Burden of proof refers most generally to the obligation of a party to prove its allegations at trial. In a civil case the plaintiff sets forth its allegations in a complaint, petition or other pleading. The defendant is then required to file a responsive pleading denying some or all of the allegations and setting forth any affirmative facts in defense affirmative defenses. Each party has the burden of proof of their allegations.
In the United States, "reasonable suspicion" is a low standard of proof to determine whether a brief investigative stop or search by a police officer or any government agent is warranted. It is important to note that this stop and/or search must be brief; its thoroughness is proportional to, and limited by, the low standard of evidence. A more definite standard of proof (often probable cause) would be required to justify a more thorough stop/search. In Terry v. Ohio, 392 U.S. 1 (1968), the United States Supreme Court ruled that reasonable suspicion requires specific, articulable, and individualized suspicion that crime is afoot. A mere guess or "hunch" is not enough to constitute reasonable suspicion.
An investigatory stop is a seizure under the Fourth Amendment. The state must justify the seizure by showing that the officer conducting the stop had a reasonable articulable suspicion that criminal activity was afoot. The important point is that officers cannot deprive a citizen of liberty unless the officer can point to specific facts and circumstances and inferences therefrom that would amount to a reasonable suspicion. The officer must be prepared to establish that criminal activity was a logical explanation for what he perceived. The requirement serves to prevent officers from stopping individuals based merely on hunches or unfounded suspicions. The purpose of the stop and detention is to investigate to the extent necessary to confirm or dispel the original suspicion. If the initial confrontation with the person stopped dispels suspicion of criminal activity the officer must end the detention and allow the person to go about his or her business. If the investigation confirms the officer's initial suspicion or reveals evidence that would justify continued detention the officer may require the person detained to remain at the scene until further investigation is complete. In some cases, the investigation may develop sufficient evidence to constitute probable cause.
Reasonable to believe
In Arizona v. Gant (2009) the United States Supreme Court defined a new standard, that of "reasonable to believe." This standard applies only to vehicle searches after the suspect has been placed under arrest and overruled New York v. Belton by saying it must be "reasonable to believe" there is more evidence in the vehicle of the crime the suspect was arrested for. Only then are police officers allowed to go back and search a vehicle incident to a suspect's arrest.
There is still an ongoing debate as to the exact meaning of this phrase. Some courts have said it should be a new standard while others have equated it with the "reasonable suspicion" of the Terry stop. Most courts have agreed it is somewhere less than probable cause.
Probable cause for arrest
Main article: Probable cause
Probable cause is a relatively low standard of proof, which is used in the United States to determine whether a search, or an arrest, is warranted. It is also used by grand juries to determine whether to issue an indictment. In the civil context, this standard is often used where plaintiffs are seeking a prejudgement remedy.
In the criminal context, the U.S. Supreme Court in United States v. Sokolow, 490 U.S. 1 (1989), determined that probable cause requires "a fair probability that contraband or evidence of a crime will be found" in deciding whether Drug Enforcement Administration agents had a reason to execute a search. Courts