that those who were most likely to vote for Barack Obama in 2012 were members of demographic groups most likely to have suffered the steepest income declines. Mr. Obama was re-elected with 51% of the vote. Five demographic groups were crucial to his victory: young voters, single women, those with only a high-school diploma or less, blacks and Hispanics. He cleaned up with 60% of the youth vote, 67% of single women, 93% of blacks, 71% of Hispanics, and 64% of those without a high-school diploma, according to exit polls.

According to the Sentier research, households headed by single women, with and without children present, saw their incomes fall by roughly 7%. Those under age 25 experienced an income decline of 9.6%. Black heads of households saw their income tumble by 10.9%, while Hispanic heads-of-households' income fell 4.5%, slightly more than the national average. The incomes of workers with a high-school diploma or less fell by about 8% (-6.9% for those with less than a high-school diploma and -9.3% for those with only a high-school diploma).

To put that into dollar terms, in the four years between the time the Obama recovery began in June 2009 and June of this year, median black household income fell by just over $4,000, Hispanic households lost $2,000 and female-headed households lost $2,300.