Derived from the Latin word scire, which means "to know", scienter refers to a knowledge or awareness of wrongdoing by the defendant in a civil or criminal trial. In order to prevail in a common law proceeding, the plaintiff or prosecutor must allege in his pleadings, and prove by the evidence, that the offending party acted with knowledge of wrongdoing. Mens rea, a legal phrase meaning "guilty mind", is often used interchangeably with scienter, usually in criminal trials. Most laws that impose severe penalties or sanctions include provisions requiring that a person must act intentionally, willfully, or knowingly to be held legally accountable. Reckless conduct, defined as behavior that reasonable persons know to be illegal or unsafe, constitutes scienter in the eyes of most courts.
Criteria for establishing scienter may be subjective, objective, or a combination of both. Examples of subjective evidence include admissions or apologies by the defendant, and past conversations or e-mails showing knowledge of wrongdoing. Objective evidence may include signs of premeditation and the defendant's past experiences with similar events. Juries know that people of average intelligence understand the laws of cause and effect, and can foresee the likely outcomes of a given action. Scienter is objectively imputed to the defendant when a rational person would have been able to predict the probable outcome or would have known the action was wrong.
In cases of limited mental capacity due to youth or mental illness, a defendant may lack an adequate understanding of the ramifications of his actions. Under these circumstances, juries must consider what a reasonable person with the same intellectual capacity as the accused would understand. For this reason, the courts, applying the doli incapax rule, presume that children under the age of ten years old cannot be held responsible for their actions under the law. When certain tests outlined by the M'Naghten Rules are satisfied, mental illness also absolves a defendant from liability. Scienter cannot be established in such cases.
Plaintiffs do not always need to prove that the defendant acted with willful intent or reckless disregard. Strict liability statutes, which govern certain cases, impose civil or criminal liability without regard to the defendant's mental state. In a strict liability law case, a plaintiff only needs to demonstrate that an injury happened and that the defendant was responsible. Most actions involving animals and vehicular traffic offenses fall into this category. The law presumes strict liability in circumstances that are inherently hazardous, thereby discouraging careless behavior and compelling potential defendants to take every possible precaution.