Multigenerational living–defined as two or more adult generations living in one household–is on its way up in the Western world.

Before World War II, American seniors often moved back in with their adult children in their later years, or even with adult nieces or nephews. Think Aunt Bee on The Andy Griffith Show. Similarly, single adult children often remained at home until they were married and could get their financial feet on the ground.

In fact, prior to 1900, researchers say 50 percent of U.S. homes were multigenerational.

But around 1945, American ideals started shifting. The growing Social Security program gave senior citizens more freedom to choose privacy and independence. Not to mention, the suburban lifestyle was more suitable for two married adults and their minor children.

If you weren’t on your own by age 18, you were considered a “late bloomer.” And a “go it alone” mindset developed: You grow up, you move out, you get married and you buy a home of your own.

As a result, multigenerational living dropped from 21 percent in 1940 to a low of 12 percent by 1980.

But that was then.

In the late 1990s, economic and cultural factors began shifting the balance back again. Younger people moved back in with parents, driven by unemployment, foreclosures and student debt. With rising health costs and diminishing pensions, more seniors chose to shack up with their children, too.

Though strongest in Hispanic, Asian and African American households, the change is spreading across all racial groups. Many who came together out of necessity now stay together by choice. Sure, smoothing out the edges between lifestyle differences can disrupt a happy home, but many families find that sharing responsibilities, from babysitting to bills, outweighs the negatives.

More and more, new-home builders are starting to field requests from families with multigenerational living as part of their long-term plans, including two master bedrooms or multiple private entrances.

An August 2016 Pew Research report says 60.6 million Americans—almost one in five—lived in multigenerational households in 2014. And those numbers are increasing.

In Washington, D.C., a national exhibit called Making Room concentrates on design solutions for this growing share of nontraditional households—or should we say, more traditional? The evolving installation features movable walls and multifunctional furniture.

Safe to say, if these demographic trends continue, the shift toward multigenerational living could redefine the real estate industry for generations to come.