Ex post facto Laws

The prohibition against ex post facto laws means that the Congress cannot pass a law imposing criminal sanctions on an action that was not criminal at the time it was committed. For example, if you spit on the sidewalk yesterday, the Congress could not pass a law making sidewalk-spitting illegal and punish you for what you did yesterday. If you spit on the sidewalk tomorrow, then you could be punished under the new law, but not for what you did before the law was passed. Similarly, the Congress cannot retroactively impose harsher penalties on an action or make it easier to prosecute a crime after it was committed. One of the key tenets of the rule of law is that laws must be widely known and understood before they are enforced. An ex post facto law would clearly violate this principle by punishing people for actions they did not know were illegal at the time they committed them. The Supreme Court has determined that retroactive tax laws or other noncriminal provisions do not violate the Constitution.

The Constitution also gives the Congress the authority to regulate interstate commerce, and forbids the states from passing any law “impairing the obligation of contracts.” While these two provisions are not generally thought of as protecting the rights of the people, one of the primary motivations for drafting and adopting the Constitution was the government’s inability to regulate commerce under the Articles of Confederation. The inability of states to alter the contracts into which individuals enter, both within and across states, also allows individuals to have confidence in their economic agreements. This confidence, together with a Congress empowered to regulate interstate commerce, provides the stability and order necessary to support a multi-trillion dollar economy in which the people are free to buy, sell, trade, and pursue their economic interests.