Helena Matute, a psychologist at Deusto University in Bilbao, Spain, and her colleagues enlisted 147 college students to take part in a computer-based task in which they each played a doctor who specializes in a fictitious rare disease and assessed whether new medications could cure it.
In phase one of the study, the student volunteers were divided into two groups — a “high illusion” one that was presented with mostly patients who had taken Drug A and a “low illusion” one that saw mostly patients who hadn’t taken the drug. Each student volunteer saw 100 patients, and in each instance, the students were told whether the patient had recovered. The student volunteers weren’t told that the drug didn’t work — the recovery rate was 70 percent whether or not patients took the drug. Yet, as expected, people in the high illusion group were more susceptible to erroneously concluding that it had an effect.
Presumably, because the student volunteers in the low illusion group had more opportunities to see the syndrome resolve without the drug, they were less prone to assuming that recovery was linked to it. Previous studies have shown that simply seeing a high volume of people achieve the desired outcome after doing something ineffective primes the observer to correlate the two.
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.
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