Editor's Note: This op-ed was co-published with The New York Times. It is not subject to our Creative Commons license.

The national furor over concussions misses the primary scourge that is harming kids and damaging youth sports in America.

The heightened pressure on child athletes to be, essentially, adult athletes has fostered an epidemic of hyperspecialization that is both dangerous and counterproductive.

One New York City soccer club proudly advertises its development pipeline for kids under age 6, known as U6. The coach-picked stars, "poised for elite level soccer," graduate to the U7 "pre-travel" program. Parents, visions of scholarships dancing in their heads, enable this by paying for private coaching and year-round travel.

Children are playing sports in too structured a manner too early in life on adult-size fields—i.e., too large for optimal skill development—and spending too much time in one sport. It can lead to serious injuries and, a growing body of sports science shows, a lesser ultimate level of athletic success.

We should urge kids to avoid hyperspecialization and instead sample a variety of sports through at least age 12.

Nearly a third of youth athletes in a three-year longitudinal study led by Neeru Jayanthi, director of primary care sports medicine at Loyola University in Chicago, were highly specialized—they had quit multiple sports in order to focus on one for more than eight months a year—and another third weren't far behind. Even controlling for age and the total number of weekly hours in sports, kids in the study who were highly specialized had a 36 percent increased risk of suffering a serious overuse injury. Dr. Jayanthi saw kids with stress fractures in their backs, arms or legs; damage to elbow ligaments; and cracks in the cartilage in their joints.

Because families with greater financial resources were better able to facilitate the travel and private coaching that specialization requires, socioeconomic status turned up as a positive predictor of serious injury. Some young athletes now face surgeries befitting their grandparents. Young hockey goaltenders repeatedly practice butterfly style—which stresses the developing hip joint when the legs are splayed to block the bottom of the goal. The sports surgeon Marc Philippon, based in Vail, Colo., saw a 25-year-old goalie who already needed a hip replacement.

In the Loyola study, sport diversification had a protective effect. But in case health risks alone aren't reason enough for parents to ignore the siren call of specialization, diversification also provides performance benefits.

Kids who play multiple "attacking" sports, like basketball or field hockey, transfer learned motor and anticipatory skills—the unconscious ability to read bodies and game situations—to other sports. They take less time to master the sport they ultimately choose.

Several studies on skill acquisition now show that elite athletes generally practiced their sport less through their early teenage years and specialized only in the mid-to-late teenage years, while so-called sub-elites—those who never quite cracked the highest ranks—homed in on a single sport much sooner.

Data presented at the April meeting of the American Medical Society for Sports Medicine showed that varsity athletes at U.C.L.A.—many with full scholarships—specialized on average at age 15.4, whereas U.C.L.A. undergrads who played sports in high school, but did not make the intercollegiate level, specialized at 14.2.

We may prize the story of Tiger Woods, who demonstrated his swing at age 2 for Bob Hope. But the path of the two-time N.B.A. M.V.P. Steve Nash (who grew up playing soccer and didn't own a basketball until age 13) or the tennis star Roger Federer (whose parents encouraged him to play badminton, basketball and soccer) is actually the norm.

A Swedish study of sub-elite and elite tennis players—including five who ranked among the top 15 in the world—found that those who topped out at as sub-elites dropped all other sports by age 11. Eventual elites developed in a "harmonious club environment without greater demands for success," and played multiple sports until age 14.

The sports science data support a "sampling period" through at least age 12. Mike Joyner, a Mayo Clinic physician and human performance expert, would add general physical literacy-building to the youth sports menu: perhaps using padded gymnastics gyms for parkour, which is essentially running, climbing or vaulting on any obstacle one can find.

In addition to athletic diversity, kids' sports should be kid-size.

In Brazil, host of this month's World Cup, kids are weaned on "futsal," a lightly structured and miniaturized form of soccer. Futsal is played on tiny patches of grass or concrete or on indoor courts and typically by teams of five players.

Players touch the ball up to five times as frequently as they do in traditional soccer, and the tighter playing area forces children to develop foot and decision-making skills under pressure.

A futsalization of youth sports generally would serve engagement, skill development and health.

USA Hockey (which has barred checking in youth games) recently invited adults to play on a 310-by-130-foot ice rink to show them what it's like for an 8-year-old to play on a regulation rink. The grown-ups' assessments: "too much time between the action"; "it's hard to communicate because everyone is spread out so far"; "you end up spending a lot of time in open space."

Futsal, basketball and padded parkour? Sounds like a strange three-sport athlete, and a perfect model for kids.