Cite as: 534 U. S. 407 (2002)
Opinion of the Court
consisted of a special and serious lack of ability to control behavior.
In recognizing that fact, we did not give to the phrase "lack of control" a particularly narrow or technical meaning. And we recognize that in cases where lack of control is at issue, "inability to control behavior" will not be demonstrable with mathematical precision. It is enough to say that there must be proof of serious difficulty in controlling behavior. And this, when viewed in light of such features of the case as the nature of the psychiatric diagnosis, and the severity of the mental abnormality itself, must be sufficient to distinguish the dangerous sexual offender whose serious mental illness, abnormality, or disorder subjects him to civil commitment from the dangerous but typical recidivist convicted in an ordinary criminal case. 521 U. S., at 357-358; see also Foucha v. Louisiana, 504 U. S. 71, 82-83 (1992) (rejecting an approach to civil commitment that would permit the indefinite confinement "of any convicted criminal" after completion of a prison term).
We recognize that Hendricks as so read provides a less precise constitutional standard than would those more definite rules for which the parties have argued. But the Constitution's safeguards of human liberty in the area of mental illness and the law are not always best enforced through precise bright-line rules. For one thing, the States retain considerable leeway in defining the mental abnormalities and personality disorders that make an individual eligible for commitment. Hendricks, 521 U. S., at 359; id., at 374-375 (Breyer, J., dissenting). For another, the science of psychiatry, which informs but does not control ultimate legal determinations, is an ever-advancing science, whose distinctions do not seek precisely to mirror those of the law. See id., at 359. See also, e. g., Ake v. Oklahoma, 470 U. S. 68, 81 (1985) (psychiatry not "an exact science"); DSM-IV xxx ("concept of mental disorder . . . lacks a consistent operational definition"); id., at xxxii-xxxiii (noting the "imperfect fit between the questions of ultimate concern to the law and
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