Monday, August 25, 2008

Props for the courtroom are called demonstrative evidence

In legal terminology props for the courtroom are somewhat grandly called demonstrative evidence. An old Chinese proverb says: “tell me and I will forget, show me and I may remember, involve me and I will understand”. The proverb is quoted is from an article by a forensic engineering company that also contains an excellent cartoon about the futility of detailed verbiage alone.

Both lawyers and expert witnesses know that most jurors think visually. They use props for showing in addition to verbally telling their story. Props were repeatedly used by the Los Angeles trial lawyer Earl Rogers, who was the basis for the TV character Perry Mason. More recently a demonstration involving gloves was an important part of the defense in the infamous OJ Simpson trial.

The abstract concepts of conservation of momentum and energy are the basis for vehicle accident reconstruction, as described in a recent Slate article on The Ferrari that split in half. Most professors of mechanical engineering could put a jury to sleep in less than five minutes by discussing the concepts starting with words, and then continuing with unfamiliar equations written on a flip chart.

About twenty years ago I saw a video of an expert testifying brilliantly on this topic. The lawyer called the professor up to the stand. He walked up with a box containing two brightly colored basketballs in one hand. The other hand was dribbling a third basketball. All the jurors woke up. Then he got down on his knees on the floor. He rolled one ball at another to illustrate a collision. Also, he told the jurors that some actually were familiar with momentum transfer from playing billiards or pool. He only started to tell them after he’d already both shown them and involved them.

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