To see if there were genetic differences between responders and non-responders, Furmark screened them for a variant of the gene for tryptophan hydroxylase-2, which makes the brain chemical, serotonin. Previous studies suggested that people with two copies of a particular "G" variant are less anxious in standard "fear" tests. Sure enough 8 of the 10 responders had two copies, while none of the non-responders did (Journal of Neuroscience (DOI: 10.1523/JNEUROSCI.2534-08.2008).

Furmark believes the effect of the gene may extend to other conditions where the amygdala is involved, such as phobias, pain disorders and even depression. However, he cautions that only further studies will reveal whether the gene influences the placebo effect more generally.

Echoing Furmark's caution is Fabrizio Benedetti of the University of Turin, Italy. "We know that there's not a single placebo effect but many." Some may work through genetics, he adds, others through the expectation of a reward.