The U.S. middle class is shrinking, but not because more Americans are poorer. Instead, more households are climbing into the echelons of the upper middle class due to income gains in recent decades, according to research from the nonpartisan American Enterprise Institute. About 31% of U.S. households earn enough to be considered upper middle class, a roughly threefold increase since 1979, making it the nation's largest economic group, the research found. Meanwhile, the share of Americans in the "core" and "low" middle class segments has declined over that time, primarily because more households in those income groups have jumped ahead...