Free Will, Determinism and Legal Responsibility

The law operates on a commonsense model of agency: individuals possess free will, the capacity to choose between courses of action, and therefore can be held morally and legally responsible for their choices. 

This model is challenged by determinism, the philosophical position that every event, including human decisions and actions, is the inevitable consequence of preceding states of affairs. A hard determinist would argue that given a person’s genetic makeup, brain chemistry, and the sum total of their environmental influences-from prenatal nutrition to childhood trauma to present stimuli-only one action was ever possible. Modern neuroscience adds force to this view, identifying neural correlates of decisions that appear in brain scans milliseconds before the conscious feeling of having decided. If our actions are the determined products of our biology and environment, the concept of desert-that someone deserves punishment-seems to collapse. How can we justly punish someone for an act they could not have avoided?

The legal system has responded to this tension not by embracing determinism, but by developing practical, rule-based proxies for moral blameworthiness. The insanity defense is the most direct concession. The M’Naghten Rules, for instance, excuse a defendant if, due to a disease of the mind, they did not know the nature and quality of their act or that it was wrong. This recognizes that certain deterministic causes, severe psychosis, destroy the minimal rational capacity required for culpability. Similarly, the law recognizes duress and coercion as mitigating factors, acknowledging that overwhelming external pressure can severely constrain choice. However, the law generally draws a bright line at the border of internal compulsion. It treats desires, impulses, and even psychopathic lack of empathy as part of the “character” for which one is responsible, while treating overt psychosis as an excusing condition. This is a pragmatic, not a philosophically pure, compromise.

The system ultimately rests on compatibilism, a philosophical position that free will and determinism are compatible. Compatibilists redefine free will not as a metaphysical ability to do otherwise given the exact same past, but as the ability to act according to one’s own desires and reasons without external coercion. Under this view, a murderer acting on his own hatred is acting freely, while one whose finger is forced onto a trigger is not. The law’s purpose, then, is not to punish metaphysical guilt but to regulate behavior in a society of causes and effects. Punishment serves forward-looking aims: deterring the defendant and others (influencing future causal chains), incapacitating dangerous individuals, and reinforcing social norms. Retribution, the backward-looking notion of just deserts, becomes more symbolic-a societal expression of condemnation that affirms shared values. 

The law thus sidesteps the unanswerable metaphysical question and focuses on the practical necessity of maintaining a rule-based order that presupposes, for its daily functioning, that most people have the cognitive and volitional capacity to understand and follow rules.