68
O'Connor, J., dissenting
Such rules may or may not help the prosecution, and when they do help, do so only incidentally. While due process does not "ba[r] States from making changes . . . that have the effect of making it easier for the prosecution to obtain convictions," McMillan v. Pennsylvania, 477 U. S., at 89, n. 5 (emphasis added), an evidentiary rule whose sole purpose is to boost the State's likelihood of conviction distorts the adversary process. Cf. Washington, 388 U. S., at 25 (Harlan, J., concurring in result). Unlike Chambers and Washington, where the State at least claimed that the evidence at issue was unreliable, Montana does not justify its rule on grounds such as that intoxication evidence is unreliable, cumulative, privileged, or irrelevant. The sole purpose for this disallowance is to keep from the jury's consideration a category of evidence that helps the defendant's case and weakens the government's case.
The plurality brushes aside this Court's precedents as variously fact bound, irrelevant, and dicta. I would afford more weight to principles enunciated in our case law than is accorded in the plurality's opinion today. It seems to me that a State may not first determine the elements of the crime it wishes to punish, and then thwart the accused's defense by categorically disallowing the very evidence that would prove him innocent.
II
The plurality does, however, raise an important argument for the statute's validity: the disallowance, at common law, of consideration of voluntary intoxication where a defendant's state of mind is at issue. Because this disallowance was permitted at common law, the plurality argues, its disallowance by Montana cannot amount to a violation of a "fundamental principle of justice." Ante, at 43-51.
From 1551 until its shift in the 19th century, the common-law rule prevailed that a defendant could not use intoxication as an excuse or justification for an offense, or, it must be assumed, to rebut establishment of a requisite mental state.
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