The power of the mind has been overestimated when it comes to fighting cancer, US scientists say.
Cancer growth was not affected by mental outlook
They said they found that a patient's positive or negative emotional state had no direct bearing on cancer survival or disease progression.
The University of Pennsylvania team followed more than 1,000 patients with head and neck cancer.
But experts said the Cancer journal study should not deter people from adopting a "fighting spirit".
Indeed, a positive outlook can help patients cope with gruelling cancer therapies and resume a "normal" life, a spokeswoman for Macmillan Cancer Support said.
Seeking emotional support may be beneficial to cancer patients, said the researchers.
Lead author Dr James Coyne said: "If cancer patients want psychotherapy or to be in a support group, they should be given the opportunity. There can be lots of emotional and social benefits.
"But they should not seek such experiences solely on the expectation that they are extending their lives.
"The hope that we can fight cancer by influencing emotional states appears to have been misplaced."
In the study, a patient's emotional status had no bearing on survival, regardless of gender, tumour site or disease stage.
Julia Frater, of Cancer Research UK, said: "People with cancer can feel under pressure to cope well with their disease and treatment and to stay on top of things. They are often urged to feel positive.
"These results should reassure them that if they don't feel like this, it's okay. Many people do feel worried or low following a diagnosis and this isn't likely to affect the outcome of their treatment."
Professor Jane Maher, of Macmillan Cancer Support, said although there was no good evidence that emotion had an impact on the cancer itself, a patient's mental outlook would have a bearing on their long-term outcomes.
"More and more people are surviving with cancer. And that survival will be affected by their emotions.
"For example, depression and anxiety can make it less likely that they are going to get back to a normal, healthy life."