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The distinction between crimes that are "mala prohibita" and those that are "mala in se" has not only shaped but, to a certain extent, also bedeviled the law. Basically, a crime is termed "malum...prohibitum" if it is "not inherently evil [but is] wrong only because prohibited by the legislature". A crime is "malum in se" if it is "wrong in [itself], inherently evil". Wayne R. LaFave and Austin W. Scott, Jr., Substantive Criminal Law (1986), § 1.6(b), Vol. 1, p. 45.
However, as LaFave and Scott point out, this terminology has never been precise. Id. at 45-48. Generally, common-law crimes are called "mala in...in se. If, on the other hand, the defined conduct is not "morally culpable" ( Perkins gives the example of overtime parking), then the offense is malum prohibitum.
The Hentzner decision itself suggests yet a fourth criterion. In the above-quoted passage, the supreme court refers to mala in se crimes as...