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The Advantage Blog

  • Tsongas Litigation Consulting


One of the most interesting components of facilitating a mock trial is seeing how jurors actually reason during deliberations. Most jurors have no training in formal logic or argumentation or in the fields in which the trial evidence is being presented (law, medicine, intellectual property, workplace safety, etc.), so they are unfamiliar with the criteria that would be used by experts in these fields. The specific jury instructions have yet to be provided, and even if they had, jurors often have a hard time following the rules as given.

This is not a totally unique situation for most people, as they are often asked to make decisions or evaluate arguments in their everyday life without the benefit of such expertise. Voting decisions are another example of where people are asked to decide which candidate presents a better set of solutions for societal problems, and yet the voters typically do not have training in the specific fields of science, social policy, or economics, or more generally formal logic and argumentation.

Even though people do not typically follow formal logic rules, they do use a more “informal” logic in making their decisions. The distinction between the type of formal logic or field specific argumentation and the more typical reasoning used by non-experts has been set out in different places, but the description I have found most useful was advanced by Chaim Perelman and Lucie Olbrechts-Tyteca in the book The New Rhetoric. Perelman and Olbrechts-Tyteca wanted to shift the focus away from formal logic and empiricism and instead create “a theory of argumentation that describes the manner in which argumentation takes place–as the complement to formal logic.”[i] In The New Rhetoric, Perelman and Olbrechts-Tyteca provide a theory of argumentation based on “informal” logic.

In their book and other articles, the authors provide a number of different categories of informal logic. One of the types of reasoning they identify that I see used in many mock trials is an argument form called “person/act.” Person/act arguments are a type of “co-existential reasoning,” or argument by sign. Sign reasoning suggests that one thing can act as a “sign” of another thing, even though no direct causal connection is observed. For example, if you step outside and see wet pavement and people lowering their umbrellas, it is fair to assume that it was raining, even though you did not actually see the drips falling from the sky.

Person/act reasoning occurs when one uses evidence of what a person is like or how they behave and then surmises the types of acts in which this person would likely engage. A juror in a recent mock trial in which I participated aptly embodied this reasoning during deliberations. Jurors were examining an email written by a key character in the case. One juror tried to discount the importance of the email, saying, “This email doesn’t prove anything about the claims in the case.” In a way, she was right; as the email did not provide direct evidence or a “smoking gun” that the person had committed misconduct. But another juror suggested it was still relevant, since it “showed what kind of person he is, which tells us that he would very likely do the thing he is accused of.”

Jurors frequently use this type of reasoning, as there is rarely a “smoking gun” that definitively proves or disproves the claims in the case. Instead, jurors look at each of the key characters in the case to try to determine what kind of person they are, and therefore what type of acts they are likely to commit.

Because of the importance and frequency of this type of reasoning, it is critical to establish that a character is or is not the kind of person who would act in a way critical to the theory of a case. There are a few strategies that can be used to make sure jurors get the right view of a character. One way to do this is to establish primary characteristics of a person and distinguish these from transitory or temporary components of a person’s behavior. This can often be done in the direct examination of a witness. Showing multiple examples of certain characteristics occurring over time can help establish the centrality and permanence of certain traits, as opposed to a one-time instance of a trait that is not truly representative. Witnesses need to practice and be encouraged to share this information; especially examples from earlier times in their life that help establish the permanence of the traits. This can also be initiated in depositions as well when questions create opportunities to share the values and beliefs that are core to a person.

Perleman and Olbrechts-Tyteca suggest other ways of creating the perception of permanence in the minds of the audience, such as using proper names when discussing the person, describing them using well-established and understood categories (i.e. “stingy”), and focusing on underlying characteristics that typically influence multiple behaviors (i.e. “generosity”).[ii]

Jurors frequently have a difficult time following the rules of formal logic and the jargon-filled jury instructions. Instead, they will use more informal ways of reasoning such as person/act inferences to help them draw conclusions. It is important to make sure the characters in your trials are seen as possessing centrally-held traits that have become grounded in their actions over time, which would lead to actions that are consistent with your case story.

[i] Frans H. van Eemeren, Rob Grootendorst, and Francisca Henkemans, Fundamentals of Argumentation Theory, (Mahway, NJ: Lawrence Erlbaum Associates, 1996), 95.

[ii] C. Perelman and L. Olbrechts-Tyteca, “Act and Person in Argument,” Ethics, Vol. 61, No. 4 (Jul., 1951), pp. 251-269


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