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The turning point, for me, was Eliud Kipchoge’s smile. In the late miles of his 2017 sub-two-hour marathon attempt at a racetrack just outside Milan, as the effort mounted, he kept flashing a beatific grin. It was a deliberate tactic to help work through the pain, he later explained. Kipchoge’s reputation as the Yoda of endurance was just taking off, and I was torn between wide-eyed admiration of his mental game and my own long-standing skepticism of anything you can’t easily measure. Then, a few months later, sports psychologists in Northern Ireland published a study in which they asked runners to smile and measured a 2 percent drop in energy consumption. Kipchoge was right—and by extension, I reasoned, sports psychology was a real and measurable thing.

Since then, I’ve become a booster. I’ve written enthusiastic articles about sports-psych topics like mindfulness, self-talk, and mental focus, touting the emerging evidence that they really can enhance athletic performance. In parallel, I’ve grown ever more skeptical of conventional sports science—ice baths, compression socks, ketone drinks, and so on. My entire column in the last issue was devoted to ripping the “weak and biased” research underpinning virtually all sports supplements. So I got a jolt of severe cognitive dissonance from a in the journal Sports Medicine. Put simply, the evidence is not that good.

A group of researchers at the Karolinska Institute in Sweden, led by Gustaf Reinebo, pooled the results of 111 studies that tested the effects of various psychological interventions on athletic performance. The studies included a wide range of sports, with outcomes like finishing time, free-throw percentage, putting performance, and so on. A few interventions, like mindfulness and mental imagery, had “moderate” effects—but when low-quality, non-randomized trials and subjective-outcome measures were removed from the analysis, the benefits disappeared. And some of my favorite approaches, like motivational self-talk, didn’t even have enough comparable evidence to merit their own meta-analyses. So am I guilty of holding a double standard, giving mental training a pass for the same research failings that I criticize in, say, supplements?

When I’m assessing new research, one of the biggest red flags is someone trying to get rich off the results—which, when you’re talking about supplements, is 100 percent of the time. In contrast, “psychological interventions are less overtly commercial,” points out Nick Tiller, a physiologist at the Harbor-UCLA Medical Center, and—most relevantly—the author of . “There’s a limit to how much one can package and sell a construct or abstract concept.” That’s not to say it doesn’t happen: companies like Calm and Headspace have made fortunes on the back of mindfulness. But it’s a little easier to take a study at face value when the key ingredient isn’t for sale on the internet.

There’s another, more fundamental reason we might cut sports-psychology research some slack: it’s uniquely hard to study. “It’s very difficult, if not impossible, to link a particular performance outcome to something that may or may not be manifesting in the brain,” Tiller says. If you’re selling a pill and you don’t have solid evidence of its purported physiological effect, that’s either because it doesn’t work or because you haven’t bothered doing the necessary research. But the absence of evidence that self-talk really boosts performance can be blamed, at least in part, on the fact that it’s almost impossible to study properly. The gold standard of evidence is a randomized, controlled trial—but how do you blind participants to whether they’re receiving self-talk? What’s the placebo? How can you get them to “unlearn” what you’ve already taught them when they switch groups?

The most damning part of the new review, to me, was that the positive effects of sports psychology had been measured subjectively, via athletes’ own performance ratings, rather than objectively, using data like race time. If you felt as if you had a great race but you didn’t go faster than normal, that doesn’t impress me.

But Carla Meijen, a sports-psychology researcher at the University of Amsterdam who edited a , urged me to think more broadly. Coaches are always most interested in tangible and immediate outcomes, she acknowledged. “As a performance director, you’re not asking your athletes, ‘Are you enjoying it a lot more?’ ” But as a sports psychologist, she’s targeting outcomes like self-efficacy, motivation, attention, and anxiety. Maybe that won’t make you measurably faster tomorrow. But if you’re thinking long-term, the less anxious you are and the more your immediate motivations align with your values, the more likely you are to improve over the coming months and years—and the less likely you are to quit altogether.

None of this means that sports psychology research is as good as it needs to be. “We’ve got a long way to go,” Meijen says. As skeptics like me are won over and the popularity of sports psychology grows, one of the challenges in scaling up its reach will be figuring out which interventions are suitable for a do-it-yourself approach and which require one-on-one work with a trained practitioner. Self-talk and goal setting are two strategies Meijen suspects people can use on their own. But that question—and many others—will take more and better studies to answer.

Despite all the gaps in our current knowledge, I remain intrigued by the role of the brain in endurance, and by the possibility that we can manipulate performance with relatively simple techniques like self-talk. After all, Tiller reminds me, the terrible quality of most sports-science research, and the massive size of the global health and fitness industry, actually demonstrate the awesome power of the mind. “If we accept the premise that 99 percent of products are not supported by evidence, then the $4 trillion worth of sales derive primarily from people convincing themselves that these interventions work,” he says. “That’s all psychology.”

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