This so-called placebo effect was greater in male recreational athletes than in females, said lead author Jennifer Hansen, a nurse researcher at the Garvan Institute of Medical Research in Sydney.

"Athletes are doping with growth hormone to improve sporting performance despite any evidence it actually improves performance," Hansen said in a statement.

Sixty-four young adult recreational athletes randomly received either growth hormone -- a substance banned in sports -- or an inactive substance, or placebo, for eight weeks.

The study found men were much more likely than women to think they had received growth hormone, but regardless of sex, athletes who took the placebo but believed they were on growth hormone or "incorrect guessers,"thought their performance improved and actually had some improvement in all measures of performance."

"The results of this study suggest that the placebo effect may be responsible, at least in part, for the perceived athletic benefit of doping with growth hormone for some people," Hansen.

The findings were presented at The Endocrine Society's 90th meeting in San Francisco.