Professor Dunbar, a leading evolutionary psychologist, said touch helps maintain relationships by triggering the release of endorphins, the feel-good brain chemicals usually associated with exercise.

The release of endorphins can relieve headaches, according to research at the headache Clinic at Southern Illinois University.

Overall, an increase in the brain's production of endorphines occurs when the body is put under stress such as long distance running, and the endorphines then cause a positive or negative mood change.