The celebrated rivalry between American swimmers Ryan Lochte and Michael Phelps came to a head this weekend when the 27-year-old U.S. teammates competed for Olympic gold in the 400-meter individual medley. Lochte beat Phelps by just 83-hundredths of second in the four-minute race at trials last month, and he won the gold handily at what had been dubbed the "duel in the pool" in London.


For years Lochte splashed around in the wake of Phelps, who won a record eight gold medals at the 2008 Olympics, but their rivalry has intensified as their finish times have converged. Lochte says this summer is his turn to shine, while Phelps wants one last hurrah. New research on rivalry suggests they're both the better athletes for their epic battle to be best.


"A rivalry in this kind of context is apt to benefit these swimmers' motivation and performance," said Gavin Kilduff, an assistant professor at New York University who studies the causes and consequences of rivalry. In fact, the swimmers could be boosting each other's performances by as much as 2 percent.


Intense rivalries develop for three main reasons, according to past research by Kilduff and colleagues. First, competitors turn into rivals when they feel similar to one another, both in terms of characteristics and abilities. Second, a rivalry builds the more times two competitors face off. It further strengthens when past contests have been closely decided.


"That could be in part because if you win or lose by a very small margin, you imagine that the opposite could have happened; that can be a very emotionally arousing situation that lives on in your mind," Kilduff told Life's Little Mysteries.


Hitting all the marks, the rivalry between Lochte and Phelps seems inevitable. Now, new, unpublished research by Kilduff and colleagues suggests it could be shaving as much as 2 percent off the swimmers' times. Looking at the performance of 82 amateur runners of 5-kilometer races over the course of six years, the researchers found that the runners, on average, ran 25 seconds faster when they competed against a runner who fit the profile of being their rival (based on the characteristics of rivals described above). Assuming experienced runners finish 5K races in 20 minutes, that's a 2 percent speed increase.


Although one cannot always reliably extrapolate that number to other sports or situations, Kilduff said the message is clear: "Rivalries produce a significant performance boost." [The 7 Biggest Mysteries of the Human Body]


That holds true for everyone, not just elite athletes. In another finding to be published as part of the same study, Kilduff found that having a rival in school, the workplace, on a sports team, or in a romantic relationship motivates and energizes us. In one experiment, he asked one group of people to write a detailed essay about a personal rival of theirs, and another group to write an essay about a non-rival competitor. When he next asked the study participants to list as many uses of an object (such as a brick or knife) as they could come up with, those who had written about their rivals persisted in the task and thought up significantly more uses for the object before moving on to another question than did the other participants. "It energizes people to think about a rival of theirs," Kilduff explained.


The results indicate that competing against a rival boosts people's performance on any tasks that require brute force or effort. There's a dark side, however: Kilduff also found that people who are "primed with rivalry" more often cheat or lie when given the opportunity. This effect manifests itself, for example, in soccer. "Games between rival teams have a significantly higher frequency of yellow and red cards given out for unethical behavior," Kilduff said.


In short, when you have a rival, "you're going to put forth more effort," he said, "but you're also going to cut corners and just do whatever it takes to win."


Follow Natalie Wolchover on Twitter @nattyover or Life's Little Mysteries @llmysteries. We're also on Facebook & Google+.


Related on Life's Little Mysteries:

Copyright 2012 Lifes Little Mysteries, a TechMediaNetwork company. All rights reserved. It is reprinted with permission here.