Because of blindness of the experimenters in the following problem, one that they posed.
Two bystanders are on a bridge above the tracks and the only way to save the five people is for one bystander to push the other in front of the train, killing the fallen bystander.
If one can push another into the path of a train one can sacrifice themselves.
*AUSTIN, OSCAR P.
Rank and organization: Private First Class, U.S. Marine Corps, Company E, 2d Battalion, 7th Marines, 1st Marine Division, (Rein), FMF. Place and date: West of Da Nang, Republic of Vietnam, 23 February 1969. Entered service at: Phoenix, Ariz. Born: 15 January 1948, Nacogdoches, Tex. Citation: For conspicuous gallantry and intrepidity at the risk of his life above and beyond the call of duty while serving as an assistant machine gunner with Company E, in connection with operations against enemy forces. During the early morning hours Pfc. Austin's observation post was subjected to a fierce ground attack by a large North Vietnamese Army force supported by a heavy volume of hand grenades, satchel charges, and small arms fire. Observing that 1 of his wounded companions had fallen unconscious in a position dangerously exposed to the hostile fire, Pfc. Austin unhesitatingly left the relative security of his fighting hole and, with complete disregard for his safety, raced across the fire-swept terrain to assist the marine to a covered location. As he neared the casualty, he observed an enemy grenade land nearby and, reacting instantly, leaped between the injured marine and the lethal object, absorbing the effects of its detonation. As he ignored his painful injuries and turned to examine the wounded man, he saw a North Vietnamese Army soldier aiming a weapon at his unconscious companion. With full knowledge of the probable consequences and thinking only to protect the marine, Pfc. Austin resolutely threw himself between the casualty and the hostile soldier, and, in doing, was mortally wounded. Pfc. Austin's indomitable courage, inspiring initiative and selfless devotion to duty upheld the highest traditions of the Marine Corps and the U.S. Naval Service. He gallantly gave his life for his country.
That's a bit silly, AndrewC. The problems are given with options. Heroic alternatives, and one can think of many, are not given as a choice. The elegance in an experiment is to get an answer with the simplest design possible. Throwing heroics into the mix, something that not everyone resorts to, is adding a complicating factor. It would be interesting for a future study, but it is certainly not a design flaw for this study.