In a series of four studies, researchers from New York University (NYU) found that the more positively participants fantasised about the future, the fewer depressive symptoms they showed at that moment but the more symptoms they showed at a follow-up session.
The pattern of results emerged when the researchers tested both adults and children and over follow-up periods that ranged from 1 month up to 7 months after the initial session.
The participants wrote down whatever thoughts and images came to mind and rated how positive and how negative these fantasies were.
They found that the college students who came up with more positive fantasies had lower scores on a scale measuring depressive symptoms - that is, at that moment, they seemed to be less depressed than their peers.
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However, when the students completed the scale again one month later, they showed higher depressive symptoms relative to students who had imagined more negative scenarios.
Additional findings indicate that individual effort may help to explain the link between positive fantasies and depressive symptoms.
College students who reported positive fantasies tended to report putting less effort into their coursework; this was, in turn, associated with lower grades and higher depression scores.
"Our findings suggest that as pleasurable and helpful as positive fantasies are for depressive mood in the moment, they can be problematic and cumbersome over time," said Gabriele Oettingen from NYU who led the study.
Investing in positive fantasies may prevent us from acknowledging the obstacles that stand in the way of reaching our goals and undertaking strategies to surmount them, researchers said.
"Positive fantasies must be complemented with a good sense of reality," Oettingen said.
The findings were published in the journal Psychological Science.