Four times as many women as men are now developing multiple sclerosis, according to research, which highlights an increasing gender gulf among those afflicted by the debilitating condition.

Though scientists have no indication why women are more susceptible to the progressive neurological condition, the latest US analysis shows the ratio of female to male sufferers has effectively doubled over the past six decades.

That MS is more prevalent in women has long been known. The new data, however, paints a stark reality. In 1940, women with MS in the US outnumbered men by about two to one. By 2000, however, the imbalance had widened to four to one.

While the findings bring scientists no closer to establishing any causes of the disease, it may herald a change in direction of research.

The author of the latest report has said more must now be done to explore unknown physiological changes that have occurred in women since the middle of the 20th century, including use of the Pill, earlier menstruation, higher obesity rates, changes in smoking habits, and women having children later in life.

The new study will nevertheless prove particularly important in Scotland. With around 10,500 here affected by the disease, the nation has the highest proportion of MS sufferers in the world.

Dr Gary Cutter, a professor of biostatistics at the University of Alabama and author of the study, said the findings "give us clues" to the cause of the disease.

"It's an increase in the ratio of women to men of nearly 50% per decade. We don't yet know why more women are developing MS than men, and more research is needed," said Dr Cutter.

"We also need to ask general questions about what women do differently than men, such as use of hair dye and use of cosmetics that may block vitamin D absorption," Dr Cutter added.

Dr Cutter's team looked at a database of 30,336 Americans with MS and determined the ratio of men and women according to age and year of diagnosis.

In some circles, it is believed the different and more variable nature of the biochemistry of women's bodies is likely to play a role in their increased susceptibility, especially since the naturally occurring sexual hormones oestrogen, progesterone and testosterone all have an immuno-modulating function.

The Multiple Sclerosis Society offered a cautious welcome to the report, and stands by its belief that the ratio of women to men with MS is about 2:1.

It points out some scientists believe female hormones - oestrogen and progesterone - may play a role in MS.

Chris Bentley, spokesman for the society, said: "The results of this study are intriguing. MS is most frequently diagnosed in pre-menopausal women and this research reinforces questions surrounding the role of hormones in multiple sclerosis.

"The idea that lifestyle changes in women over the decades may play a part in multiple sclerosis is interesting, but further research is needed to explore this level of influence and its bearing on the prevalence of MS."

MS Society Scotland