Definition - What does Proportionality mean?
When used in the context of criminal law as practiced in the United States and some European countries, proportionality is the idea that the severity of a punishment should not be greater than the severity of the crime. In the United States, this belief can likely be traced to the Eighth Amendment of the Constitution, which bans cruel and unusual punishment.
Justipedia explains Proportionality
In the United States, there is no specific state or federal law that mandates proportionality. However, as noted above, the foundations for this concept may be linked to the stance against cruel and unusual punishment expressed in the Eighth Amendment.
One of the earliest cases in which the Supreme Court interpreted the Eighth Amendment and addressed the issue of proportionality was O'Neil v. Vermont, 144 U.S. 323, 339-40 (1892). In that case, a dissenting justice took a broad view of its meaning, arguing that the Amendment effectively banned not only punishments deemed "barbarous and inhumane," but also, "all punishments which by their excessive length or severity are greatly disproportionate to the offenses charged."
Since then, the Supreme Court has acted on the belief that sentiment regarding what constitutes reasonable or unreasonable punishment is not set in stone. As a result, it has historically given state and federal governments tremendous leeway with regard to sentencing guidelines.