Findings based on hundreds of skeletons recovered from Wharram Percy, which have been documented in a new book, have shown heads becoming rounder, rather than longer and narrower, between the 11th and 13th centuries.

Past explanations for the change had focused on immigration introducing different characteristics. But Simon Mays, human skeletal biologist for English Heritage, said the work questioned that view.

He said: "Our work has yielded few clues on why skulls changed, but we have cast serious doubt on some of the current theories.

"Despite the best efforts of science, we're still in the dark to explain why it happened. If immigration was responsible, we would expect both sexes to be affected. We do know that male skulls shapes eventually reverted back, becoming similar to those we have today."

The trend was not restricted to Wharram, though, and another theory suggested a colder climate, where people tend to have rounder skulls, may have been a factor.

But research has shown the climate of Wharram was warming up during the period.

Mr Mays said: "What we do know is that male bones more readily display the effects of environmental changes in their early development than those of women.

"But to confuse matters further, as the weather got much colder in the later medieval period, skulls started to become longer and narrower again.

"Perhaps the plague could be a factor, since it struck across Western Europe, but at this stage that's little more than a guess."

The book also reveals there was a higher proportion of left-handed people in medieval times than today, at around 15 per cent, compared to around eight per cent today.

It is thought the figure is probably due to the suppression of left handedness in more recent literate times.

Though infant mortality was high by modern standards, it was lower than other historic populations.

But villagers also tended to be more vulnerable to new diseases due to their isolation than poor city folk in York, who built up more resistance by being regularly exposed to germs.

The new material has been published in Wharram XI: The Church, which is now available from Oxbow Books.