There are now more low-income and high-income Americans combined than there are people in the middle class, a study released Wednesday found. 

According to a Pew Research Center report, there were 120.8 million adults living in middle-income households and 121.3 million in lower- and upper-income households combined in early 2015, marking the first time in the center's four decades of tracking this data that the size of the latter groups has transcended that of the first. 

The study defines middle income as adults earning two-thirds to double the national median, which translates today to somewhere between $42,000 and $126,000 a year for a three-person household.

Since 1971, the percentage of adults living in the low income bracket has increased from 25 percent to 29 percent, and the percentage of adults living in the highest income bracket has shot up from 14 percent to 21 percent. The middle class, meanwhile, has shrunk from 61 percent to about 50. 

That's not entirely bad news. The shrinking of the middle class is due more to the rising number of high-income Americans, which increased 7 percentage points, than it is to the increasing number of low-income Americans, which increased by just 4 points.

"In at least one sense, the shift represents economic progress," the study notes. 

On the other hand, the rich are getting richer at a pace much faster than the middle and lower classes. According to the study, the median income of people in the high-income bracket shot up 47 percent between 1970 and 2014; meanwhile, the income of the middle class jumped 34 percent, and that of the low-income bracket increased by 28 percent.

The widening wealth gap remains a hot topic on the presidential campaign trail. Vermont Sen. Bernie Sanders has made reducing income inequality a pillar of his bid for the Democratic nomination, while candidates from both parties have acknowledged the problem of the middle class's decline.

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