Xjyuk

I'm wondering about the limits put on which behaviors the legislature can criminalize in US law. I'm wondering about things like, could clearly harmless behavior (say, lifting a pack of gum with your left hand on a Tuesday) be criminalized without the law being unconstitutional, or how much discretion does the legislature have to determine whether it wants to consider behavior harmful.

As an example, in German law the Grundgesetz starts with an enumeration of fundamental rights that get divided into liberty rights and equality rights by legal theorists depending on what the individual right protects. Art. 2 GG acts as a generic right to liberty in case any of the specific liberty rights (such as freedom of assembly, of the press, of religion) don't cover something. A law criminalizing wearing blue lipstick for example would be a clear violation of Art. 2 GG, so the legislature is limited in that sense.

Question: what constitutional limits exist for US lawmakers to criminalize behavior?

There are a number of specific limitation on what can be made criminal in US law, derived from constitutional protection. Expressions of free speech, for example, cannot be made criminal, although there can be laws which regulate or impact speech to a degree. Similarly, the establishment clause of the First Amendment prohibits a law requiring people to attend a particular church, and he free Exercise clause prohibits any criminal penalties for any religious practice or lack of practice.

Most of the other provisions of the Bill of right would impose similar limitations. The legislature cannot criminalize what people have a constitutional right to do.

Beyond that the Due Process clause of the 5th and the parallel clause of the 14th impose limits on criminal law generally. Criminal laws must have (at least) a rational basis. The legislature cannot just prohibit picking up papers because they felt like it, they must have articulated a problem and a plausible reason why a given law would address it. If they don't the law may be overturned as unconstitutional. Similarly, vague laws are prohibited. If a reasonable person cannot tell from the law what is or is not illegal, the law can be overturned for vagueness.

Then there are procedural limits, also largely derived from the Due Process clause(s). A law cannot automatically convict people without a genuine hearing, in which a person can present a defense to an independent, impartial judge or jury. A law cannot impose duties which are flatly impossible to perform. A law cannot impose punishments which are considered unreasonable for the seriousness of the crime (as evaluated by judges).

There are other limits as well, it would take a book to list them all in detal. But those are some of the more frequently applied ones.

The basic limit is whatever right is protected by the US or state constitution. Really obvious examples pertain to the first amendment, so racial insults cannot be criminalized. We also have a notion that "Every person shall have the right to life and physical integrity. Freedom of the person shall be inviolable. These rights may be interfered with only pursuant to a law". An example of such a right being interfered with pursuant to a law is that in many states and at the federal level, any possession or consumption of marijuana is a crime.

There is a concept "strict scrutiny" according to which the constitutionality of a law is subject to strong rules in case a fundamental constitutional right is infringed, or else when a government action involves "suspect classification" such as race or national origin. Fundamental rights may be specifically protected by the Bill of Rights, or could be rights recognized by the Supreme Court as a right protected by the Due Process Clause or the liberty clause of the 14th Amendment. Even when a right is a fundamental right, it does not mean it is absolutely uninfringible. The government can infringe your fundamental rights if it does to for reasons of a compelling governmental interest, the law is narrowly tailored to only that purpose, and it is the least restrictive way to achieve that purpose.

If you are not dealing with a fundamental right, there are still purported restrictions on what the government can do, because the law still has to pass the rational basis test. Your proposed ridiculous law probably would not pass a rational basis examination, although it is hard to know for sure because there don't seem to be any cases of laws that have been overturned solely because they lack a rational basis.

This may depend on who is interpreting The Constitution.

But one of the "originalist" interpretations made by Barnett in Reason's "Has The Constitution Lost Its meaning debate" is that The Constitution is not the law that governs the people, but rather it is the law which governs the people who govern the people. His argument for it is that the people don't consent to the Constitution, but all public servants' oath of office requires them to follow the Constitution.

As such, the Constitution, in its entirety, is a restriction on what various branches of the government may do and when additional restrictions come into being.

While it was thought, at the time of the writing, that it is unnecessary to put any additional restrictions on government actions (because it would be understood that anything which wasn't explicitly allowed, would be forbidden for the government to do) in order re-enforce this point on some scenarios in which a government may be tempted to overreach, "The Bill of Rights" in the form of additional 10 amendments was added. The Bill of Rights specifically forbids legislature which would curtail some specific rights.