Wearable activity monitors can count your steps and track your movements, but they don’t, apparently, help you lose weight. In fact, you might lose more weight without them.

The fascinating finding comes from a study published today in JAMA that found dieting adults who wore activity monitors for 18 months lost significantly fewer pounds over that time than those who did not.

The results suggest that activity monitors may not change our behavior in the way we expected, and raise interesting questions about the tangled relationships between exercise, eating, our willpower and our waistlines.

There have been tantalizing hints in a few studies recently that new technologies such as wearable activity monitors, which tell us how much we are moving and how many calories we have burned during the day, might help some people to drop pounds.