Alternatively, chewing might reduce how much a person is paying attention to something stressful out in the world, which influences the activity of the hypothalamic-pituitary-adrenal (HPA) axis, the body’s main stress response system. Attempts to measure cortisol levels of gum chewers, a hormone related to stress, have been mixed too, Haskell-Ramsay said. Sometimes, cortisol has been higher, reflecting higher attention. But chewing could also reduce cortisol at other times, particularly in the morning when cortisol is higher already.
Could the answer lie in evolution? Animals often chew when they’re stressed. It’s possible there’s something primal and comforting about the act of chewing, passed down from our common ancestors with other animals. Adam van Casteren, an evolutionary biomechanic, has studied chewing, but has suggested the opposite: it was human’s ability to chew less that made us the species we are today.
Humans chew much less than non-human primates; chimpanzees chew between four and five hours a day, and gorillas can chew six. Humans chew food, on average, for just 35 minutes. “Before the invention of fire and the use of tools, our ancestors would have also had to have chewed for similar amounts of time,” he said. “It would have been a significant time investment to sit there and chew.”
Gum’s appeal is probably not about an ancestral link to a chewing-heavier era, Van Casteren said. It’s not a vestigial habitat left over from caveman times. His best guess: “Humans just like to do repetitive things,” he said. “I’m a foot shaker when I’m thinking.”
Consider other repetitive motions like tapping your toes, squeezing a stress ball, or clicking a pen. In research on fidgeting, Human Computer Interaction and Games researcher at UC Santa Cruz Katherine Isbister has found that people engage in fidgeting when they’re trying to pay attention to a task that’s taking a long time, or in a long meeting (even if at the annoyance of those around them). Others use fidgeting objects to calm themselves down.