According to Livescience, because placebos relate to the power of suggestion, there has been a great deal of interest in finding whether certain groups of people are more susceptible to the placebo effect.

But while researchers have looked at age, gender and religiosity, among other factors, findings have been inconsistent.

"No one has been successful in identifying a profile of a placebo responder," said one researcher.

If such a group were found, she added, "drug companies would be excluding those people [from clinical trials], so they’d be able to test their drugs more correctly."