Jurors use mental state information to assess breach in negligence cases.

To prove guilt, jurors in many countries must find that the criminal defendant acted with a particular mental state. However, this amateur form of mindreading is not supposed to occur in civil negligence trials. Instead, jurors should decide whether the defendant was negligent by looking only at his actions, and whether they were objectively reasonable under the circumstances. Even so, across four pre-registered studies (N = 782), we showed that mock jurors do not focus on actions alone. US mock jurors spontaneously rely on mental state information when evaluating negligence cases. In Study 1, jurors were given three negligence cases to judge, and were asked to evaluate whether a reasonably careful person would have foreseen the risk (foreseeability) and whether the defendant acted unreasonably (negligence). Across conditions, we also varied the extent and content of additional information about defendant's subjective mental state: jurors were provided with evidence that the defendant either thought the risk of a harm was high or was low, or were not provided with such information. Foreseeability and negligence scores increased when mock jurors were told the defendant thought there was a high risk, and negligence scores decreased when the defendant thought there was a low risk, compared to when no background mental state information was provided. In Study 2, we replicated these findings by using mild (as opposed to severe) harm cases. In Study 3, we tested an intervention aimed at reducing jurors' reliance on mental states, which consisted in raising jurors' awareness of potential hindsight bias in their evaluations. The intervention reduced mock juror reliance on mental states when assessing foreseeability when the defendant was described as knowing of a high risk, an effect replicated in Study 4. This research demonstrates that jurors rely on mental states to assess breach, regardless of what the legal doctrine says.