§ 19.01 General Rule

§19.01 General Rule

Generally speaking, a person is justified in using force to protect a third party from unlawful use of force by an aggressor.¹ The intervenor's right to use force in such circumstances parallels the third party's right of self-defense;² that is, the intervenor may use force when, and to the extent that, the third party would apparently be justified in using force to protect herself.³ Thus, deadly force is justified if the intervenor has reasonable grounds for believing that such force is necessary to prevent the danger of imminent death or grievous bodily injury to the innocent third party.⁴

Some potential limits to this rule exist. First, the defense originally was limited to protection of persons⁵ related to the intervenor by consanguinity, marriage, or employment relation.⁶ This limitation no longer applies.

Second, a majority of jurisdictions once applied the "alter-ego rule": An intervenor could only use force to defend a third party if the latter party in fact would have been justified in using force, and force in the same degree, in self-defense.⁷ This means that D, the intervenor, was placed in the shoes of X, the person being defended, and acted at her peril. If X in fact had no right of self-defense, even though a reasonable person observing the situation would have believed that X did have a right of self-defense, D was not justified in using force to protect X. The alter-ego doctrine, when applied, represented an exception to the common law rule that an actor is justified in using force based on reasonable, albeit mistaken, appearances.

The justification for the alter-ego rule is best understood if one considers the following not-uncommon scenario: D comes upon an apparently unlawful attack by V on X; D defends X; D subsequently learns that V was an undercover police officer using lawful force against an unlawfully resistant X. Permitting D in such circumstances to act on reasonable appearances, it is said, creates "a dangerous precedent . . . that plain-clothes police officers attempting lawful arrests over wrongful resistance are subject to violent interference by strangers ignorant of the facts."⁸

Largely due to the influence of the Model Penal Code, however, the overwhelming⁹ majority view today is that an intervenor may use force to the extent that such force reasonably appears to the intervenor to be justified in defense of the third party, even if those appearances prove false. As one court has nicely put the two...