Oftentimes the prosecution will want to admit a defendants prior bad conduct in a pending criminal case to show he or she is a bad dude. This practice is limited to specific acts and only if they are relevant to something other than propensity.

In People v. Ewoldt the Supreme Court noted the degree of similarity varies depending on the issue sought to be proved by the prior conduct evidence. The least degree of similarity is required when the issue to be proved is whether the defendant harbored the requisite intent for the charged crime, because of the recurrence of similar results tends increasingly to negate accident or inadvertence, self-defense, or other innocent mental states — and instead tends to show the presence of the criminal intent which normally accompanies criminal acts.

A greater degree of similarity is required when the prior conduct evidence is used to establish a common scheme or plan. In this context, the evidence must show more than merely similar results: instead, there must be such a concurrence of common features that the various acts can be explained as being caused by a general plan of which events are but individual manifestations. Evidence that the defendant has committed uncharged criminal acts that are similar to the charged offense may be relevant if these acts demonstrate circumstantially that the defendant committed the charged offense pursuant to the same design or plan he or she used in committing the uncharged acts. The common features must show the existence of a plan, moreover, rather than a series of spontaneous acts.

The greatest degree of similarity is required when the prior conduct evidence is used to establish identity: to be relevant on the issue of identity, the uncharged crimes must be highly
similar to the charged offenses. Evidence of an uncharged crime is relevant to prove identity only if the charged and uncharged offenses display a “‘pattern and characteristics . . . so unusual and distinctive as to be like a signature.’ (quoting 1 McCormick on Evidence (4th ed. 1992) § 190, pp. 801-803.) “The strength of the inference in any case depends upon two factors: (1) the degree of distinctiveness of individual shared marks, and (2) the number of minimally distinctive shared marks.” (People v. Thornton (1974) 11 Cal.3d 738, 756, emphasis in original, disapproved on other grounds in People v. Flannel (1979) 25 Cal.3d 668, 684, fn. 12.)

Under section 1102, specific instances of conduct are inadmissible under section 1102 to prove character: the statute permits, in a criminal trial, evidence “in the form of an opinion or . . . reputation”; it does not, however, permit evidence of specific instances of misconduct. (People v. Felix, supra, 70 Cal.App.4th at 431.) Moreover, evidence of a defendant’s bad character may only be offered by the prosecution under section 1102 in rebuttal to similar evidence presented by the defense. (People v. McFarland (2000) 78 Cal.App.4th 489, 495; People v. Bunyard (1988) 45 Ca1.3d 1189, 1216.) Accordingly, such evidence cannot be offered during the prosecution’s case in chief.

(b) Under section 1103, evidence of a criminal defendant’s character for violence or trait of character for violence (in the form of an opinion, evidence of reputation, or evidence of specific instances of conduct) is not made inadmissible if the evidence is offered by the prosecution to prove conduct of the defendant in conformity with the character or trait of character after evidence that the victim had a character for violence or a trait of character tending to show violence has been adduced by the defendant. Thus the evidence cannot be offered until the prosecution’s rebuttal, and only may be offered if the defense introduces evidence of defendants character for violence or trait of character tending to show violence – which he has no intention of doing.