Katie Roper's daughter Ellen likes to play lots of sports--soccer, basketball, softball and swimming. But even at Ellen's tender age of nine, that's getting hard to do.

Kids in her swim program are dropping other sports to specialize in competitive swimming year-round, and Ellen is having a tough time keeping up. The year-round swimmers tend to beat others badly, says Ms. Roper, of Los Altos, Calif. "I'm sad that soon she'll be discouraged and probably want to quit a sport she loves."  

As millions of kids take to fields, courts and rinks this fall, as many as half to two-thirds are destined to quit sports by their teens, largely because they're not having fun, studies show. A trend toward specialization--pressure for kids to play just one competitive sport year-round--is one reason, researchers say.  

Many kids thrive in competitive year-round programs, learning advanced skills and enjoying expanded opportunities. But the programs can be a bad fit for others, fostering stress and an overemphasis on winning, says a recent focus-group study of 67 school officials, coaches, parents and teens led by Daniel Gould, director of the Institute for the Study of Youth Sports at Michigan State University. Such strains are linked to higher injury risk, reduced motivation and burnout.  

For parents who want their kids to embrace sports as a path to lifelong fitness and fun, the trend calls for new vigilance. Here are some early-warning signs that your child may be burning out:  

It's No Fun

A child who grows bored, resists practices and games, feigns illness or injury, or retreats to the back of the line in drills, is at high risk of dropping out. Jeffrey L. Brown, a Harvard Medical School psychologist, also cites fatigue, performance worries or a lack of concentration in other activities.  

The challenge for parents is to help children hold on to the joy of participating while others are focused primarily on winning. Switching to a community, YMCA or recreational league program can help. But it's usually best to finish the season, says Marty Ewing, an associate professor of kinesiology at Michigan State. Exceptions might include cases when a child is clearly in over her head and can't focus, or is so mismatched with a team that she's at risk of injury.  

Teammates Aren't Compatible

Camaraderie is a big source of the fun in sports and its absence can signal trouble. Joy Wyatt's daughter showed early aptitude for gymnastics and worked hard, at age 8, to prepare for her first competition. But when she was forced to sit out with an injury, her teammates were unsupportive and unkind, says Ms. Wyatt of New York. Put off, she decided to quit at the end of season.  

Cheating Rears Its Head

If your child complains about cheating or starts cheating himself, burnout may loom. Blaming a bad call for a loss or error may signal that a child is frustrated, anxious or too focused on winning, Dr. Ewing says. It also may be a clue that you're hovering too much: An in-depth 2006 study of four junior tennis players at North Carolina State University, Raleigh, found parental pressure to win was by far the biggest reason players cited as the cause of cheating.  

Your Involvement Eclipses Your Child's

A 2006 Norwegian study of 677 soccer players ages 10 to 14 found children who felt heavy performance pressure from parents and coaches were over-concerned about making mistakes and harsher in their self-criticism.  

If you see such emotions in your child, Michael Checchi advises, "take a hard look in the mirror." When Mr. Checchi's son showed early talent for baseball, he immersed him in the sport, enrolling him in off-season training and doing drills at home. But to Mr. Checchi's dismay, his son began at age 13 to lose interest. "The harder I pushed, the more resistance he gave me," recalls the San Ramon, California, father. Finally, his wife "told me I was being a jerk," he says. He backed off, and his son's interest in baseball soon reignited.