In some people the placebo effect has a lot of strength and in others nothing. To clarify the reason they have conducted an investigation at the University of Michigan, led by Jon-Kar Zubieta.
According to this study, it seems that the placebo effect on people to whom the possibility of obtaining the prize produces great pleasure is greater than in others. To find out, they have measured the amount of dopamine that flows a certain brain nucleus, which is the molecule that forms when the prize is expected. Dopamine is precisely the one that produces the sensation of pleasure.