Here’s more proof that it’s all in your head.

Guys who receive an inert shot that might or might not be growth hormone are more likely than women to believe it’s the real thing, a new study finds. What’s more, men who got fake shots actually scored higher on a jumping test than they did in previous tests, researchers from Australia report.

Ken Ho, an endocrinologist at the Garvan Institute of Medical Research and at the University of New South Wales in Sydney, and his colleagues randomly assigned 64 recreational athletes to receive a series of injections over eight weeks. Half got growth hormone, the others a placebo.

At the end of the study, among people receiving placebos, 81 percent of men but only 31 percent of women believed they were getting the real thing. Also, those men scored significantly higher on a jump height test than they had before getting the fake shots.

When asked to fill out a questionnaire, men who had gotten a placebo but thought it was real growth hormone were also likely to assess their own scores as improved. Ho presented the findings June 17 in San Francisco at a meeting of the Endocrine Society.

“It’s clear in our study that the mind plays amazing tricks,” Ho says. “I just wonder whether a lot of the achievements in sports are likely to be due to the power of the mind rather than substances that have been dished out to athletes.”

It also says something about the male mind, he says. “When we started recruiting for this study, we easily filled the slots for men. It was very difficult to fill the slots for women,” he says. “I think women are very protective of their bodies.”

The study team is still evaluating the participants who received real growth hormone to see whether they benefited.

Growth hormone has legitimate uses in certain groups — elderly people, patients with HIV and people who have a natural growth hormone deficiency. But no legitimate study has shown it helps young athletes in the prime of life, says Richard Auchus, a steroid biochemist at the University of Texas Southwestern Medical Center, in Dallas.

On the other hand, placebos have shown effects in study after study, he says.