Nervy individuals may be more at risk of aggressive cancer, new research suggests.
Scientists found that anxious mice were worse affected by skin tumours than less fearful animals. Stressed and worried humans could be susceptible in the same way, they believe.
Researchers in the US first identified laboratory mice with nervous personalities that avoided the dark or open spaces.
The hairless animals were then exposed to ultra violet radiation at levels equivalent to those experienced by humans who spend too long in the sun.
After a few months, the mice developed skin tumours - as did a group of non-anxious mice.
The difference was that the nervy mice grew more tumours, and only they went on to develop invasive skin cancer.
Anxious mice had higher levels of regulatory T-cells, whose job is to dampen down overblown immune responses.
They also produced fewer of the chemical signals needed to fire up an immune attack on tumours.
Levels of the stress hormone corticosterone were also raised in the anxious mice.
The research appears in the online journal Public Library of Science ONE.
Lead author Dr Firdaus Dhabhar, from Stanford University Medical Centre in California, said: "Anxiety may be defined as increased sensitivity to physically existent, or non-existent but perceived or anticipated, stressors.
"Identifying a psychological trait right at the beginning - before any experimental manipulation - and seeing that it can be associated with increased tumours months later, and with biology that can begin to explain mechanisms, was a rewarding surprise."
Further research now needs to be carried out on human patients, said Dr Dhabhar.
He added: "It's bad enough that cancer diagnosis and treatment generates stress and anxiety, but this study shows that anxiety and stress can accelerate cancer progression, thus perpetuating a vicious cycle.
"The goal is to ameliorate or eliminate the effects of anxiety and chronic stress, at least at the time of cancer diagnosis and during treatment."
Chronic stress has already been linked to cancer and other problems. But the latest study is said to be the first to show a biological connection between having an anxious disposition and greater threat from cancer.
Dr Dhabhar now wants to see whether countering the effects of anxiety and stress can improve the effectiveness of cancer treatment.
Taking an anxiety medication such as valium for limited periods of time might be helpful, he said.
"Ultimately, we really want to harness the patient's mind and body while doing everything that medicine can from the outside to maximise treatment success," said Dr Dhabhar.
