A 2015 study published in the journal Pain analyzed 84 clinical trials of pain medication conducted between 1990 and 2013 and found that in some cases the efficacy of placebo had grown sharply, narrowing the gap with the drugs’ effect from 27 percent on average to just 9 percent. The only studies in which this increase was detected were conducted in the United States, which has spawned a variety of theories to explain the phenomenon: that patients in the United States, one of only two countries where medications are allowed to be marketed directly to consumers, have been conditioned to expect greater benefit from drugs; or that the larger and longer-duration trials more common in America have led to their often being farmed out to contract organizations whose nurses’ only job is to conduct the trial, perhaps fostering a more placebo-triggering therapeutic interaction. - www.nytimes.com