INCOME inequality in the United States has been growing for decades, but the trend appears to have accelerated during the Obama administration. One measure of this is the relationship between median and average wages.

The median wage is straightforward: it’s the midpoint of everyone’s wages. Interpreting the average, though, can be tricky. If the income of a handful of people soars while everyone else’s remains the same, the entire group’s average may still rise substantially. So when average wages grow faster than the median, as happened from 2009 through 2011, it means that lower earners are falling further behind those at the top.

One way to see the acceleration in inequality is to look at the ratio of average to median annual wages. From 2001 through 2008, during the George W. Bush administration, that ratio grew at 0.28 percentage point per year. From 2009 through 2011, the latest year for which the data is available, the ratio increased 1.14 percentage points annually, or roughly four times faster.

The reasons for the widening income gap aren’t entirely clear. Yes, the nation has had a big recession, but recessions typically tend to lessen inequality rather than increase it.