People making travel plans may unwittingly heed a strange rule of thumb — southern routes rule. In a new experiment, volunteers chose paths that dipped south over routes of the same distance that arched northward, perhaps because northern routes intuitively seem uphill and thus more difficult, researchers suggest.

Volunteers also estimated that it would take considerably longer to drive between the same pairs of U.S. cities if traveling from south to north, as opposed to north to south, says psychologist and study director Tad Brunyé of the U.S. Army Research, Development, and Engineering Command in Natick, Mass., and Tufts University in Medford, Mass. For journeys that averaged 798 miles, time estimates for north-going jaunts averaged one hour and 39 minutes more than south-going trips, he and his colleagues report in an upcoming Memory & Cognition.

“This finding suggests that when people plan to travel across long distances, a ‘north is up’ heuristic might compromise their accuracy in estimating trip durations,” Brunyé says.

Only individuals who adopted a first-person, ground-level perspective treated southern routes as the paths of least resistance, he notes. From this vantage, one moves forward and back, right and left.

No southern leaning characterized those who assessed routes from a bird’s-eye view. This type of navigation uses the directional terms north, south, east and west.

Real-world experiences underlie avoidance of northern routes, Brunyé proposes. Young children learn that as objects and locations get higher, they become harder to attain. Examples include reaching for a toy on the counter, climbing the stairs and jumping.

An ingrained notion that “up is difficult” then gets applied to other situations. When someone imagines traversing a northern and a southern path, the northern way feels higher and more physically demanding, Brunyé suggests.

Another phenomenon might account for the new findings, remarks psychologist Stella Lourenco of Emory University in Atlanta, who was not involved in the study. From infancy on, people categorize different quantities — say, the numbers 2 and 4 or a big and a small object — as instances of “less than” and “more than.” Also, adults tend to associate larger numbers with “up” and smaller numbers with “down.”

If volunteers equated a northern route’s greater height on a computer screen with “more than” and a southern route’s lower position as “less than,” that could explain a southern bias, Lourenco says.

Brunyé’s group first presented 160 college students with a series of maps on a computer screen showing parts of Pittsburgh or Chicago. Each map contained icons for various fictional landmarks, including an information booth and subway stops. Different-colored lines portrayed routes from one landmark to another, going north to south, east to west, or at angles.

An experimenter asked participants to choose the shorter, faster route to a destination. Some participants took whatever perspective they wanted; others were instructed to take a first-person or a bird’s-eye outlook.

Participants who assumed a first-person stance chose southern routes two-thirds of the time. Most reported no awareness of having favored southern routes.

Students had no preference for eastern or western routes, or for routes that angled in any particular direction.

Further experiments ruled out the possibilities that participants favored left or right turns, perceived northern routes as longer than southern routes or chose southern routes because they liked information located toward the bottom of the computer screen.

Instead, participants rated northern routes as potentially more scenic and requiring more calories to walk or fuel to drive than southern routes — all signs of perceiving northern routes as elevated, Brunyé suggests.

His team is now examining whether volunteers wearing head-mounted devices that place them in virtual settings prefer southern over northern routes.

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