Cite as: 521 U. S. 346 (1997)
Kennedy, J., concurring
ex post facto concerns. Moreover, the Act clearly does not have retroactive effect. Rather, the Act permits involuntary confinement based upon a determination that the person currently both suffers from a "mental abnormality" or "personality disorder" and is likely to pose a future danger to the public. To the extent that past behavior is taken into account, it is used, as noted above, solely for evidentiary purposes. Because the Act does not criminalize conduct legal before its enactment, nor deprive Hendricks of any defense that was available to him at the time of his crimes, the Act does not violate the Ex Post Facto Clause.
III
We hold that the Kansas Sexually Violent Predator Act comports with due process requirements and neither runs afoul of double jeopardy principles nor constitutes an exercise in impermissible ex post facto lawmaking. Accordingly, the judgment of the Kansas Supreme Court is reversed.
It is so ordered.
Justice Kennedy, concurring.
I join the opinion of the Court in full and add these additional comments.
Though other issues were argued to us, as the action has matured it turns on whether the Kansas statute is an ex post facto law. A law enacted after commission of the offense and which punishes the offense by extending the term of confinement is a textbook example of an ex post facto law. If the object or purpose of the Kansas law had been to provide treatment but the treatment provisions were adopted as a sham or mere pretext, there would have been an indication of the forbidden purpose to punish. The Court's opinion gives a full and complete explanation why an ex post facto challenge based on this contention cannot succeed in the action before us. All this, however, concerns Hendricks alone. My brief, further comment is to caution against dangers in-
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