Phase two of the study is when things got interesting. The experiment was repeated, except this time some patients simultaneously received two drugs — the ineffective one from phase one and a second drug that actually worked. This time, volunteers from both the high and low illusion groups were presented with 50 patients who’d received the two drugs and 50 who received no drugs. Patients in the drug group recovered 90 percent of the time, while the group that didn’t get meds continued to have a 70 percent recovery rate. Volunteers in the “high illusion” group were less likely than participants in the “low illusion” group to recognize the new drug’s effectiveness and instead attributed the benefits to the drug they’d already deemed effective. The prior belief in the first drug’s potency essentially blocked acquisition of the new information.

“You have to be sure before you’ll destroy what you already know and substitute it with something new,” Matute told me.

This finding might seem like nothing more than an interesting psychological quirk if it didn’t make us so vulnerable to quackery. Many so-called “alternative” remedies exploit the illusion of causality, Matute said, by targeting conditions that naturally have high rates of spontaneous recovery, such as headaches, back pain and colds. Quack cures remain popular in part because they bestow a sense of empowerment on people who are feeling miserable, by giving them something to do while they wait for their problem to run its course.