UntitledPew Research Center% of adults in each income tier. (Note: the share of adults living middle income is dropping.)
The middle class has been shrinking.
The share of US adults in the upper-income tier has increased to 21% today, up from 14% in 1971, and the share of adults in the lower-income tiers also went up, from 25% to 29% today. 
Technically, the difference between those two groups (3 percentage points) is a net gain. However, there was considerable variation for this measure among different demographic groups.
In a recent report, the Pew Research Center determined the "demographic winners and losers" over the last 44 years in terms of income status by calculating the change in a group's share that is upper income minus the change in the group's share that is lower income in percentage point change.
People aged 65 and up were the biggest winners as the poverty rate among them fell to 10% in 2014, down from 24.6% in 1970. Notably, they are the only age group with a smaller share in of folks in the lower-income tier today than forty-four years ago.
"Evidence shows that rising Social Security benefits have played a key role in improving the economic status of older adults," the report added.
Other "winners" included married people with and without children (especially if both people worked), blacks, whites, Asians, and women.
On the flip side, the biggest "losers" were high school graduates. Other groups who were worse off included Hispanics, people aged 18-29, those without high school diplomas, and unmarried people.
However, it's important to note that some of the changes among racial and ethnic groups don't tell the entire story.
"Although blacks advanced in income status, they are still more likely to be lower income and less likely to be upper income than whites or adults overall," the report notes. "For Hispanics, the overall loss in income status reflects the rising share of lower-earning immigrants in the adult population, from 29% in 1970 to 49% in 2015. Considered separately, both US-born and foreign-born Hispanics edged up in the economic tiers."