She wrote, too, about the Navajo Indians, a group she studied with her husband, Walter Dyk, an anthropologist.

ECONOMIST: Ruth Dyk

Psychologist Dr Brent Scott studied a group of bus drivers over a two week period who were asked to smile when dealing with the public.

FORBES: Faking It Makes You Miserable

Instead, Dr Miller studied 32 depressed women who were not in hospital, and matched them with a control group of healthy women.

ECONOMIST: The power of negative thinking