People who are friendly and outgoing and those prone to be nervous and moody have at least one thing in common: their personalities do not seem to be connected to their risk of developing or dying from cancer, a new study finds.

The findings, from two long-running studies of Finnish and Swedish twins, add to evidence refuting the theory that there are certain cancer-prone personalities, and that personality is an important factor in the odds of surviving cancer.

Researchers found that among the nearly 60,000 adults across the two studies, there was no association between cancer risk and either of two personality traits: extraversion, a tendency to be friendly and sociable; and neuroticism, a proclivity toward worry, anxiety and emotional ups and downs.

Nor were the two traits connected to cancer patients’ survival odds, according to findings published in the American Journal of Epidemiology.

For people with cancer, the findings mean that they “should not think that their personality traits may have affected their cancer or cancer prognosis,” write the researchers, led by Dr. Naoki Nakaya of the Danish Cancer Society in Copenhagen.

Dr. James C. Coyne, the co-author of an editorial published with the study, agreed.

The notion that having a “fighting spirit” will affect your cancer prognosis is an appealing one, said Coyne, who directs the behavioral oncology program at the University of Pennsylvania School of Medicine in Philadelphia.

But there is also a bright side to the lack of an association between personality and survival: People with cancer do not have to feel they should “adopt” an outlook that is not true to them, Coyne told Reuters Health.

“Cancer patients don’t have to feel like they should blame themselves when things go wrong,” he said.

The idea that personality plays a role in cancer risk has been around since ancient times. But much of the research evidence of a link comes from the work of one team of European investigators, whose studies in the 1980s connected certain personality traits, including a tendency to repress emotions, to substantially increased risks of cancer death.

Other researchers, Coyne said, subsequently questioned the validity of the studies, because of suspected manipulations of the data and other inconsistencies.

Some small studies since then have found associations between personality factors and cancer risk or survival, but many others have not. And where there have been links seen, both neuroticism and extraversion seem to have different relationships with cancer risk, on the one hand, and cancer survival, on the other.

Some researchers have suggested that people who are outgoing and consistently calm - high on extraversion, low on neuroticism - have increased risks of developing cancer; the theory is that these individuals may seek out stimulation and potentially stressful situations, while their cool demeanor denies them an emotional outlet for the stress.

On the other hand, some studies have linked high levels of neuroticism and low levels of extraversion to poorer cancer survival.

According to Coyne, the strengths of the current study include its large population sample and long-term follow-up.

The 59,548 study participants had their levels of extraversion and neuroticism assessed in the mid-1970s, when they were young to middle-age adults. Over the next 30 years, 4,631 study participants were diagnosed with cancer, and 1,548 died of cancer.

When Nakaya’s team factored in a number of other variables - like study participants’ smoking and drinking habits, body weight and education level - they found no association between the two personality traits and people’s risk of developing or dying from cancer.

There was a link between higher levels of both neuroticism and extraversion and the risk of lung cancer. However, when the researchers took a closer look at smoking history - including how heavily a person smoked - the relationship between lung cancer and the personality traits faded.

That, the researchers write, indicates that “people with a certain set of personality traits are more likely to smoke and, by smoking, would have a higher risk of lung cancer.”

The findings do not mean that psychological and emotional well-being are unimportant in cancer. Coyne said that patient support groups, for example, are “great” if they help improve the quality of a cancer patient’s life.

But he added that people should not feel they have to adopt a particular attitude toward their disease in order to extend their lives.

“People have a right to their anger - that may be just who they are,” Coyne said.

He pointed out, however, that even if personality traits do not have a strong influence on cancer risk or prognosis, they may be more of a factor in a condition like heart disease.

People with heart disease may need to spend years, or decades, following a regimen of medication, diet changes and exercise, and traits like conscientiousness, Coyne said, may be important in their ability to do that.

SOURCE:  American Journal of Epidemiology, online July 16, 2010.