Men have a higher risk of developing Parkinson's disease, a study suggests.
Parkinson's is a disease of the brain but causes are unknown
A team at the University of Virginia School of Medicine found men are 1.5 times more likely to develop the disease, but are not sure why.
They suggest increased exposure to toxic chemicals and a higher rate of head injuries among men could explain the statistics.
The report was published in the Journal of Neurology Neurosurgery and Psychiatry.
Parkinson's occurs when the balance between two chemical messengers in the brain is upset, affecting a person's ability to co-ordinate movements.
But it is currently unknown what causes Parkinson's disease.
Statistics from the UK charity the Parkinson's Disease Society do not suggest that men are more prone to the disease.
Rate of incidence
However, the researchers say their new study differs from previous ones, in which statistics on gender were gathered on death rates alone.
The researchers argue that concentrating solely on death rates alone can be inaccurate as it does not give a flavour of the number of new cases, and the cause of death on certificates can sometimes be incorrect.
In the new study, Dr Fred Wooten and colleagues looked at the actual incidence of Parkinson's disease among men and women.
They collated data from seven separate population-based studies, conducted between 1989 and 1999 covering the US, China, Italy, Spain, Poland and Finland.
They used these studies to show the rate of Parkinson's per 100,000 people, and then broke them down to reveal the difference in incidence of the disease between men and women.
The researchers say nobody can be sure why men may be more vulnerable to the disease.
However, the speculate that the 'male lifestyle' and 'male roles', such as farm work and labour could put men at a higher risk by making them more prone to head injuries and exposure to toxic chemicals, which have previously been suggested as risk factors for the disease.
Another possibility, they say, is that the female hormone oestrogen helps to protect women's brains from neurological damage.
Dr Yoav Ben Shlomo, Senior Lecturer in Clinical Epidemiology at the University of Bristol said the study could provide "useful clues" to the causes of Parkinson's.
But he told BBC News Online the results should be treated with some caution as it is possible one gender is more likely than another to seek health care or diagnosis.
"Future research should try to understand what may cause these gender differences," he said.
Around 120,000 people in the UK to have Parkinson's disease, with ten thousand being diagnosed each year.
Symptoms include shaking, muscle stiffness and slowness of movement. It can affect a person's ability to walk, talk, write, speak, smile and swallow.