But the U.S. wasn’t always so profusely bathroomed. In the past half century, the number of bathrooms per person in America has doubled. “We went from two people per bathroom to one person per bathroom in the last 50 years,” says Jeff Tucker, an economist at Zillow. “That’s amazing, because postwar America was already rich and booming, and we just, you know, kept building more bathrooms.” Across the country, bathrooms are multiplying—including in apartments and condos—even as American families and households are getting smaller.

Read: Why are American homes so big?

Where did this American obsession with bathrooms come from? The full answer takes us back centuries and involves some bad scientists, some good inventors, and a dash of extremely American notions about space and luxury. What used to be the smallest room in the house now holds the key to our anxieties about hygiene, cleanliness, consumerism, and the power of a room of one’s own.

Baths—large public spaces—are thousands of years old. Bathrooms—small private chambers—are far more recent inventions.

The ancient Romans filled their capital with more than 1,000 public baths. Those privies weren’t remotely private. They “generally had twenty seats or more in intimate proximity, and people used them as unselfconsciously as modern people ride a bus,” Bill Bryson writes in his history of the modern house, At Home.

This type of bath went out of style in Europe for almost a millennium after the fall of Rome, thanks in part to Dark Age scientists’ developing the very unscientific idea that bathing in water invites a host of awful diseases into the body’s pores. For most of the Middle Ages, “most [European] people didn't wash, or even get wet, if they could help it,” Bryson writes. When John Wesley, the founder of Methodism, declared that “Cleanliness is next to Godliness” in a 1778 sermon, he was talking about our garments, not our armpits.

It was another unscientific idea that led to the creation of the bathroom as we know it. In the mid-19th century, American sanitarians came to believe that disease stemmed from “sewer gas” emitted by toilets, which encouraged home builders to cram tub, sink, and toilet into one well-ventilated room with exposed pipes, in order to limit the spread of disease. While the sewer-gas theory would be overturned by the science of contagion, the three-fixture bathroom remained a staple of the modern American home. (Elsewhere around the world, the toilet is far more commonly found in its own chamber, separate from the bath.)

In 1940, only half of Americans had a three-fixture bathroom in their home. After World War II, several developments set the stage for the bath boom.

First, as The Atlantic’s Joe Pinsker has written, new highways, pro-sprawl laws, tax preferences, and zoning rules “steered Americans toward living in detached single-family homes” in the suburbs, which have space for more than one full bath.