Lex Mitior: Converse of Ex Post Facto and Window into Criminal Desert

New Criminal Law Review, Vol. 18, No. 2, pp. 167-213, 2015

U of Michigan Public Law Research Paper No. 455

48 Pages Posted: 2 Apr 2015 Last revised: 22 May 2017

See all articles by Peter K. Westen

Peter K. Westen

University of Michigan Law School

Date Written: March 20, 2015


In 2009 New Mexico prospectively repealed the death penalty. Three years later in 2012, New Mexico prosecuted a defendant for a capital murder that was committed before repeal, and it sought to subject him to the death penalty. If state prosecutors had prevailed with the jury, they would have secured the very kind of sentence – death – that state officials had been lauded in Europe for outlawing three years earlier. A prosecution like New Mexico’s could never occur in Europe, and not merely because Europe has long outlawed the death penalty. It could never occur because, in contrast to the law of most American jurisdictions, European states embrace a doctrine known as “lex mitior” (“the milder law”). The latter doctrine is a counterpart to the ex post facto prohibition. Both doctrines both concern retroactivity in criminal law, but they are the converse of one another. The ex post facto doctrine prohibits retroactivity by prohibiting the state from prosecuting persons under criminal statutes that either retroactively criminalize conduct that was hitherto lawful or retroactively increase penalties for conduct that, while unlawful all along, was hitherto punishable less severely. In contrast, lex mitior mandates retroactivity by mandating that criminal defendants receive the retroactive benefits of repealing statutes that either decriminalize conduct altogether or reduce punishments for it. After surveying laws in the United States regarding the retroactive effect of ameliorative repeals, the author addresses whether punishing offenders under harsher laws that obtained at the time of their conduct can serve consequentialist and/or retributive purposes of punishment. He concludes that, although doing can be morally justified under limited circumstances, typically it is not – a conclusion that bears upon lex mitior’s proper scope, whether it consists of a binding norm (as it is among European nations), a nonconstitutional norm (as it presently is within the United States), or, when legislative intent is uncertain, a function of the rule of lenity.

Keywords: ex post facto, lex mitior, retribution, condemnation, hard treatment

Suggested Citation

Westen, Peter K., Lex Mitior: Converse of Ex Post Facto and Window into Criminal Desert (March 20, 2015). New Criminal Law Review, Vol. 18, No. 2, pp. 167-213, 2015, U of Michigan Public Law Research Paper No. 455, Available at SSRN: https://ssrn.com/abstract=2588143 or http://dx.doi.org/10.2139/ssrn.2588143

Peter K. Westen (Contact Author)

University of Michigan Law School ( email )

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