If a cop is factually mistaken (but sincerely mistaken, not a liar) in his belief that X is true, and if X would typically constitute grounds for reasonable suspicion (were it to be true, which it isn’t), does the cop have reasonable suspicion?


A cop pulls a driver over because he thinks the driver turned right without signaling. The cop smells something strange, conducts a search, finds a crack pipe, and arrests the driver. Dash cam footage from the cop car is later reviewed, and shows that the driver did in fact use his turn signal properly when turning.

Did the mistaken cop have a "reasonable suspicion", and thus, grounds for pulling the car over? Is the crack pipe admissible as evidence?

A Spicier Example:

A cop thinks he hears a woman screaming from inside a house, repeating the phrase, “Help! They’re stabbing me!”. He calls for backup and has the house surrounded. Officers enter the house and find it completely empty, other than a small meth lab in the basement.

They find no people in the house, no amplification devices, and absolutely nothing capable of producing the sound of a woman screaming for help.

Witnesses who were standing next to the cop are interviewed, and all report that they heard nothing. Digital forensics experts later conclude that the officer’s body cam audio shows no traces of a woman screaming, and that there is no way a normal human ear placed within three feet of the camera would have heard anything other than silence.

There are fingerprints all over the equipment in the lab. They belong to the owner of the house.

Did the cop - provably mistaken in his sincerely held belief that there was a woman screaming for help - have reasonable suspicion that a crime was being committed, and thus, grounds for entering the house, admitting the meth lab into evidence, and arresting the owner?

Would the answer be different if the cop’s medical history was deemed admissible, and it turned out that he was a schizophrenic with a history of hearing voices?

1 Answer 1


A fact cannot by itself constitute reasonable suspicion; the word "reasonable" describes not only the relationship between the fact and the possible existence of a crime, but also the officer's knowledge about the fact. The officer must not only sincerely believe the truth of the facts constituting reasonable suspicion, but must also reasonably believe it.

I do not know whether a hallucinatory experience may be found reasonable for this purpose; I suspect that it would be a matter of dispute at trial. The first example is perhaps more straightforward, as there are probably a few possible explanations for the officer's failure to notice the turn signal. Still, are they reasonable? If the officer could not see the turn signals because something blocked the line of sight, it would not be reasonable for the officer to conclude that the driver had failed to use the signal. Rather, the officer has no evidence one way or the other, and absence of evidence is not evidence of absence.

The question deals in hypotheticals, where we can assume that the officer is sincere. But in a trial, the jury (or judge in a bench trial) cannot do that. They will look at the evidence, including the officer's testimony, and assess the officer's credibility. They will form an opinion about whether the belief was sincere and reasonable before they look at whether the facts, as the officer believed them to be, reasonably indicated that a crime was being committed or was imminent.

  • If the cop's knowledge of the fact is relevant to reasonable suspicion, and the cop must reasonably believe that X... Is there a general legal definition of what it means to say that some subject "S reasonably believes that X"? Is saying "S reasonably believes X" the same as saying "S knows that X")? (Typically in epistemology, a necessary condition of "S knows that X" is that X is true, which would mean that the cop had no knowledge of X, since X is not actually a fact; not-X is a fact). Nov 27, 2021 at 2:07
  • Just to flesh out the hypothetical, suppose that instead of something blocking the officer's line of sight in the turn signal example, it was later demonstrated by expert witnesses that on that particular day, in that place, and at that time, the bright dessert sun would have been hitting the regulation blinker light in a way that would have made it difficult to see when it was blinking - and that the cop testifies that this is indeed the reason for his error. Would these details effect the answer? Nov 27, 2021 at 2:08
  • @user10264746 the general legal approach is the reasonable person hypothetical, taking into account the officer's knowledge and training. Reasonable belief is not the same as knowledge, among other reasons because reasonable belief may be mistaken, whereas, as you note, you can't "know" something that is false. If the officer's vision was obscured by the sun, the question becomes whether the officer should have recognized that and recognized that the failure to perceive the turn signal didn't imply that the driver failed to signal.
    – phoog
    Nov 27, 2021 at 18:18
  • @user10264746 so yes, varying facts such as the weather and the position of the sun will likely affect the outcome, but every case is different. Even if conditions existed that could reasonably explain the officer's failure to see the signal, it never means that the trier of fact will accept that they do explain it. For example, if the officer's story changes over time, the officer's credibility might come into question, and then the trier of fact might find that the claim of reasonable suspicion fails, regardless of the weather or any other fact.
    – phoog
    Nov 27, 2021 at 18:22

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