Presumably, this happens, because thinking about a positive event brings you mentally closer to that happy time in your past, and being close to something happy makes you happy. Likewise, thinking about negative events brings you mentally closer to sad events.

An interesting paper in the January, 2013 issue of Psychological Science by William Hart examined this question of mental closeness using language.

The complex grammar of language allows us to take all kinds of vantage points on events. We are all familiar with basic components of grammar like tense. We can talk about events that happened in the past as well as events that will happen in the future.

Another (and less obvious) element of grammar is aspect. Aspect allows us to describe an action as if it is extended in time or as an action that has a clear endpoint. The imperfective aspect describes events that extend over time (I was shopping for a shirt). The perfective aspect marks that the event has ended (I shopped for a shirt).

Hart suggests that when people talk about an event as if it is extended in time (using the imperfective aspect), then they will feel mentally nearer to that event than if they describe it as completed (using the perfective aspect). That mental nearness can influence mood. So, when thinking about a positive past event, people should be happier when they describe it using the imperfective aspect (which brings them mentally closer to it) than when using the perfective aspect. When thinking about a negative past event, people should be sadder when they describe it using the imperfective aspect (which brings them mentally nearer to it) than when using the perfective aspect.

In one study, people were asked to describe a past event that was either negative or neutral (neither positive or negative). They were cued to talk about the event either in the imperfective aspect (What was happening?) or in the perfective aspect (What happened?). After describing the event, people also rated their mood.

People who described a neutral event were not affected by the aspect they used to describe the event, meaning that particular grammatical forms do not influence mood on their own. However, those who described a negative event were sadder when they used the imperfective aspect than when they used the perfective aspect. That is, when the language made people feel close to the event, they were sadder than when it made them feel further away.

In a second study, people did either an easy or frustrating task. The easy task involved solving simple anagrams (unscramble the letters LGRAE into LARGE). The difficult task involved some difficult anagrams and some that were actually impossible.

After doing this task, people were asked to describe it. As before, they were prompted to use either the imperfective or perfective aspect when describing what they did. Finally, people rated how happy they were feeling.

Those who did the easy task felt positively about it. When they described the task using the imperfective aspect (which made them feel close to the event), they rated themselves as happier than when they described it using the perfective aspect. Those who did the hard task felt negatively about it. When they described the task, using the imperfective aspect, they rated themselves as sadder than when they described it using the perfective aspect.

This research is fascinating, because it demonstrates how the language we use affects our sense of closeness to the past. Simply by describing events as extended in time we can bring ourselves closer to the past. This effect happens, even though most of us are not explicitly aware of these elements of our grammar.

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