Praying for a sick cardiac patient may feel right to people of faith, but it doesn’t appear to improve the patient’s health, according to a new study that is the largest ever done on the healing powers of prayer.

In fact, the researchers from Harvard Medical School and five other U.S. medical centers found – to their bewilderment – that coronary bypass patients who knew strangers were praying for them fared significantly worse than people who received no prayers. The team speculated that telling the patients about the prayers may have caused “performance anxiety,” or perhaps a fear that doctors expected the worst.

“Obviously, my colleagues were surprised by the unexpected and counterintuitive outcome,” said the Rev. Dean Marek, director of chaplain services at the Mayo Clinic in Rochester, Minn., and a co-investigator for the project.

It was a strange end for the prayer study, which cost $2.4 million and enrolled 1,802 patients who had bypass surgery. Most of the funding came from the British-based John Templeton Foundation, which supports research at the intersection of science and religion.

The new study, which appears in the April issue of the American Heart Journal, was designed to be large enough to see if patients who knew they were being prayed for had better recoveries.