Lie-detector tests take off in India

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Media captionDeepti Puranaik puts the BBC's Rajini Vaidyanathan to the test

An increasing number of companies in India are offering polygraph tests to individuals and companies trying to catch out deceitful spouses and dishonest employees, prompting critics to accuse them of taking the law into their own hands.

Scientist Deepti Puranaik's job as a polygraph tester puts her in a position somewhere between judicial inquisitor and nosey friend asking personal questions.

She is paid to probe. Working with the Mumbai-based company Helik Advisory, she conducts several tests each week for private clients who pay anything from $150 to $300 (£94 to £188) to determine whether someone is telling the truth.

"There have been some cases of infidelity where a wife might suspect her husband is cheating on her. In other cases a theft might have taken place and the employer wants to know who may be involved in it," says Puranaik.

The company conducts polygraph tests in a variety of cases.

Sometimes an employer wants to interview test the integrity of prospective employees before hiring them.

Or the test may be designed to catch a culprit - in an infidelity case for example, a robbery in a company, or a theft in a household where the domestic help is suspected.

The company, which has been operating for a year, says it is getting more and more enquiries every day. Often the desire for confidentiality is a factor that leads people to seek its services.

"Many people don't want to go to the police because it's very time-consuming and anything which goes to the police is made public," says Helik chairman and former head of government forensic services for Maharashtra, Rukmani Krishnamurthy.

"Some clients come to us because they don't want outsiders to know that a particular robbery has happened in their organisation."

Anyone who is put forward for a lie-detector test has to consent to it beforehand and undergo a series of preliminary interviews.

Puranaik, who points out that polygraph tests can determine someone's innocence as well as their guilt, concedes that they are not totally foolproof.

"They are about 80% accurate," she says.

The polygraph works by measuring physiological changes such as blood pressure, sweating and breathing.

It was invented in 1917 by American William Marston, who was also the creator of the Wonder Woman comic character.

The equipment has been used in India for many years, including by the police.

But in 2010 the country's Supreme Court deemed polygraph evidence from suspects as invalid.

The ruling stated that such tests were an infringement of an individual's personal liberty, with the result that no-one can be forced to take one as part of an investigation.

Consensual tests are permitted, although such evidence could not alone secure a conviction, merely be used as supporting evidence.

Helik is one of a small number of private companies in India which have started to offer private polygraphs in the past five years.

The wider private forensic market in India is also growing, with companies offering services such as signature verification (cheque fraud is a big problem in India), handwriting analysis and even personality tests.

With a growing population and an already stretched police force, these firms are filling a gap in the market, and they say their work enables the police to concentrate on more serious crimes.

But some in the legal profession are dubious about private firms taking on these roles.

Bharat Chugh, an advocate at India's Supreme Court, says better regulation has become essential.

"I think it's a dangerous trend... private players are in it to make money, they don't have the justice system to deliver to," he says.

"People are practically taking law into their own hands - they are running a parallel machinery in which they are deciding disputes. This is not very good."

But Krishnamurthy argues that until the government can build more forensic labs in India, companies like hers can serve both private and public needs, and ultimately reduce the backlog of criminal cases.