From an article in Smithsonian:
Scientists testing emotions in research subjects have resorted to a variety of techniques, including playing emotional music, exposing volunteers to hydrogen sulfide (“fart spray”) to generate disgust or asking subjects to read a series of depressing statements like “I have too many bad things in my life” or “I want to go to sleep and never wake up.” They’ve rewarded test subjects with money or cookies to study happiness or made them perform tedious and frustrating tasks to study anger.
“In the old days, we used to be able to induce fear by giving people electric shocks,” Levenson says.
Ethical concerns now put more constraints on how scientists can elicit negative emotions. Sadness is especially difficult. How do you induce a feeling of loss or failure in the laboratory without resorting to deception or making a test subject feel miserable?
“You can’t tell them something horrible has happened to their family, or tell them they have some terrible disease,” says William Frey II, a University of Minnesota neuroscientist who has studied the composition of tears.
But as Gross says, “films have this really unusual status.” People willingly pay money to see tearjerkers—and walk out of the theater with no apparent ill effect. As a result, “there’s an ethical exemption” to making someone emotional with a film, Gross says.
Read the rest of the article by Richard Chin at Smithsonian.