Two copies of the "val" gene makes powerful COMT that mops up the chemicals rapidly.

BBC: Gene controls pain threshold

The COMT enzyme breaks down chemicals such as dopamine which transmit messages between brain cells.

BBC: Gene controls pain threshold

But two copies of the "met" form produce a weaker version of COMT that is not so good at clearing away the chemicals.

BBC: Gene controls pain threshold

The researchers found individuals with a COMT variant that triples the amount of dopamine in the front of the brain felt no improvement without treatment but an improvement with the placebo acupuncture.

BBC: Placebo effect may be 'down to genes'