CHENNAI: Monday's rumour mills on mehendi causing death sent over 600 harried hennaed women to government hospitals in the city. Last week's rumour about the threat of violence against people from the northeast sparked India's largest internal exodus.

The rumours may be new, but the mills have been churning for centuries, say psychologists. In India, research has been on for over 75 years with psychologist Jamuna Prasad having studied the psychology of the spread of rumours during earthquakes in north India in 1935.

According to Prasad's study, rumours spread the most when anxiety levels are at their peak. As uncertainty and anxiety levels rise, those who find importance in the subject find it difficult to analyse the rumour's credibility. Add internet and instant messaging to that and the rate of transmission increases manifold - rumours used to spread a person at a time, but now it's literally a horde at a time.

A month ago, it was the mass-circulated rumour of HIV-positive blood being injected into a popular mango drink that had mothers in Chennai thinking twice before reaching for a tetra pack of juice. A few months ago, Chennaiites beat each other up at petrol bunks because they were told the city was running dry. Years ago, a rumour about the polio vaccine causing impotence triggered panic across the country.

According to author and psychoanalyst Sudhir Kakar, "Rumours are a way of both provoking and mastering anxiety in a situation of threat and violence." Canberra-based professor Prashant Bordia, the author of "Rumour Psychology: Social and Organisational Approaches", seems to agree with Kakar. He believes they merely give meaning to pre-existing anxieties.

Bordia says people may act on the basis of a rumour even if they don't believe it. "It is a 'better safe than sorry' approach," he says. He adds that people tend to spread rumours not just to cause unsettlement but also to seek more information.

Also, there is no one type of person who is affected by rumours or spreads it. It tends to be situational - a parent may be involved in a rumour about a school building collapsing, while anxious employees may be connected to rumours spreading about downsizing.

Bordia says the rumours that spread the fastest are almost always related to ethnic conflicts and rioting, issues of personal safety or health, and crime. "And the only way to control or rebut it is to have the rumour denied by a source that is trustworthy to the target audience," he says.

In Chennai, over the last week, police have been visiting establishments that employ people from the northeast and reassuring them ofthe city being a safe place. Suspected rumour-generators have been arrested. Phone service providers have been instructed to restrict customers to five SMSs a day. Over 100 panic-inducing websites have been blocked. But it may not help, says Bordia, if people affected by the rumour do not trust the spokesperson. "You need to get someone from the community that has been affected, someone people actually trust, to rubbish the rumour," he says.

When that happens, the results are positive. As it seems to be with the case of the violence against people the northeast rumour. "Thanks to the way the police spoke to my staff, many of those who had run away have returned to work," says restaurant owner Vipin Sachdev.

But when it doesn't happen, well, that's the reason you have people like a Rani S, still refusing to touch a mango drink, or a Mumtaz J staying up all night furiously scrubbing her daughter's mehendi-ed palms.