An "age of paranoia" is dawning at the start of the 21st century, according to a leading psychiatrist.

Dr Daniel Freeman, who has spent a decade conducting pioneering research into paranoia, believes one in four people regularly suffer irrational fears of being threatened or in danger.

Paranoia is far more common among the general public than had previously been suspected, and on the increase, he claims.

"These days we daren't let our children play outside; we're suspicious of strangers; security cameras are everywhere," said Dr Freeman, from the Institute of Psychiatry at King's College London.

"We seem to have entered an age of paranoia. And the indications are that things may only get worse."

Dr Freeman, who recently developed a "virtual reality" method of diagnosing paranoia, believes a combination of factors has helped to create an "era of suspiciousness".

One is the increasing number of people living in cities.

For the first time, 2008 has seen urban populations in the world outnumber rural.

In 1800, just 5% of people around the world were city dwellers. By 2030, this figure is expected to have risen to 65%.

Rates of paranoia are known to be twice as high in cities than in rural communities, said Dr Freeman.

Unequal distribution of wealth also played a major role in fostering paranoid feelings, Dr Freeman added.

Research in the US had shown that populations with the widest income inequalities also had the lowest levels of trust - and the highest death rates.