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Many people who self-identify as middle class think of themselves as lower or upper middle class, yet this would classify nearly all Americans as middle class. Researchers often use other concepts to define the term: the middle 60 percent of the income distribution and earning thresholds, such as 75 to 200 percent of median income. This Perspective describes how, under these definitions, the U.S. middle class has been either receiving less income (as a share of total) or shrinking in size (proportion of the population) since the 1970s and is now smaller than middle-class populations in comparable countries. Another point of concern is research showing an increasing likelihood that lower-income Americans and their children will remain stuck outside the middle class with limited opportunities for upward mobility.