Michael Phelps says his rivalry with fellow Olympic gold medalist Ryan Lochte is a great motivator. But what motivates the average non-Olympian to take part in athletic events?

According to a Harris Interactive Survey commissioned by Eventbrite (an online event registration and ticketing service), the top reason people might take part in marathons, 10K runs, bike races and other “endurance events” is ... charity.

Of 2,209 U.S. adults surveyed in June, 34 percent said that raising money for a good cause would be their biggest motivation. But while 71 percent of respondents said they’d like to participate in such events, only 32 percent said they actually had done so.

Why not? Thirty-seven percent cited lack of time; 32 percent cited the cost of participating and 27 percent blamed a lack of motivation.

Other motivators for participating included improved health and fitness (32 percent), the sheer challenge posed by the event (28 percent) and the desire to lose weight (27 percent).

Single people were more likely than married folk to report that they didn’t have time to participate in such events (43 percent vs. 34 percent), while parents with kids younger than 18 were far more likely than people without children to cite family commitments as obstacles to taking part.

More than half (56 percent) of respondents thought going for a run would be more therapeutic than talking with a therapist or psychologist. More than 40 percent of those surveyed said they figured training for a marathon would make them more attractive.

Not only that, about a third of respondents (37 percent of the men and 25 percent of the women) thought training for a marathon would make them better in bed.