You may have noticed it the last time you went on a long journey -- by foot, by car or by plane: the outbound portion of your trip seemed to take a lifetime, while the (more or less identical) leg that brought you home felt like it flew by.
Scientists have noticed this "return trip effect" too, and are beginning to hone their understanding of why we experience it.
In past years, researchers have suggested that it has to do with the way our bodies experience and measure time as it passes, or the way we remember the trips we take after the fact, or perhaps a bit of both. On Wednesday, a team in Japan released a new report in the journal PLOS ONE detailing the latest effort to solve the mystery. This group's take? That the return trip effect is created by travelers' memories of their journeys -- and those memories alone.
"The return trip effect is not a matter of measuring time itself. Rather, it depends on time judgment based on memory," said Ryosuke Ozawa of the Dynamic Brain Network Laboratory at the Graduate School of Frontier Biosciences at Osaka University.
To test out what is going on when the trip home seems shorter, Ozawa and colleagues, then at Kyoto University, created an experiment in which 20 healthy men, between 20 and 30 years of age, watched varying combinations of movies filmed by an experimenter who held a camera in front of the chest while walking two different routes. Half of the group viewed an outbound and return roundtrip on a single route; the other half, walking videos of two different routes in separate locations.
The videos were all approximately 26 minutes long, and the participants viewed them in individual sessions, seated in a chair. Researchers asked test subjects, who were not allowed to have access to clocks, to tell them each time they thought three minutes had passed, and monitored subjects' heart activity electrocardiograms to assess whether the autonomic nervous system plays a role in the effect. The team also administered a questionnaire at the end of the two movies to see if participants perceived that one trip took longer than the other.
In the end, only that last test -- the after-the-fact questionnaire -- revealed strong evidence of the return trip effect.
"During the initial and return trip, [participants] do not seem to experience the passing time any differently, but when asked afterwards, they have a strong feeling that the return trip felt shorter than the initial trip did," explained psychologist Niels van de Ven, of Tilburg University in the Netherlands, who has studied the return trip effect in the past but was not involved in Ozawa's research.
In an email, Van de Ven told the Los Angeles Times that he thought the new study supported his own finding that the return trip effect originates from "a violation of expectations."
"People are often too optimistic about an initial trip after which it [feels] quite long," he said. "When heading back we think, 'It's going to take a long time again,' after which it feels not as bad."
Or perhaps the return trip effect exists simply because people believe it does and respond in kind, he speculated.
Ozawa said he would like to examine the effect in further detail -- analyzing what happens when a filmed traveler returns to his original station via a different route, for instance. He said he had experienced the phenomenon himself during daily activities and had wanted to know more about it for many years.