Even before a game begins, an athlete’s body changes: heart rate increases, hormones surge and beads of sweat dapple the skin. Competition is such a visceral experience that the mere anticipation of a challenge excites our instincts to fight. These biological responses are even more pronounced when people face an opponent they have come to know and despise, an opponent they must battle again and again—a rival. In a 2003 study psychologists at Northumbria University in England found much higher testosterone levels in soccer players preparing to play against a team they considered an extreme rival than in those matched up with a moderate rival.

Rivalry differs from other kinds of competition in its intimacy. It offers contenders a psychological prize people cannot win in other contexts: the chance to beat someone obnoxiously familiar, someone whose abilities and traits are frustratingly matched with their own.

Whether on the field, in a classroom or at work, rivalry changes more than our body chemistry. Researchers are now finding that it also sways our minds, changing how we think and behave during competition—and outside of it. Rivalry not only boosts motivation but also can disrupt rational thinking, bias memories and encourage unethical behavior.

Improving Performance
Although competition has long interested social psychologists, only recently have scientists looked at situations involving true rivals. They are discovering that the psychology of rivalry differs in important ways from that of ordinary competition.

On the positive side, rivalry can be highly motivating. In unpublished work social psychologist Gavin J. Kilduff of New York University’s Stern School of Business analyzed six years’ worth of race results archived by a running club in the Northeast to identify rival racers—runners who were evenly matched, similar to one another in age and gender, and who frequently competed against one another. Kilduff found that runners consistently ran faster when competing against rivals. The mere presence of a rival could trim between 20 and 30 seconds off a runner’s total race time in a five-kilometer race. Above and beyond ordinary competition, rivalry delivered a measurable boost to motivation and performance. Rivalry changes behavior by endowing competition with a significance that exceeds the objective stakes of the contest, Kilduff believes. “Defeating a rival feels good in and of itself, beyond what else is tangibly at stake,” he says.

Creating rivalries could improve athletic performance, Kilduff suggests. In practice scrimmages, for example, instead of mixing up the players each time, coaches could create stable teams to build up a rivalry. Coaches might also repeatedly pit two individuals with similar abilities against each other. “If you can play up the rivalry with someone you are training with, it could certainly improve motivation and performance,” Kilduff says.

Bad Decisions
Rivalry can often hamper performance, however, especially when it comes to decision making. In a 2005 study negotiations expert Deepak Malhotra of Harvard Business School and his colleagues asked participants to imagine themselves at an auction for a one-of-a-kind item for which they agreed to pay no more than $150. In the final round of bidding, some of the participants were told there were eight other contenders for the item, whereas others were told they were up against only one, to simulate a type of rivalry. Then the researchers told all participants that a competitor had bid $150 and that they had to decide whether to bid higher. Participants facing a single bidder rated their excitement and anxiety as much higher than those bidding against a group and were far more likely to exceed the preset bidding limit. This behavior is economically irrational, because the more bidders remaining in an auction’s final round, the more the contested object is likely to be worth.

“People seem to start out with reasonable goals,” Malhotra says. “But in the heat of the moment their motivation shifts, and they want to win at any cost.” Similar feelings of rivalry can overpower rational thinking during an argument with a close friend or family member, making what might have been a productive debate into a useless verbal sparring match that, of course, neither party can really win. On a larger scale, corporate rivals may engage in egregiously expensive marketing campaigns to outdo one another or to tarnish a competitor’s reputation, Malhotra says.

Rivalry impairs not only judgment but also people’s actual memories. In a study published in February, psychologist Kevin S. LaBar of Duke University invited male fans of the Duke men’s basketball team and male fans of Duke’s rival, University of North Carolina at Chapel Hill, to watch their teams face each other on a big screen TV. Each participant watched the game with two or three other fans of the same team. Later, LaBar asked the fans to view segments of the game while lying in a functional MRI machine. Each segment focused on a single play whose outcome clearly benefited either Duke or U.N.C.—but the clip always ended just before the play did, at which point the fan tried to recall how the play ended.

LaBar found that fans remembered outcomes that favored their team far more accurately than those benefiting the rival team. Accordingly, brain regions implicated in emotion, attention and memory responded more vigorously to plays that fans interpreted as helping, as opposed to hurting, their team. Usually negative stimuli elicit the most intense emotional responses, leading to the strongest memories. But rivalry seems to shift the brain’s focus to the rewards of winning, making triumph more memorable.

Success at Any Cost
Rivalries not only bias our thought processes but also can corrupt our moral code. We have all witnessed rivals trading put-downs and playing practical jokes on each other. But more surprisingly, the unsavory behavior spawned from rivalry can spill over into other, totally unrelated domains. In another study, also as yet unpublished, Kilduff and his colleagues—social psychologists Niro Sivanathan of the London Business School and Adam Galinsky of Northwestern University—asked people to recall a personal encounter with a rival, a recent ordinary competition or a recent collaborative effort from their everyday experiences. Participants then completed a series of puzzles and scored their own success. Students who recalled competing against a rival were more likely to deliberately exaggerate their performance. In other words, remembering a rival motivated them to lie about their performance on a subsequent unrelated task. Kilduff suspects that conjuring up memories about rivalry leaves a psychological residue that taints other choices we make. It makes us want to succeed at any cost and more likely to deceive and cheat.

Because we encounter people we consider rivals quite often—both in and outside of direct competition—rivalries may alter our motivation and moral code on a regular basis, Kilduff believes. Logging onto Facebook in the morning and scrolling through your news feed only to stumble on a personal rival’s obnoxious status update or vain photos could influence your behavior and decisions throughout the day. You might be more likely to, say, run that red light, cut in line at the movie theater, claim a co-worker’s idea as your own or tell a white lie to excuse a transgression against someone you love.

In related work, also unpublished, Kilduff tested the relation between rivalry and unethical behavior by simulating rivalries in the laboratory. He set up two contests. In the rival condition, students repeatedly faced the same opponent and experienced narrow margins of victory and defeat; in the ordinary competition situation, participants faced different opponents and experienced more lopsided margins. The students who faced a rival later scored higher on a test of Machiavellian attitudes, which measures whether people endorse selfish, devious and manipulative behavior. High scores on this scale are correlated with unethical actions such as cheating, lying and exploitation. Competing against a rival, Kilduff says, may bring out the inner Machiavelli in people. “Rivalry opens up the possibility you might behave irrationally or unethically based solely on the relationship you have with your competitor,” Kilduff says. “It just changes everything.”