Automatism is a state in which a person performs actions without conscious control or awareness of what they are doing. The term appears most often in criminal law, where it serves as a defense arguing that a person cannot be held responsible for actions that were involuntary. But automatism also has roots in neuroscience and medicine, where it describes real biological states in which the brain carries out complex movements without input from conscious decision-making.
The Legal Definition
The most widely cited legal definition comes from Lord Denning in the 1963 case Bratty v. Attorney General for Northern Ireland. He described automatism as “an act which is done by the muscles without any control by the mind such as a spasm, a reflex action or a convulsion; or an act done by a person who is not conscious of what he is doing such as an act done whilst suffering from concussion or whilst sleepwalking.” That definition still anchors how courts in England, Canada, Australia, and the United States handle automatism claims today.
The core principle is straightforward: criminal law requires that a guilty act be voluntary and willed. A person who acts mechanically, without exercising free will or even knowing what they are doing, has not committed a voluntary act in the legal sense. The U.S. Model Penal Code spells this out explicitly, listing reflexes, convulsions, bodily movements during unconsciousness or sleep, and conduct during hypnosis as examples of involuntary acts that do not satisfy the requirements for criminal liability.
Sane vs. Insane Automatism
Courts divide automatism into two categories based on what caused the involuntary state, and the distinction has major consequences for the person claiming it.
Sane automatism results from an external factor: something that happened to the person from outside their body. A blow to the head, an injection of a drug, or a hypoglycemic episode caused by taking insulin all qualify. If a court accepts sane automatism, the result is a complete acquittal. The person walks free because the law treats their actions as genuinely involuntary.
Insane automatism results from an internal factor: a disease or condition of the brain itself. Epilepsy, a brain tumor, or a psychotic episode would fall into this category. A successful insane automatism defense does not lead to a simple acquittal. Instead, courts typically return a special verdict of “not guilty by reason of insanity,” which can result in mandatory hospitalization or psychiatric supervision. The logic is that an internal condition may recur and could lead to future violence, so the person needs oversight even though they were not acting with criminal intent.
The diabetes cases illustrate how fine this line can be. A diabetic who takes insulin and then experiences a hypoglycemic episode (low blood sugar triggered by the external act of injecting insulin) has a claim to sane automatism. But a diabetic whose blood sugar spikes because they failed to take insulin (hyperglycemia caused by the internal disease process itself) would fall under insane automatism. The medical reality is nearly identical, yet the legal outcomes are entirely different.
Common Triggers Courts Recognize
Sleepwalking is one of the most established grounds for an automatism defense. The Latin legal principle “in somno voluntas non erat libera,” meaning a sleeping person has no free will, captures the reasoning. Courts have consistently held that a sleepwalker acts involuntarily regardless of how complex their behavior appears. Sleepwalkers can walk, open doors, drive cars, and even carry out violent acts, all without conscious awareness. Determining the person’s level of consciousness during the episode is critical, because it helps a jury decide whether the behavior is consistent with a genuine parasomnia.
Epileptic seizures are another recognized trigger. During certain types of seizures, a person may pick at their clothes, walk aimlessly, pick up objects, or mumble, with no memory of the episode afterward. These actions are involuntary, and the person cannot appreciate what they are doing or its consequences. Courts have increasingly classified seizure-related automatism as non-insane (sane) automatism, though the categorization varies by jurisdiction.
Concussions, severe physical trauma, and drug reactions round out the list. In one North Carolina case, a court found that a defendant in a dissociative state caused by PTSD should have received an automatism instruction from the judge. Dissociative states sit in a gray area, though. Courts are generally more skeptical of psychological triggers than physical ones, and simply claiming an “abnormal state of consciousness” is usually not enough to meet the legal criteria on its own.
How the Brain Produces Automatic Actions
Neuroscience confirms that the brain can generate complex movements without conscious involvement. Several brain regions work together to plan, initiate, and execute motor actions, and much of this machinery operates below the level of awareness.
The basal ganglia, a cluster of structures deep in the brain, play a central role. These circuits sit at the intersection of movement, cognition, and emotion, and they help initiate, inhibit, and switch between behaviors. The thalamus, which relays information between brain regions, works closely with the basal ganglia in these processes. Together, they can drive habitual or automatic motor sequences without requiring a conscious “go” signal.
Research using brain imaging has shown that motor preparation areas, particularly the supplementary motor area near the top of the brain, become active before a person is consciously aware of deciding to move. In a famous experiment by Benjamin Libet, researchers recorded electrical brain activity that preceded voluntary movement by a measurable interval, yet participants reported deciding to move only after that activity had already begun. Later studies extended this finding, showing that patterns of activity in the frontal cortex could predict a person’s decision to move several seconds before they reported being aware of making it.
This does not mean every action is unconscious. But it does show that the brain has robust pathways for generating actions that bypass or precede conscious awareness. In states like sleepwalking, seizures, or severe dissociation, the higher-level brain regions responsible for conscious monitoring and decision-making are effectively offline, leaving these automatic motor systems to operate on their own.
Proving Automatism in Court
Raising an automatism defense is not as simple as claiming you don’t remember what happened. Courts treat automatism as an affirmative defense, which generally means the burden falls on the defendant to prove it to the jury’s satisfaction. There is one exception: if the prosecution’s own evidence raises the possibility of automatism, the burden shifts to the state to prove the defendant was conscious beyond a reasonable doubt.
Expert medical testimony is typically essential. A defendant usually needs a physician, psychiatrist, or neurologist to explain the specific medical condition involved and to testify that the defendant was in an automatistic state at the time of the alleged offense. Courts have rejected automatism claims where the only evidence was the defendant’s own statement that they could not remember their actions. In one case, a defendant who told police a detailed account of events shortly after the crime but later claimed poor memory at trial was denied an automatism instruction, since the court attributed the memory gap to the passage of time rather than unconsciousness.
Even with expert support, success is rare. In England and Wales, fewer than 30 insanity-based defenses (which include insane automatism) succeed in any given year. Sane automatism claims are even less common. The bar is deliberately high: courts want to prevent the defense from becoming a loophole for anyone who claims a blackout after committing a crime.
Why the Distinction Matters
The stakes of automatism are unusually binary compared to most legal defenses. A successful sane automatism claim results in a full acquittal with no conditions attached. The person is treated as though no crime occurred, because in legal terms, no voluntary act took place. Insane automatism, by contrast, triggers the mental health system. The person avoids a criminal conviction but may face indefinite hospitalization or a supervision order, depending on the jurisdiction and the severity of the offense.
This creates a difficult choice for defendants with conditions like epilepsy or dissociative disorders. Arguing automatism means submitting to medical evaluation and potentially being labeled as having a “disease of the mind,” a legal term that does not map neatly onto modern psychiatric diagnosis. Some defendants choose not to raise automatism at all rather than risk a special verdict that leads to forced treatment. The legal framework, rooted in rules from 1843, has been widely criticized for failing to keep up with current medical understanding of the brain and consciousness.