There's no single, universal definition of what counts as "middle class." A new report from the Pew Research Center defines the middle class in the United States as any adult in a household whose income sits between 66 percent and 200 percent of the national median (adjusted for the size of the household). That encompasses about half of all American adults.

Middle class or not middle class? That's the question! (Damian Dovarganes / AP)

And yet, as Catherine Rampell perceptively notes over at Economix, Americans in different income groups can have wildly varying perceptions of whether they're in the middle class. Overall, Pew found that 49 percent of Americans considered themselves "middle class." That's about what you would expect. But there were significant variations within income tiers.

For instance, about 46 percent of adults making more than $100,000 a year considered themselves middle class, even though that income level would technically place them in the top 20 percent of earners. Likewise, nearly half of all adults who were actually in the broad middle (with incomes ranging from $30,000 to $99,000) categorized themselves as either upper or lower class.

One reason why the answers may vary so much is that there are very different ideas about what income is actually necessary to achieve middle-class status. Here's another interesting Pew chart:

Americans with college educations tend to have higher standards for middle-class life than those who never finished high school. What's more, the higher up the income ladder you go, the higher your standards rise for what counts as middle class. Americans making more than $100,000 a year tend to think that $100,000 is what it takes to keep a middle-class existence. There are regional differences, too: Midwesterners have far lower expectations for what counts as middle class (possibly because housing costs are frequently lower in the region).

In any case, this is just one facet of the Pew report, titled "The Lost Decade of the Middle Class." The report focuses a lot on the fact that median U.S. household incomes for the middle tier have stagnated over the past decade. Median net worth has also plummeted, and 85 percent of those who are in the middle class say it's harder to maintain standards of living than it was 10 years ago. The full report is here.