The composition of the food and the surface on which it falls matter as much if not more than the length of time it remains on the floor, the study found. Watermelon, with its moisture, drew the highest rate of contamination and the gummy candy the least.

In an interview, Professor Schaffner said, “I will tell you on the record that I’ve eaten food off the floor.” He quickly added: “If I were to drop a piece of watermelon on my relatively clean kitchen floor, I’m telling you, man, it’s going in the compost.”

Where did the rule get its start?

The history of the five-second rule is difficult to trace but it is attributed apocryphally to Genghis Khan, who declared that food could be on the ground for five hours and still be safe to eat, Professor Schaffner said.

Why do people do this anyway?

William K. Hallman, an experimental psychologist and a professor at the Department of Human Ecology at Rutgers University, said people do not put every decision through a risk-benefit filter and instead rely on cognitive shortcuts called heuristics to help in their daily lives.

“It’s a way of making a very quick decision with whatever data is available,” he said in an interview.

But sometimes those shortcuts can be based on flawed assumptions or missing information.

For instance, germs are invisible and so they are easy to ignore when “something of particular value, like a yellow peanut M&M” falls to the floor, he said. Because germs are out of sight, the belief is there is no harm in picking up the M&M and popping it in your mouth.