That fitness tracker you got for Christmas promises to make you healthier — but it doesn’t promise weight loss. And now it looks like there’s a good reason why: people in a weight-loss study who used fitness trackers dropped about five pounds less than those who didn’t, according to a study published this week in JAMA.

For the study, 471 overweight people ages 18 to 35 were randomly assigned to one of two groups. Both groups received diet advice, instructions to exercise, and group counseling sessions. After six months, one group started using Fit Core by BodyMedia (which has since been discontinued) and the other didn’t. Everyone ended up thinner and healthier than before, though most people gained some weight back. But at the two-year mark, the group with the trackers had lost about 5.3 fewer pounds on average than the group blissfully free of electronic monitoring.

Participants wore an outdated, bulky armband monitor

That doesn’t mean the devices are harmful, says lead researcher John Jakicic, a professor in the department of health and physical activity at the University of Pittsburgh. He suggests that the real problem is how companies design the apps that present us with information — and the inflated expectations we get from using them.

Weight loss is complicated. A glance at the study data suggests that there was actually little difference in how much the two groups ate or exercised, but this is impossible. The difference makes sense once you realize people reported their own results — a method that is notoriously inaccurate. When it comes to the trackers, it’s possible that people who use these devices get a false sense of security, Jakicic says. They think it’ll solve all their problems, so they pay less attention to how much or what they eat. And everyone likes novelty, so it’s common for people with a new device to get “tunnel vision” and focus on that instead of on the counseling or dieting, says Sherry Pagoto, a professor of medicine at the University of Massachusetts Medical School.

Another possibility is that having this information is just plain discouraging. It’s 8PM, you’re exhausted after a long day at work, and your fitness tracker says you’ve only taken 1,000 steps. In this situation, a lot of people think that there’s no way they’ll make the 10,000 daily step goal, so they might as well not even try.

Companies making these trackers need to pay more attention to how people actually behave

We already know many people lose interest in fitness trackers after a few months. The study participants were no exception. Since the devices report when they’re being worn, the researchers knew that the group with the trackers did wear them more at first before trailing off.

Besides the self-reported data, the study has a couple notable limitations. First, everyone in the study was young, so the results might not be applicable to older people. Also, the participants didn’t get to choose which wristbands they wore, which sounds minor until you remember that “the tech moves faster than the research,” as Jakicic puts it. The study started in 2010 and used the Fit Core, a large, bulky armband that many people wouldn’t want to wear for an extended period of time.

The capabilities of the Fit Core are far behind the technology of today’s wearables, according to Shelten Yuen, Fitbit’s vice president of research. The Fit Core doesn’t have a display, and users had to log into a website to see their stats. Today, practically every tracker has a monitor that shows numbers in real time. The Fitbit lets users connect with friends with the device and “challenge” them to walk more. It has reminders to walk at least 250 steps each hour and offers “virtual tours,” all of which Yuen says address the motivational issues that come with exercise.

Fitbits have been used in studies from institutions including the Mayo Clinic and Johns Hopkins University. Companies including BP, Bank of America, and Kimberly-Clark have created corporate wellness programs that integrate Fitbit, and many of their employees have shed pounds and seen good results.

But these participants usually weren’t studied as closely as the ones in the two-year JAMA study. Participants in the JAMA study were randomized — which is the gold standard of clinical trials — and the researchers tried to follow up with everyone. “If you’re just tracking the people who stuck around and not that many people stuck around, your outcomes might look better,” says Pagoto from U-Mass, “but in randomized trials like these, you’re committing to including everyone in your analysis. And so to do that for two years, you’re gonna get a different picture than you might if you’re just looking at users of a device in a company.”

Still, the biggest takeaway, according to Jakicic, is that the companies making these trackers need to pay more attention to how people actually behave. Most apps give information, maybe with a prompt that goes “Hey, get up and move” (or, if you’re one of the wearers of the new Apple Watch: “Are you running today?”). It’ll do this even when it has the data to know that you haven’t budged from the couch for the last four days.

What the software doesn’t do is say something like, “It looks like you’re on a bad track right now, why is that? What’s going on? What strategy might be most helpful to get you moving?” That might be what’s needed rather than the reprimands or nudges. “I think that the interfaces don’t use very strong theoretical approaches to changing behavior,” says Jakicic. “How it talks to you is critical. I think if we were to build it, we would build all of that in to make it more like a human was actually counseling you and we could make it much more effective.”

Jakicic has approached several companies (he declined to name which ones) about building this type of interface, but they declined. Fitness tracker companies have, however, been working on different ways to get us to move. The Apple Watch has frequent reminders to stand, for example. And companies like Mio abandoned step counters in favor of tracking heart rate because, according to chief executive Liz Dickinson, scientific research suggests that intensity — more than just steps — is the key to effective exercise.

It’s important to remember that everyone in the study did lose weight in the end. And certainly the outdated Fit Core looks more like a giant medical device when compared to today’s increasingly stylish and powerful trackers. But the results do point at something important: data alone isn’t enough.