When I was a young university student in a large sociology class in criminology, the Professor told us that we were going to participate in an experiment on the reliability of witnesses, and that we should come to the next class with five dollars.
When we arrived the next day, he put our money in a pot and said that after the experiment -- he was about to show us a brief film of a chaotic crime scene -- it would be divided and given equally to all those who got the correct answer to a single question about a detail of the film. As young students without much money, we all got excited, each of us secretly calculating how much food (or beer) getting the right answer was going to bring. We were motivated to observe carefully.
Then he showed us a grainy, amateurish, thirty-second film of a loud, poorly-lit party scene, in which a lot of people were crowded together, milling about, drinking and dancing. Suddenly, a man with what looked like a stocking over his head burst through a door, waved a weapon at the crowd - which caused a lot of panic and screaming - and as suddenly left the room. The clip ended. It was actually a little scary. And there was a lot of buzz in the class right afterward.
Then came the question: "What kind of weapon was the masked man waving at the crowd?" (a pistol, a knife, a pipe, a sawed-off shotgun?)
A few said the guy either didn't have a weapon, or they didn't see one. Everyone else was sure - absolutely sure - of what they had seen. And most gave different answers: He had a handgun. He had a sawed-off shotgun. He had a long knife. A short knife. A pipe. A short baseball bat. And so on. Students who saw one thing, were in disbelief, to the level of scorn, that others saw something else.
What was so striking was the certainty of each witness. "Are you absolutely certain?" "Would you swear on a Bible?" Yes. Yes. And again, Yes. And I was no different. I grew up learning about and using guns, and said I was so sure it was a certain kind of revolver that I was willing to put more money into the pot.
Then he showed us the film clip again, but this time in slow motion.
You had to be there to understand the level of disbelief and how shocked we all were to see that what the guy was actually waving at the crowd was ... a banana!
The film was a set-up. It had been made as an object- lesson in human misperception for criminology students. So at first we felt a bit tricked. But we all agreed that there was never so persuasive and effective a lesson on our own misperceptions, misplaced confidence, and the unreliability of sworn witnesses.
Then he gave us back our money. We were no richer or poorer, but a lot wiser and more skeptical.