You're driving down the turnpike. That painful pressure below your navel has grown too intense to ignore, but you've got another hour and a half left to go before you get home. A blue sign flashes through your headlights, a rest stop. You pull in, walk to the public bathroom, and wrinkle your nose. A row of stalls stretches from wall to wall. Which should you choose?
Most of us enter this situation armed with nothing but our eyes, noses, and intuitions. But science can help. Research suggests you should avoid the middle stalls at all costs.
A wealth of research shows that, given several equally good (or gross) options, people tend to choose the middle one. Psychologists call this "centrality preference." That means that most people who entered the bathroom before you probably went for the center stalls and avoided the sides. With any luck, that could leave the side stalls a bit less nasty.
RELATED: See images of bathrooms all around the world
Bathrooms around the world
Bathrooms around the world
CHENGDU, CHINA - NOVEMBER 07: (CHINA OUT) A man walks out of a five-star public toilet at Qingcheng mountain resort on November 7, 2015 in Chengdu, Sichuan Province of China. The World Toilet Day falls in November 19 each year and will kick off Thursday this year. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
TO GO WITH AFP STORY BY MAUDE BRULARD A view of the bathroom at the world's first real-life 'sand castle hotel', constructed from tonnes of sand and enforced with wood, in the small southern Dutch city of Oss, on October 2, 2015. Curious tourists and the young at heart this summer flocked to Oss to spend a night, for 150 euros ($167) per night, in a specially-built room deep inside the heart of the massive castle sculpture made entirely of sand. Two sand hotels were built in the Netherlands at venues where annual sand art sculpture festivals are being held: one in Oss and the other in the northern Frisian city of Sneek. AFP PHOTO / EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)
CHONGQING, CHINA - MAY 02: (CHINA OUT) Toilet troughs at a public toilet which is honoured as giant 'castle' toilet in Yangren Jie (also known as Foreigner Street) in Nan'an District on May 2, 2015 in Chongqing, China. The World Toilet Day falls in November 19 each year and will kick off Thursday this year. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
TAIYUAN, CHINA - MARCH 25: (CHINA OUT) A man pees at a toilet where three female models in the provocative attire in a restaurant on March 25, 2015 in Taiyuan, Shanxi Province of China. The World Toilet Day falls in November 19 each year and will kick off Thursday this year. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
CHONGQING, CHINA - FEBRUARY 07: (CHINA OUT) Two citizens watch a camera-shaped public toilet in Shiqiaopu Street on February 7, 2014 in Chonqging, China. The World Toilet Day falls in November 19 each year and will kick off Thursday this year. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
NINGBO, CHINA - MARCH 13: (CHINA OUT) A public toilet is seen beside Beidou River on March 13, 2012 in Ningbo, Zhejiang Province of China. The World Toilet Day falls in November 19 each year and will kick off Thursday this year. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
TOKYO, JAPAN - JANUARY 6: EXCLUSIVE A urinal that has a video game console above it in the SEGA World complex in Akihabara Electric Town, Tokyo, Japan. For men a stroll to the gents has become a leap into the twenty first century, thanks to the SEGA video games corporation. The company has developed a new entertainment system which is incorporated into a public lavatory. Now rows of peeing men can spend a penny and get a great video game experience while they are at it. The 'Toylet' male urinal video game provides a choice of sumo wrestling, erasing graffiti and dousing an exploding volcano. The 'Toylet' works by a pressure sensor in the base of the urinal measuring the strength and location of the urine stream as it hits the basin. An LCD screen displays the game graphics and rewards the strength, length and accuracy of the pee through a typical video game points system. There are currently no plans for a multiplayer version of the 'Toylet'. (Photo by Matthew Tabaccos / Barcroft Medi / Getty Images)
Toilets In The Middle Of Nowhere At Star Wars Movie Set Near Tozeur, Tunisia. (Photo by Education Images/UIG via Getty Images)
ZHENGZHOU, CHINA - JUNE 12: (CHINA OUT) A elephant-shaped public toilet is seen on June 12, 2015 in Zhengzhou, Henan Province of China. The World Toilet Day falls in November 19 each year and will kick off Thursday this year. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
Heerlen, NETHERLANDS: This picture taken 12 July 2006 shows urinals shaped like a woman's mouth at The McDonalds in Heerlen. McDonalds is going to remove its urinals after complaints by a tourist, declared the owner, Giel Pijpers, 12 July 2006. In his opinion it is art, but in America there are different opinions. The urinals will be sold. 'I'm not going to be pissed off about a couple of urinals' , he declared on Wednesday. AFP PHOTO/ANP PHOTO MARCEL VAN HOORN (Photo credit should read MARCEL VAN HOORN/AFP/Getty Images)
Sketch, London, United Kingdom, Architect Architect Unknown, Sketch Egg Pod Toilets Wider Angle. (Photo by View Pictures/UIG via Getty Images)
Skeptical? Psychologists have shown that centrality preference applies to public bathrooms.
In a paper published in 1995 in the journal Psychological Science, the psychologist Nicholas Christenfeld presented a series of short experiments in centrality preference. For one of them, he examined the habits of beachgoers in a coastal Californian public washroom.
It would have been at the very least impolite to stand around in there all day with a notebook recording which stalls people used. So Christenfeld came up with a useful proxy: toilet paper use. He enlisted the help of the bathroom's custodian, who kept track of how often toilet paper needed changing in each of the four stalls for 10 weeks.
The results: Far more people used the middle stalls than random chance would predict — 60% of finished rolls came from the central stalls, with only 40% from the end stalls.
An important caveat here is that just because fewer people use an end stall, that doesn't necessarily mean it's cleaner. Maybe the sort of people who use end stalls also make more of a mess. Maybe custodians know that outer stalls see less traffic and clean them less often. In any given bathroom, an unseen cue or quirk of design may shift people toward outer stalls.
But still, at my next rest stop I'll be locking the door as close to the wall as possible.