That there is an intense rivalry between two of the top male triathletes in the world should come as a surprise to no one. What's odd is that the two men are brothers. Alistair Brownlee, 24, typically takes the gold, but he suffered a recent Achilles-tendon injury, so his younger brother, Jonathan Brownlee, 22, has a serious shot at the top spot on the podium at the Olympics this year. Rare coincidence? Not at all.

In the past decade alone, we've seen Venus and Serena Williams (Sydney, 2000), the uber-gymnastic Hamm brothers (Athens, 2004), and the super-rowing Winklevoss brothers (Beijing, 2008). Let's not forget the triple-threat tae kwon do kids—Steven, Diana, and Mark Lopez—who each medaled in Beijing. (Badass brothers in professional sports is a whole other story; think Eli and Peyton Manning.) All of which raises the question: Are super-siblings a product of nature or nurture?

Psychologists and scientists bemoan the fact that the field of sibling athletes has not been studied enough. One of the best studies on the subject, in the British Journal of Sports Medicine, concludes that genetics are paramount: "Although deliberate training and other environmental factors are critical for elite performance, they cannot by themselves produce an elite athlete. Rather, individual performance thresholds are determined by our genetic make-up, and training can be defined as the process by which genetic potential is realized."

Other studies indicate that the second most important factor is likely the degree to which the family has wholly dedicated itself to the children's athletic pursuits. Raising an Olympic-level athlete requires huge sacrifices of time and money, so if you're going to drag a kid to early morning swim practice, or hire a private trainer for your son, why not throw his brother into the pool as well, as long as he shows promise?

Interestingly, good friends trained by the same coach will not see the same results as blood relatives in that environment. "Siblings tend to look upon each other as competitors differently than how they look at other competitors," says sports psychologist Christopher Carr, who consulted with American athletes at the Beijing Games. "It actually has a way of relieving some of the externalized pressure. And in a way, they can be happier for a sibling's victory, as opposed to a stranger's, even when it's at the cost of their own success."

Birth order proves to be equally as important in determining which sibling outperforms the pack. A 2002 study published in The Journal of Sport Behavior found that firstborn athletes, regardless of gender, reported higher levels of anxiety—which can prove immobilizing during a race—before an event than later-born siblings, which the study's authors speculated could be due to the fact that firstborns perceive a greater responsibility to achieve in competitive situations. And a 2010 study from the Personality and Social Psychology Review found that for more than 90 percent of sibling pairs who played major league baseball, the younger brother exhibited more risk-taking behavior, stealing base more often than his older brother. Yet another study, sponsored by Victoria University, concluded that elite athletes are more likely to be later-born children.

So what, if any, moral can be extracted from this story? If you're a parent who wants to raise the next Tiger Woods, you can double your odds pretty easily by throwing another kid into the mix. And if you're a sibling with an eye on the gold, it turns out that—at least when it comes to birth order—sometimes it pays not to be first.

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