Demonstrative evidence and substantive evidence
Substantive evidence is what proves a fact: a medical record, a maintenance log, a witness's account of what happened. Demonstrative evidence helps the jury follow it.
The California Supreme Court put it directly in People v. Duenas: "a computer animation is not substantive evidence used to prove the facts of a case; rather it is demonstrative evidence used to help a jury to understand substantive evidence." (People v. Duenas (2012) 55 Cal.4th 1, 21.)
Federal courts now have a rule for this. Federal Rule of Evidence 107, in effect since December 1, 2024, covers "illustrative aids." A court can allow one if its usefulness in helping the jury understand the evidence isn't substantially outweighed by the risk of unfair prejudice, confusion or delay. An illustrative aid isn't evidence, and it doesn't go to the jury room during deliberations unless all parties agree or the court orders it for good cause. When practicable, it's entered into the record. The rule's terminology separates illustrative aids from demonstrative exhibits that are admitted as evidence, so it's worth knowing which one a given exhibit is meant to be.
Common types of demonstrative evidence
Attorneys have used charts, timelines, photographs, maps and scale models for a long time. Courts have compared computer animations to those same tools, describing them as a modern way to illustrate expert testimony. (People v. Hood (1997) 53 Cal.App.4th 965, 969.)
Two newer forms are worth knowing about. A forensic animation shows how an event unfolded, based on the evidence and the experts' opinions. An interactive courtroom exhibit lets a mediator, judge or jury explore a reconstructed space, for example by standing where a witness stood and checking what that person could actually see.
Animation and simulation
Courts treat these two differently, and the difference matters for admissibility. Duenas adopted this description from Harris v. State:
"Animation is merely used to illustrate an expert's testimony while simulations contain scientific or physical principles requiring validation. Animations do not draw conclusions; they attempt to recreate a scene or process, thus they are treated like demonstrative aids. Computer simulations are created by entering data into computer models which analyze the data and reach a conclusion."
Because a simulation reaches its own conclusion, California courts may require the method behind it to have gained general acceptance in the relevant scientific community under the Kelly test. (People v. Kelly (1976) 17 Cal.3d 24, 30.) In Hood, the defendant argued that a computer animation should face the same test. The court disagreed, treating the animation as an illustration of the expert's opinion, the same way it would treat a chart or diagram.
Caustic Studio builds animations. Each one illustrates the opinions of the experts on a case and is built from the evidence they rely on. The analysis stays with the expert, and the animation shows the jury what the expert means.
What California courts look for
A computer animation is admissible if it is "a fair and accurate representation of the evidence to which it relates." (Duenas, 55 Cal.4th at 20.) When the animation illustrates expert testimony, Duenas framed the question this way: "the relevant question is not whether the animation represents the underlying events of the crime with indisputable accuracy, but whether the animation accurately represents the expert's opinion as to those events." (Id. at 21.)
The court then worked through the objections a defendant raised against the animation, and rejected each one. It found the animation wasn't speculative, didn't create an improper air of scientific certainty, and wasn't inadmissibly cumulative. It also found that it was appropriate for the animation to match the other evidence offered. (Id. at 25.) Those objections are a useful checklist for anyone preparing an animation, since opposing counsel is likely to raise them.
Trial courts still weigh any exhibit under Evidence Code section 352, which allows exclusion when the risk of undue prejudice, confusion or wasted time substantially outweighs its value. California's civil jury instructions also include CACI 5020 on demonstrative evidence, which tells jurors how to treat it.
What helps an exhibit hold up
From our side of the work, a few habits make a real difference:
- We scan the scene or object with lidar, blue laser scanning or photogrammetry, so dimensions come from the site itself.
- Camera position, timing and motion should trace back to the expert's opinion and the evidence behind it.
- If the exhibit is challenged, the expert can explain how each part was made.
- Showing the exhibit to the other side well before trial gives time to resolve objections before they become a motion in limine.
- Sound effects, emotional framing and invented detail invite a prejudice objection and add nothing to the expert's opinion.
At mediation and at trial
Demonstrative evidence isn't only for juries. A mediator weighing both sides' positions has the same problem a juror does: understanding something they didn't see. In one of our cases, a mediator cited an interactive sightline exhibit as a key factor in the outcome. In another, a reconstruction of a mechanical failure played a significant role in settlement negotiations. Both cases resolved on confidential terms.
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