Though popularized in the 1998 finale of Seinfeld, there is generally no requirement to intervene or even report a crime you witness. “Good Samaritan” laws provide a defense to those who attempt to render aid in an emergency, but do not impose a requirement on anyone.
There are some situations that may trigger what is called “mandatory reporting”, but this usually only applies in incidents of child abuse or to professionals in certain fields. A member of the general public is unlikely to find themselves sharing a cell with Jerry, Elaine, George or Kramer.
It’s illegal, in most cases, to actively try to help someone conceal a crime.1 You may also be compelled to appear as a witness at a trial if the police or prosecutor thinks you have information that may be relevant. However, only a small minority of states require the general public to report crimes, and only in a few situations.2 Even in those states, prosecution is rare except in extreme circumstances.
Certain professionals may have a greater duty to report crimes. For instance, doctors, teachers, and even photo processors must report suspicions of child abuse.3 Additionally, accountants, lawyers, and doctors must report suspicions of elder abuse.4 Veterinarians may be required to report suspicions of animal abuse.5
Often, reasonably believing that a crime didn’t occur — and even unreasonably, but still honestly believing that a crime didn’t occur — is enough to prevent prosecution, even in cases of mandatory reporting.
Individual employers may also set reporting requirements for their employees. These would never result in your arrest, but could result in your firing even if you broke no laws.
And finally, although arguable, there may be a moral requirement to report your knowledge of a crime. There are times that the needs of a victim outweigh our desire to protect a friend or family member. That tipping point may vary from individual to individual, but it is always important to consider how our actions or inactions might affect innocent victims.