Cheerful extroverts and neurotic introverts have the same risk for developing cancer, according to a new study, which lays to rest earlier research that suggested that certain personalities are more likely to develop cancer. It also means that cancer patients should not blame themselves for their disease.

Years ago, researchers had suggested that people who developed cancer tended to be more outgoing and less neurotic. The explanation for these findings was that both groups of people were more stressed: outgoing people "seek stimulation and thus experience high levels of stress"; and less neurotic people were stressed because they didn’t have an "outlet" to release it. But happily, based on new evidence from the largest cancer-personality study to date, these ideas do not seem to hold much weight.

There was also no relationship between one’s personality and their risk of dying from cancer.

The new work, by Naoki Nakaya and colleagues the Danish Cancer Society, followed almost 60,000 Swedish and Finnish twins born between 1926 and 1958. In the mid-1970s the twins were asked all sorts of different personality questions to asses how extraverted or introverted they were. Over the next thirty years, just over 4,600 of the participants were diagnosed with cancer, and 1,548 died of it.

There was no evidence that people had any greater risk for cancer depending on their personality types: the authors say that "neither extraversion nor neuroticism was significantly associated with risk of cancers". There was also no relationship between one’s personality and their risk of dying from cancer.

The researchers did find a relationship between both extroversion and neuroticism and lung cancer, but when they looked at how many cigarettes the participants smoked, these relationships all but disappeared. "We interpret these results to suggest 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", they say. There was a slight inverse relationship between neuroticism and liver cancer, but because the number of participants who developed liver cancer was so small, this finding was likely due to chance alone.

The research was published in the July 16, 2010 online issue of the American Journal of Epidemiology. A Commentary in the August 15th issue of that journal by Adelita V. Ranchor, Robbert Sanderman and James C. Coyne sums up the message, [W]e now can be reasonably confident that the overall effect size for a personality-cancer causal association is much too small to have clinical and public health implications, if it exists at all."