You've heard the saying "seeing is believing." That's the theory behind the use of demonstrative evidence, tangible items like photographs, models, and charts that can aid a jury's understanding of the facts. Unlike spoken testimony, demonstrative evidence can appeal to all of a juror's senses.
Long ago, demonstrative evidence was limited mostly to criminal cases. It was Melvin Belli who popularized the use of demonstrative evidence in civil cases. Here's what Belli says in his autobiography about his first use of demonstrative evidence:
I had learned a valuable lesson, one taught by Captain Kidd at Boalt, but only half realized: that jurors learn through all their senses, and if you can tell them and show them, too, let them see and feel and even taste or smell the evidence, then you will reach the jury. "A good trial attorney," said Captain Kidd, "is a good teacher. He doesn't overestimate the jury's knowledge, either. He has to show them." A lot of lawyers knew this, of course, and some of those who did were even using the knowledge in court. But, for some reason, they only saw it as a tool in criminal trials--when someone was accused of breaking a law, murder or robbery or rape. The D.A. would bring in the lethal weapon, the autopsy picture, even the aborted fetus, the blood-spattered scene of the crime. All would be admitted, no matter how grisly. But they weren't using demonstrative evidence nearly enough in civil cases. (It would be years before I could get a picture of the deceased admitted in a civil case.) It didn't take long before I would step into the breach. Today even more demonstrative evidence is being used in civil cases than in criminal cases, as I was to advocate in my six-volume Modern Trials.
This passage was written about thirty years ago. The use of demonstrative evidence continues unabated today.
Next: The use of demonstrative evidence in modern times.