This leads to a lot of odd phenomena, including the “return trip effect.” You’re probably familiar with the sensation: When you go to an unfamiliar destination and come back again, it often seems like the way back takes less time than the initial trip, even though you traveled the same distance.

Now, new research published in the journal PLOS ONE adds weight to the idea that the return trip effect actually exists.

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To carry out their study, the researchers, from Japan, had people sit in a dim room and watch a 20-minute movie recorded while the cameraman was walking around a city. Some subjects were asked to report each time they thought three minutes had passed. They watched two movies, and then were asked to rate which one was longer.

One group "took" a round-trip, from point "S" to point "E" in the lefthand image below and back again. The second group took two one-way trips: from "S" to "E" in the lefthand image, then from "S" to "E" in the righthand image.

The study was set up to examine at how we perceive time in two ways: how we perceive time as it is going along, and what we think retrospectively once a period of time is over. And the results were different for these two scenarios.

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The researchers found that the two groups did a roughly equal job at predicting how time was passing during the experiment. But when they recalled the experiment afterward, the people who did the return trip remembered the second leg to be shorter, while those who took two one-way trips did not experience the phenomenon.

As Joseph Stromberg of Vox writes, the study suggests that the return trip effect has something to do with hindsight and storytelling — the way people use language to look back on an experience and remember. The idea is that, in order to experience the “return trip effect,” you need to know that you're taking a return trip.

It’s not clear why this happens, though psychologists have several theories.

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One explanation is that the return trip effect has to do with paying attention to time itself. When you pay more attention to time passing — let’s say you’re late, and keep checking your watch or phone — time seems to take forever. But when you’re distracted by other, more interesting things, time passes quickly.

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This idea is certainly present in old adages — "a watched pot never boils," and "time flies when you're having fun."' It also helps to explain the phenomenon of time seeming to slow down when our lives are at risk. This also apparently happens in our memory: When we devote more attention to a period of time, we tend to remember that period of time as being longer.

This leads to an interesting idea: That by "mindful," or paying attention to the here and now, we can actually slow the brain's perception of time and make our lives seem longer.

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Another common explanation has to do with familiarity. On the way there, you don’t yet know the route; on the way back, you recognize landmarks and other familiar sites, which makes the trip seem to go faster. It's true that people don’t experience the return trip effect on journeys they take often, like their daily commute. That offers some evidence for the idea of familiarity.

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There is some evidence that time seems to slow down when we confront the unfamiliar, and whiz by when we're engaged in routine. This is why time appears to go by faster as we get older, and we confront fewer new experiences. That feeling of your birthday arriving "one day earlier each year" is expressed beautifully in this video by The Dictionary of Obscure Sorrows:

But this may not be the whole explanation for the return trip effect. Some research has found that people can experience the effect even when they're in unfamiliar territory. In a 2011 study, psychologists asked a group of cyclists to ride to a fair. They then split the group in half; one half rode home by the same route, while the other group took an equidistant, unfamiliar route. Both groups reported feeling the return trip effect.

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The conclusions of the 2011 study were that people are often too optimistic on their outbound journey, which makes that portion of the trip seem to take longer. On the way back, they revise their expectations too far the other direction. They expect the trip to be shorter than it actually is, which then it seems longer. According to the author of the study, this is why we don't experience the return trip effect on our daily commute: We've taken the route so often that we have accurate expectations about how long it will take.

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In the end, the return trip effect may be a combination of these things, and probably some dynamics that psychologists have yet to discover. But the current research makes clear that time is a subjective experience, stretching and contracting in ways that don't conform with a clock.