Evidence of prior criminal offenses, whether charged or not, are generally not admissible in Court in an attempt to prove disposition to commit a crime pursuant to California Evidence Code section 1101.
Justice Bernard S. Jefferson in his treatise entitled California Evidence Benchbook, explains, "the reason for this exclusionary rule of evidence is that evidence of the fact of a defendant’s disposition to commit criminal acts has very slight relevancy when offered to prove that defendant committed this specific crime charged, compared with the grave danger of prejudice to a defendant from such evidence. If evidence that a defendant committed an uncharged offense were admissible on the criminal-disposition theory of relevancy, defendants would be convicted primarily because of their criminal character traits rather than because of substantial evidence that they committed the charged offenses." Jefferson, Cal. Evidence Benchbook, Seq.2Ed., Section 33.6, p. 1199. See also, People v. Tassell (1984) 36, Cal.3d 77 and People v. Thompson (1980) 27 Cal.3d 303.
Prior bad acts can be used to establish something other that disposition, however strict rules apply. Admission of other crimes evidence cannot be justified merely by asserting an admissible purpose. People v. Guerrerro (1976) 16 Cal. 3d 719, 724. Evidence of other crimes must "logically, naturally and by reasonable inference" establish the material fact for which it is offered. People v. Schader (1969) Cal. 2d 761, 775. In light of its inherently prejudicial effect, the evidence should be admitted only when its connection to the charged crime is "clearly perceived". People v. Durham (1969) 70 Cal. 2d 171, 186-187.
For example in People v. Gibson (1976) 56 Cal. App. 3d 119, the prosecution introduced prior bad acts of the defendant to establish motive. In that case, the defendant was charged with murder. The prosecution evidence established that the victim was killed by being beaten to death in a home occupied by the defendant and his girlfriend. There was evidence that all persons in the home that day had been drinking heavily. The defendant's girlfriend and a second witness testified that the defendant committed the beating that caused the victim's death. The defendant's defense was that he had gone to the store and when he returned he found that the victim had been beaten to death. The prosecutor admitted evidence over the defense objection to show that several months before the charged offense, the defendant had struck his girlfriend in the head with a crutch and had taken her purse. The prosecutor also admitted evidence to establish the defendant had struck a second person in the face. Lastly, the prosecutor admitted evidence to establish that the defendant, three months earlier had taken the clothes of a person visiting him at his home. The prosecutor admitted the evidence on the grounds that it established motive of the defendant's intent to kill the victim for purposes of robbery.
The court held that admission of the evidence was error, that the defense objection should have been sustained and the proffered evidence refused admissibility. The court pointed out that even when the commission of a criminal act is a disputed issue, evidence of motive may become relevant to that issue. The court found however that other crimes evidence, when offered to prove a defendant's motive is much closer, however, to its use as character trait evidence than when it is offered solely to prove defendant's intent. The court determined, that in terms of prejudicial consequence, there is very little difference, between other crimes evidence that is introduced to establish a defendant's motive, and thus to the inference that the charged offense was committed by defendant in accordance with such motive, and other crimes evidence as character trait evidence that leads to the same inference - that a defendant acted in accordance with such character trait and committed the charged offense.
The Gibson held concluded that the evidence with respect to the prior criminal acts of defendant led inexorably and inevitably to the defendant's prejudice. The court concluded that the jury could not be expected to undertake the "Herculean task" of not using this evidence - and thus would use it, either to establish that the defendant committed the murder or to find defendant guilty simply because he possessed character traits for violence and theft. The expectation that the jury could limit its use of such evidence as circumstantial evidence of the defendant's intent and motive, after applying other evidence in the case to reject defendant's alibi defense in accordance with the courts limiting instructions and admonitions, was considered to be an exercise in futility