We rank physicians among the most trustworthy members of our society, but a new survey finds the respect isn't always mutual.

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Is your doctor telling you the truth? Possibly not, according to a new survey in Health Affairs of nearly 1,900 physicians around the country.

The researchers found that 55% of doctors said that in the last year they had been more positive about a patient’s prognosis than his medical history warranted. And 10% said they had told patients something that wasn’t true.

About a third of the MDs said they did not completely agree that they should disclose medical errors to patients, and 40% said they didn’t feel the need to disclose financial ties to drug or device companies.

Really? The study’s lead author, Dr. Lisa Iezzoni, a professor medicine at Harvard Medical School and director of the Mongan Institute for Health Policy at Massachusetts General Hospital, was surprised to learn how mendacious her colleagues were. “Some of the numbers were larger than I expected they might be,” she says.

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Why the white lies? In some cases, Iezzoni says it was for self-protection. Nearly 20% of the doctors admitted that they didn’t disclose a medical error to their patients because they were afraid of being sued for malpractice.

In other cases, it may have been for the patient’s benefit. MDs might spare an anxious patient from hearing about the slightly abnormal results of a lab test, for example, if it has no impact on the patient’s health. Conversely, the doctor might exaggerate a health result in hopes of motivating a patient to take better care of himself.

Doctors who sugarcoat patients’ prognoses may do so to soften the blow of bad news and help them remain hopeful about their potential recovery. Also, short office visit times may preclude the long and emotional discussions that accompany the delivery of difficult medical news, so doctors may say what they can to avoid causing the patient pain.

“The [doctor-patient] relationship is a human interaction, and physicians are human too,” says Iezzoni. “They don’t want to upset their patients, they don’t want their patients to look unhappy or burst into tears. But they also need to be professionals, so they need to tell themselves that if there is a difficult truth they need to tell their patient, they need to figure out a way of communicating that effectively.”

That’s critical for doctors to appreciate, because as well-intentioned as their fibs may be, other studies consistently show that patients prefer the truth, and would rather hear harsh news than remain ignorant about a dire medical condition. Being fully informed is a way that patients can cope and prepare for whatever might occur.

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As for the failure to disclose medical errors, Iezzoni says doctors’ fear of malpractice suits may often be misplaced. Studies suggest that in cases where physicians are open about their mistakes, patients are more likely to be understanding and refrain from suing.

So how can doctors learn to be more honest with their patients? More training about how to communicate with people about their health is critical — especially when it comes to delivering bad news. Patients also need to be clear and firm about how honest they want their doctors to be. Communication is a two-way street, after all, even in the doctor’s office.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.