Samuel McNerney in Scientific American:
In 1996, Lyle Brenner, Derek Koehler and Amos Tversky conducted a study involving students from San Jose State University and Stanford University. The researchers were interested in how people jump to conclusions based on limited information. Previous work by Tversky, Daniel Kahneman and other psychologists found that people are “radically insensitive to both the quantity and quality of information that gives rise to impressions and intuitions,” so the researchers knew, of course, that we humans don’t do a particularly good job of weighing the pros and cons. But to what degree? Just how bad are we at assessing all the facts?
To find out, Brenner and his team exposed the students to legal scenarios. In one, a plaintiff named Mr. Thompson visits a drug store for a routine union visit. The store manager informs him that according to the union contract with the drug store, plaintiffs cannot speak with the union employees on the floor. After a brief deliberation, the manager calls the police and Mr. Thompson is handcuffed for trespassing. Later the charges were dropped, but Mr. Thompson is suing the store for false arrest.
All participants got this background information. Then, they heard from one of the two sides’ lawyers; the lawyer for the union organizer framed the arrest as an attempt to intimidate, while the lawyer for the store argued that the conversation that took place in the store was disruptive. Another group of participants – essentially a mock jury – heard both sides.
The key part of the experiment was that the participants were fully aware of the setup; they knew that they were only hearing one side or the entire story. But this didn’t stop the subjects who heard one-sided evidence from being more confident and biased with their judgments than those who saw both sides. That is, even when people had all the underlying facts, they jumped to conclusions after hearing only one side of the story.