Figure this one out: The fake drugs we know as placebos seem to be getting more and more effective in the US. That strange finding comes out of a new analysis in the journal Pain, which looked at medical studies going back to 1990.

It turns out that people who take placebos in studies today experience a 30% decrease in pain levels, a decrease in pain that has dramatically increased from nearly a quarter century ago, say researchers out of McGill University in a press release.

The scientists theorize that people today have higher expectations about drugs, and when they're participating in a big, fancy study—studies have grown in scope over the last two decades—they expect to feel better.

It's no mere quirk, as Nature explains. To get their drugs approved, manufacturers need to show that they outperform placebos, and the improved placebo performance is making it harder to do that.

"The data suggest that longer and larger trials are associated with bigger placebo responses," says the author of the paper. "This, in turn, tends to result in the failure of those trials—since it makes it harder for pharmaceutical companies to prove that the drug being tested is more effective than treatment with a placebo." Forbes notes that the finding applies only to the US, and not to other countries studied.

One theory on why: The US allows drug companies to advertise directly to consumers, perhaps again raising those aforementioned expectations. Popular Science draws a lesson: "For drug companies looking to get their products on the market, they may have to find a different way to conduct clinical trials, like making them shorter and smaller." (You'll perform better if you think you slept well, even if you didn't.)

This article originally appeared on Newser: Something Weird Is Happening With Placebos in America

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