Cite as: 518 U. S. 37 (1996)
O'Connor, J., dissenting
the State's purpose in trying to improve its likelihood of winning convictions. The final justification proffered by the plurality on Montana's behalf is that Montana's rule perhaps prevents juries, who might otherwise be misled, from being "too quick to accept the claim that the [drunk] defendant was biologically incapable of forming the requisite mens rea," ante, at 50-51. But this proffered justification is inconsistent with § 45-2-203's exception for persons who are involuntarily intoxicated. That exception makes plain that Montana does not consider intoxication evidence misleading—but rather considers it relevant—for the determination of a person's capacity to form the requisite mental state.
A State's placement of a significant limitation on the right to defend against the State's accusations "requires that the competing interest be closely examined." Chambers, 410 U. S., at 295. Montana has specified that to prove guilt, the State must establish that the defendant acted purposely or knowingly, but has prohibited a category of defendants from effectively disputing guilt through presentation of evidence relevant to that essential element. And the evidence is indisputably relevant: The Montana Supreme Court held that evidence of intoxication is relevant to proof of mental state, 272 Mont., at 122-123, 900 P. 2d, at 265, and furthermore, § 45-2-203's exception for involuntary intoxication shows that the legislature does consider intoxication relevant to mental state. Montana has barred the defendant's use of a category of relevant, exculpatory evidence for the express purpose of improving the State's likelihood of winning a conviction against a certain type of defendant. The plurality's observation that all evidentiary rules that exclude exculpa-tory evidence reduce the State's burden to prove its case, ante, at 55, is beside the point. The purpose of the familiar evidentiary rules is not to alleviate the State's burden, but rather to vindicate some other goal or value—e. g., to ensure the reliability and competency of evidence or to encourage effective communications within certain relationships.
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