Why does criminal liability usually require a voluntary act?

In criminal law, the requirement for a voluntary act—known as actus reus—is a fundamental principle because criminal liability is generally predicated on human agency and choice. By definition, an act must be a form of voluntary human behavior to be punishable.1

This requirement ensures that individuals are only held accountable for conduct they have the capacity to control. Consequently, involuntary movements, such as those occurring during an epileptic seizure or sleepwalking, do not constitute criminal acts, even if they result in harm to others. This principle upholds the ethical standard that justice should not penalize individuals for events beyond their conscious volition.1

While the law requires a voluntary act, liability can also arise from an omission—a failure to act—provided the accused was under a specific legal duty to act and was reasonably capable of doing so. This duty may be established by statute or by the nature of a relationship, such as the obligation of parents to provide for their children.1