I was among the many people excited by fitness trackers and purchased one soon after they came out. It was fun. Look, 20,000 steps in one day (trip to a new city)! Two nights this week of uninterrupted sleep and 21 miles walked!

There's something captivating about numbers, which can explain the existence of the quantified-self phenomenon—that is, people who measure many things about themselves. Nowadays a smartwatch or even just your phone can keep track of a wide variety of markers, including heart rate, sleep patterns, steps in a day and even arrhythmias. People who run, swim or bike can measure their pace, distance covered, calories burned or total exercise time. A clip on your lapel can monitor your exposure to the sun. And perhaps warming the heart of every parent who got tired of reminding their children to “stand up straight,” you can even wear a device that buzzes you if you slump for more than, say, 15 seconds!

You can even spice it all up with “gamification,” setting up daily or weekly goals, and the program will award you badges and play celebratory tunes when you hit them. You can also upload your data to share with others—perhaps in friendly competition. Meanwhile workplace wellness programs that offer incentives or discounts on health insurance to employees who use such tracking devices and meet certain goals are spreading.

Unfortunately, despite some early encouraging studies that suggested that wearers of such devices were healthier than those who were not, the first large-scale experimental study, where people were randomly assigned to wear a fitness tracker, showed no difference in outcomes. Findings are similarly discouraging for workplace wellness programs: early research hopefully suggested that they were effective for lowering health care expenditures. But once again, better-designed studies showed practically no difference in outcomes over time.

What's going on? In fact, probably something very common. Early studies for new treatments or devices tend to be observational: they compare individuals who have chosen to take a specific action (eat a healthy diet; exercise regularly) with those who do not. Yet that engenders confounding biases because of the way people self-select into the groups, a problem that can be resolved only with true randomized experiments.

Should you sport one anyway? One concern is that these tracking devices are ... tracking devices. Many also track location, and they've already been invoked in court cases. In one, the unfortunate victim's heart rate spiked significantly and then stopped while the suspect was with her despite his claims he had left before she died. Solving a murder is good, but it's easy to imagine health insurers or employers requiring a certain number of steps a day or using such health data for making decisions.

When I first got my tracker, I tried to hit 10,000 steps a day. It felt gratifying when it catalogued 10-plus-day streaks of meeting my goals. This is called the Hawthorne effect, after experiments in a relay factory outside Chicago, Hawthorne Works, showed productivity increased when the lights were turned up—but also when the lights were turned down. A change, any change, and a feeling of being observed seem to put us on alert and better behavior—but only for a time. The novelty does wear off, and then we return to our baseline behavior.

Still, could it hurt to know the number? Maybe. Employees who were offered financial rewards along with a tracker fared worse after the trial ended and the cash dried up as compared with those who had never been offered incentives. External rewards seem to “crowd out” internal motivations—and once they go away, we don't always get the internal motivation back.

When my wrist tracker broke down after about a year, I just didn't feel like shelling out for another one. Anyway, even by the end of the third month, I could pretty reliably guess my steps or hours of sleep for the day without checking the app. Instead I took inspiration from people who argue that exercise is also about the right ecology—individuals in walkable cities get a lot more than those in suburbia, for example. So I now have a treadmill desk at work, and I set things up at home so that almost all my TV viewing is done on an elliptical. And honestly, I get cranky if I don't get some movement in during the day. That seems to be the best motivation to keep turning those pedals.