Heredity is usually singled out as the main factor behind the disease.

But research has shown that environment is the likely cause of Parkinson's disease in most patients over the age of 50.

The researchers suggest that there could be two separate conditions - one caused by genetic factors that starts before 50, and another caused by environmental factors that starts after.

Degenerative disease

The research looked at almost 20,000 twins who were veterans of World War II.

It was carried out by researchers at the Parkinson's Institute in Sunnyvale, California.

The findings were published in the Journal of the American Medical Association on Wednesday.

Parkinson's disease is a progressive, degenerative brain disease, which slowly reduces the ability of the nerves to control the muscles.

Famous Parkinson's patients include Muhammad Ali, and Michael J. Fox, Enoch Powell and Pope John Paul II.

The potential causes of Parkinson's have been a matter of dispute among scientists for years.

In recent years, research has turned up evidence of specific genetic defects that can cause Parkinson's.

Some scientists have thus suggested that heredity is a principal cause for the condition.

Twins

The California study examined both identical and fraternal sets of twins.

Muhammed Ali has the degenerative disease

Because identical twins share the exact same genetic profile while fraternal twins are no closer genetically than any other siblings, the researchers had a clear view of how closely the disease is tied to genetic factors.

It found that, in the rare instances of "early onset" Parkinson's striking before the age of 50, both members of each of the identical twin sets in the study came down with the disease, meaning the cause was almost certainly genetic.

But in the "typical" cases of Parkinson's, diagnosed after the age of 50, the study found no statistical difference between identical and fraternal twins. In each case, only about 10% of the twins came down with Parkinson's.

The researchers said this means environmental factors were almost certainly to blame.

Research focus

Dr J. William Langston, president of the Parkinson's Institute, led the research.

He said: "The results appear to be fairly clear-cut. They tell us that in typical Parkinson's disease, occurring over the age of 50, there does not appear to be a genetic component. There was no evidence of a hereditary factor."

Dr Langston said research into the disease could now focus on external factors such as herbicides, pesticides, and other chemicals.

"If it's not inherited, that really points very strongly to something in the environment," Dr Langston said.

"From a scientific angle, we really know where to invest our money in research."

Dr Langston said the results of the study pointed to such a major difference between "early-onset" Parkinson's and the typical form the disease that the two could be entirely separate conditions.

"Our suspicion is that typical Parkinson's disease is environmental in origin, but that 'young-onset' cases belong to a group of genetic disorders.