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Long-term and short-term relationships are obviously different from each other. Some people are the type you'd want to marry; others are good primarily for the sex.

At least, that's how conventional wisdom goes. But new research out of the University of California, Davis, suggests that—at first—long-term and short-term relationships may look more or less identical.

When you survey the complete time course of a short-term and long-term relationship—from the moment you meet someone until the moment the relationship is over for good—it takes a while for the differences in short-term and long-term relationships to emerge.

"Long-term and short-term trajectories typically pull apart after you've known someone for weeks or months," said Paul Eastwick, an associate professor of psychology at UC Davis who is the lead author on a new study published in the Journal of Experimental Psychology: General.

"In the beginning, there is no strong evidence that people can tell whether a given relationship will be long-term and serious or short-term and casual."

More than 800 people surveyed

Eastwick and his co-authors surveyed more than 800 people from a wide range of ages. They used a state-of-the-art "relationship reconstruction" survey in which people reproduce the events and experiences they had in their prior real-life short-term and long-term relationships.

Importantly, Eastwick and his colleagues asked the participants to reconstruct these relationships from the very beginning. This procedure differs from the standard "relationship science" approach, which starts studying people once they are already in a dating relationship.

"Some of the most interesting moments in these relationships happen after you meet the person face-to-face, but before anything sexual has happened," Eastwick added. "You wonder 'is this going somewhere?' or 'How much am I into this person?' It is somewhere around this point that short-term and long-term relationships start to diverge, and historically, we have very little data on this particular period of time."

The researchers found that romantic interest rises at the same rate in both short-term and long-term relationships. But at some point, romantic interest tends to plateau and decline in short-term relationships, while in long-term relationships, it continues to ascend and reaches a higher peak.

What is the moment when the two trajectories start to diverge? On average, it happens at about the time that the relationship starts to become sexual.

"People would hook up with some partners for the first time and think 'wow, this is pretty good.' People tried to turn those experiences into long-term relationships," said Eastwick. "Others sparked more of a 'meh' reaction. Those were the short-term ones."

The study offers a new twist on the distinction between the stable, long-term partner and the exciting, short-term partner. In real life, people may end up in short-term relationships when they are "just a little" attracted to the other person—enough to keep having sex, but maybe not for very long. Long-term relationships may be the ones that start especially exciting and sexy and grow into something stable and lasting.

More information: Paul W. Eastwick et al. What do short-term and long-term relationships look like? Building the relationship coordination and strategic timing (ReCAST) model., Journal of Experimental Psychology: General (2018). Journal information: Journal of Experimental Psychology: General Paul W. Eastwick et al. What do short-term and long-term relationships look like? Building the relationship coordination and strategic timing (ReCAST) model.,(2018). DOI: 10.1037/xge0000428