Introductory note: Grouped within this chapter are a number of affirmative defenses (that is, defenses as to which, generally, the defendant must bear the burden of proof) that will allow the defendant to escape conviction, even though the prosecution may be able to prove all the elements of the crime. These defenses are: (1) duress; (2) necessity; (3) self-defense; (4) defense of others; (5) defense of property; (6) law enforcement (arrest, prevention of crime and escape); (7) consent; (8) maintenance of authority; and (9) entrapment. While the underlying rationale varies somewhat from defense to defense, there are two recurring reasons for exculpating the defendant: (1) because his conduct was a choice of the lesser of two evils; and (2) because his conduct, even if not a choice of the lesser of two evils, was all that a person of ordinary firmness or courage would do in the situation.
A. Justification vs. excuse: Courts occasionally denominate some of the defenses discussed in this chapter as being “justifications,” or, on the contrary, “excuses.” Thus the defense of “necessity” (infra, p. 111) is usually thought to be a justification, whereas duress is generally thought of as an excuse. The theory behind the two labels is that “justification” applies where the defendant took the better, more socially useful, and morally defensible of two actions; “excuse” applies where he did not necessarily do so, but did all that he could have been expected to do. For instance, if the defendant is forced by terrorists to join in a bank robbery or else be killed, his doing so would be “excused,” not “justified,” under the doctrine of duress.
1. Significance of distinction: Generally, there is no great significance to the distinction. However, it has been suggested that a claim of justification is transferable to a third party, whereas a claim of excuse is not. Suppose that a starving woman steals bread to feed herself and her child; if her claim of necessity is a justification, then a third person ought to be able to take the bread on her behalf. But if her conduct is merely excused, then the defense would be personal to her, and a third person could not commit the act for her and claim the defense. See Fletcher, pp. 761-62. It is not clear that courts would recognize this distinction, however.
B. Effect of mistake of fact: One problem that arises with respect to almost all of the defenses in this chapter is the effect of a mistake of fact by the defendant. That is, if the defendant is mistaken about the need for, say, self-defense, does he lose his right to assert that defense? The courts have frequently divided the problem into two categories: (1) the effect of a reasonable mistake; and (2) the effect of an unreasonable one.
1. Common-law approach has no unified rule: In general (but not always), the common-law approach has been that a reasonable mistake will not negate the privilege. (But see the privilege of arrest, and that of prevention of crime by private citizens, both of which are voided by a reasonable mistake; infra, p. 136). An unreasonable mistake, however, will negate virtually all of these defenses, under the common law.