Fitness- and life-tracking gadgets were some of the most ubiquitous products at this year’s Consumer Electronics Show. Companies that weren’t into it before joined up; companies that had failed at it were getting back in the ring; and companies with proven success showed no sign of flagging.

As these life-tracking devices and services race toward commodity status, it’s worth asking whether they actually work. As a motivator, they can be great initially. But lots of the trackers have novelty, which I can attest to from my time spent comparing a few fitness-tracking bracelets. I didn’t realize how few steps I took on days that I didn’t make the effort to work out, and I was fascinated to see the graphs of my sleep and wake times from past nights.

The effectiveness of the data “reward”

There is some science to back up the efficacy of these trackers. One small study at Indiana University showed even a simple pedometer helped participants lose an average of 2.5 pounds over 12 weeks and significantly increased participants’ active hours.

Dieting and exercise in general form a kind of reward-based system: eat this (or don’t eat this, as the case may be), lose weight. Swing dumbbells and barbells around, gain muscle.

Fitness bands are an easy way to track the effort put forth into the “rewards” of fitness. They draw the line between effort exerted and results achieved more thickly and darkly than does, say, logging steps from a pedometer or writing down a fitness routine in a journal. Plus, apps and automation of the data collected by fitness trackers can be cross-referenced and rearranged in any number of graphs and charts. The novelty of the information itself becomes a reward alongside the actual reward of results.

And rewards can be an important component of achieving goals. Studies repeatedly indicate the effectiveness of rewards in many different forms: awarded in direct proportion to the effort, at random, at increasing or decreasing quantity or frequency, and everywhere in between.

In the introduction of the book Drive, author Daniel Pink describes a study where scientists look at the role of rewards in motivating monkeys to solve puzzles. The monkeys first learned to solve the puzzles by themselves because they are curious, and working out the problem of the puzzle gives them intrinsic satisfaction, the scientists theorized. When they introduced raisins as an extrinsic reward for solving the puzzles, the monkeys made more errors and solved the puzzles less frequently. Being good at puzzle-solving became secondary in terms of a reward to being good at raisin-getting.

...until the “reward” wears off

Whether the “reward” of a fitness tracker is just the novelty of seeing one’s efforts accounted for or the concreteness it gives to weight-loss or fitness, both of these things are external rewards. Eventually they wear off, and customers are left holding the $100 networked silicone bracelet.

The nature of fitness and the demonstrated ineffectiveness of achieving a certain physique as a “reward” suggest that lasting changes need an intrinsic motivation. This isn’t something a fitness-tracking device, with its external rewards, can provide.

Extrinsic rewards can provide ongoing motivation, as other research shows. But for the reward effect to continue motivating people, it has to keep coming and get better every day. The rewards, both long- and short-term, need to be at least comparable to the short-term reward of that delicious-looking donut over there. But the problem is that extrinsic rewards can undermine intrinsic ones, and it’s literally impossible to keep the extrinsic rewards of fitness up indefinitely.

The New York Times documented in a 2011 feature, The Fat Trap, how difficult it is to keep lost weight off. Even a year later, it appears that metabolisms don’t adjust to a slimmer, less calorically demanding body. So after a certain point in weight loss, the literal reward stops coming.

Fitness bands don’t measure weight loss itself, so the problem of tapering weight loss or fitness results isn't logged there. But at some point the “rewards” of seeing a certain amount of steps or calories burned will plateau. The NYT’s piece touches on a few cases of people who have managed to keep weight off in the long term, and universally, it’s the case that they remain extremely vigilant about their diets and amount of exercise. “They never don’t think about their weight,” Kelly Brownell, director of the Rudd Center for Food Policy and Obesity at Yale University, tells the NYT.

That will lead most people, per the NYT article, into a natural decline when their diet or fitness routine stops delivering. Once someone has cycled through the decline, they’ve rebuilt their reward potential, and the fitness tracking can come back into function.

Provided the right motivations and intrinsic rewards are in place, fitness trackers can be helpful for losing weight. But still, for most people in the context of life-tracking, fitness, weight loss, and rewards, a device that aids it is not only bound to stop delivering on the good feelings, information, and reinforcement that it gives at the outset, but it cannot effect true, lasting change any more than a new diet or exercise can. That isn’t the fault of the fitness band. But while the devices remain a significant money investment, it’s worth being familiar with their limits.