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Why "Not Guilty by Reason of Insanity" Exists

Animals used to be tried for murder. Changing that changed the legal process.

Key points

  • "Not Guilty by Reason of Insanity" represents a change from viewing guilt based on "what" someone to also considering "why."
  • Humans have a capacity for abstract reasoning that animals do not. This reasoning is important for human environments.
  • Moral reasoning is one type of abstract reasoning deemed essential for human environments.
  • Two centuries ago, legal authorities determined that moral reasoning must be intact for someone to be criminally guilty.

Over the past several months I took a break from writing to start providing “Not Guilty By Reason Of Insanity” evaluations in a neighboring state. Doing this led me to realize that many people question why there is any need for this sort of evaluation.

It is understandable why people question the legitimacy of this defense. We all want to see people answer for their crimes, and the idea that someone could just say, “Sorry, I didn’t know what I was doing,” and get away with hurting someone seems wrong.

So, it is interesting to look at where the logic for legal plea comes from. And a good place to start is the 1400s, because that is when there are records of animals being put on trial. In the magazine History (“Pigs Might Try," November 2020), Alexandar Lee writes about how pigs, horses, dogs, and sheep were put on trial for crimes ranging from damaging property to murder from around 1266 through the 1400s (when the largest number of such trials were recorded) and beyond.

Areas that allowed for animal trials and those that did not differed on one issue: When animal trials were allowed, these were in areas (mostly in France and surrounding regions where the legal focus was entirely on behavior. There was no focus on “why” something occurred; only on “what” happened. In regions where laws did not allow for animal trials, laws reflected expectations that individuals could reason their behaviors.

In 1843, British law formally introduced the “McNaghten rule," which created a presumption that a person was guilty of their crimes unless the defense could prove that “at the time of committing the act, the accused was laboring under such a defect of reason…that he did not know what he was doing was wrong." Evidence supports that this legal approach was used throughout Europe and was at least informally accepted in places where animal trials had been prominent.

So, what is this “reason” that became the essential factor in deciding whether an individual should be found guilty of their crimes? There are several types of reasoning, and most are possessed by both animals and humans, but this reasoning type is an important one they do not share. Animals can make rational choices about which behaviors are most likely to obtain a goal. They are able to adapt behaviors based on their perceptions of what others are feeling. They can recall specific events, use tools, and solve basic problems. Each of these represents a reasoning type both humans and animals share. In fact, the only reasoning types animals do not show is reasoning associated with verbal language. This includes “moral reasoning," which requires distinguishing between verbal concepts like “right” and “wrong." These are abstract constructs reflected in symbolic communication (i.e. “words”) understood by members of a human community.

Several hundred years ago it was determined that animals did not have the capacity to understand abstract concepts necessary for moral reasoning, and this is still the view today. Animals can understand the differences between “good” and “bad” outcomes (i.e. getting things to turn out in positive or negative ways) and “effective” and “ineffective” approaches (i.e. approaches that minimize the amount of effort required and those that waste effort). These are both examples of concrete reasoning that humans and nonhumans share. What animals do not share is an ability to understand more abstract constructs reflecting societal definitions of morally “right” or “wrong."

Humans need this type of reasoning to function in human environments. Animals do not because they live in different environments. When a human loses that type of reasoning ability, they lose the ability to function adequately in human environments. Having lost the ability to rationally decide if they are doing something society considers “wrong," as opposed to recognizing something is “wrong” but choosing to ignore that conclusion, is the reason people may not be held responsible for those crimes.

This, of course, does not mean that humans all agree on the definitions of “right” and “wrong." What moral reasoning means is that humans can infer, based on available evidence, a general definition of what is morally “right” or “wrong." This represents an ability to generalize global definitions based on pieces of individual evidence. What is important is that the person has the ability for this type of reasoning, not that their reasoning leads to any particular decision. An individual who decides robbing a bank is OK in their particular situation, despite knowing it is morally wrong, is found guilty. Knowing something is wrong but choosing to do it anyway does not reflect problems in moral reasoning.

When in the midst of a major psychiatric disorder like schizophrenia, some humans lose the ability to make decisions about whether something is morally “right” or “wrong." What happens is basically that parts of the brain important for recognizing moral concepts either break down or stop communicating with other parts of the brain. These brain areas may still be there but the intense neurological activity associated with the psychiatric disorder overrides the ability to communicate with those areas. In these cases, the person becomes only able to see what they determine to be practical in the moment and not to define their actions in any more global ways.

This is when a person may be found “not guilty” for criminal behavior due to their being unable to make moral decisions. It is important to note here that the person is not then set free but is, rather, typically sent to a guarded setting where they receive the treatment needed for their serious psychiatric condition.


Lee, Alendar (2020). "Pigs Might Try". History, November 2020 edition