THERE is an old dieters’ joke that anything eaten directly from the fridge has no calories. Wrong. But surely anything in a teensy snack pack must help you lose weight, right? Wrong again.

Grocery aisles are full of small “diet packs” of candy, cookies or fried snacks, advertised as a guilt-free way of helping you eat less. But Rik Pieters and colleagues at Tilburg University in the Netherlands suspected that diet packs might in fact make people drop their guard and eat more.

They had 140 students watch TV – to rate advertising, they were told – and gave them …