Paris (AFP) - MDMA, the main ingredient in ecstasy, makes humans more likely to cooperate -- but only with trustworthy people -- researchers said Monday in the first study into how the drug impacts our willingness to help others.

Despite its status in Britain as a Class A drug, MDMA is widely consumed due to the heightened sense of energy, empathy and pleasure it arouses in users.

It contains neurotransmitters -- chemical messengers for the brain -- that are known to be linked to behaviour and mood, but scientists currently understand very little about how these affect social interactions.

Researchers at King's College London studied 20 healthy adult men who were given a typical recreational dose of MDMA or a placebo pill and then asked to complete a set of tasks while images of their brain activity were taken with an MRI scanner.

One of the mind exercises they were given was the Prisoner's Dilemma -- an example of so-called "game theory" in which an individual is asked to choose between cooperating or competing with another, unknown person.

Both players get an equal share of the points on offer if they choose to cooperate, but if one player chooses to compete they get all the points, while the other player gets nothing. They however risk getting nothing if the other player chooses to compete.

The team found that participants on MDMA were more willing to cooperate than those given the placebo.

But, to their surprise, the MDMA takers only became more cooperative with players they perceived as trustworthy, based on observations of the previous choices of the other player.

"We thought MDMA might make you think that other people are more trustworthy. And we were wrong," Mitul Mehta, from King's Institute of Psychiatry, Psychology and Neuroscience, told AFP.

"Actually it didn't change one bit what you thought of the other person, but it did change your behaviour towards them.

"It's not that MDMA is giving you rose-tinted glasses and therefore you're behaving differently, it's actually keeping your glasses clear and you still think an untrustworthy person is untrustworthy."

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- Clinical trials -

The team found that MDMA increased activity in the superior temporal cortex and mid-cingulate cortex, two areas of the brain known to be important for empathising with the thoughts and intentions of others.

They also observed noticable differences in activity in the right anterior insula -- a section of brain important in appraising risk and uncertainty -- when participants on MDMA processed the behaviour of trustworthy and untrustworthy players.

"In other words, MDMA did not make participants naively trusting of others," said Mehta.

"For the first time ever we are showing that MDMA isn't having a global effect on brain behaviour, it's actually having quite a specific effect and that's really useful."

The team behind the study, which was published in The Journal of Neuroscience, said their findings could help the treatment of psychiatric conditions such as PTSD.

"Patients with psychiatric disorders have a lot of difficulties with social interactive behaviour. We now can ask the question about what specifically you have trouble with in social behaviour -- is it your behaviour, or how you evaluate others?", said Mehta.

Despite being legally classified in Britain as a schedule 1 drug -- meaning it is deemed to have no medical use -- MDMA has been given Breakthrough Therapy designation by the US Food and Drug Administration.

It's currently undergoing phase 3 clinical trials for treating PTSD and Mehta said the drug should be more widely available for medical research.

"It's about access to treatment that is so promising that it seems unethical not to give people access," he said. "There's a renewed interest in MDMA, how it works and how it affects the brain."