Occasionally, in child or adult sex crimes cases, prosecutors will request permission from the court to use evidence of a defendant's prior involvement in a similar offense, or even go so far as to introduce past unsubstantiated allegations that had been brought against the defendant.
In most non-sex related cases, this type of prior offense "innuendo" or "character evidence" is precluded by the Arizona Rules of Evidence. But the Arizona courts and rule makers have made a special exception in sex crimes cases in order to allow the use of this "prior act or accusation" evidence against people accused of sex crimes.
The court's reasoning is fairly simple and goes like this – in normal cases the courts think that it is too prejudicial to say that because someone has stolen in the past they must be stealing again in the present. So the prosecutor can't argue "we know from his past that this person is a thief, therefore he is guilty of the theft charges in this case." The courts have determined that the value of the first offense in determining that the defendant is actually a "thief," is not sufficient in comparison to the prejudice that results from bringing in the prior offenses or accusations. The court is essentially saying "people can change." Or, "once a thief, not always a thief."
However, in sex crimes cases, the courts have taken the opposite approach in evaluating whether past sex crimes accusations are meaningful evidence on the new charges. The court has determined that there is so much value in learning of the previous offense, that it is not outweighed by the prejudice in the case (and obviously, there is a lot of prejudice). Rule 404C reads, in pertinent part, as follows
(c) Character evidence in sexual misconduct cases
In a criminal case in which a defendant is charged with having committed a sexual offense, or a civil case in which a claim is predicated on a party's alleged commission of a sexual offense, evidence of other crimes, wrongs, or acts may be admitted by the court if relevant to show that the defendant had a character trait giving rise to an aberrant sexual propensity to commit the offense charged. In such a case, evidence to rebut the proof of other crimes, wrongs, or acts, or an inference therefrom, may also be admitted.
Essentially the court is saying that a sexual interest in children (pedophilia), teens (hebephilia), or nonconsensual sense is so deviant that it is highly probative evidence of intent in the case at hand. The court is allowing the prosecutor to argue, literally, that "we know from his past that this person is sexually deviant, therefore he is guilty of the sex crimes charges in this case."
In determining whether to admit the evidence, the court will evaluate the evidence in relation to a variety of questions, such as how close in time the past offense occurred? How similar is the past accusation or offense to the present charges, especially in relation to the nature of the victims and the nature of the acts? If these factors are at all close, the courts tend to admit the evidence. The evidence can often make or break a case in the sex crimes context