The victim's and defendants' patterns of sexual behavior in rape trials are not covered by the usual rules. There are special rules in some states (not all) that treat sex crime victims more favorably than other kinds of victims; and treat sex crime defendants more harshly than other kinds of defendants. Usually, the 404(a) exception permits the defendant to introduce evidence of patterns of a victim's behavior in support of his case. Without rule 412, defendants could introduce evidence of promiscuity on the part of a rape victim if it rises to the level of "usual" behavior. Rule 412 excludes such evidence. Without Rules 413-415, the prosecution would be prohibited from proving a defendant's pattern or specific prior acts of pedophilia to prove an act of child molesting. Rules 413-415 admit such evidence.
These rules were not part of the original Federal Rules of Evidence and are inconsistent with them. They were added by Congress under political pressure. Sex crimes are different because the usual political balance between "conservatives" (get tough on crime) and "liberals" (defendants have rights) does not exist, as many liberals joined the conservative arguing we needed to get tough on sex crimes to make the world safer for women.
Some relatively feeble arguments have been made to justify these rules on some rational basis other than politics. The usual argument is that patterns of sexual behavior by defendants are more probative of conduct than other criminal patterns -- e.g., that child molesters have an especially high recidivism rate and are particularly unable to control their conduct. There are 2 problems with this argument. First, if sexual behavior is especially likely to fall into a predictable pattern, then it would hold true for women as well as men, and it should be easier for both sides to introduce sexual character evidence. Second, the argument is belied by statistics showing the highest rates of recidivism occur in crimes involving theft, drugs, and alcohol, and that perpetrators of sex crimes are no more likely to be "serial" criminals than perpetrators of other crimes of violence.
I realize that many of you may have strong personal feelings about the admissibility of character evidence in rape cases, especially questions concerning the prior sexual behavior of the victim, and therefore like Rule 412 making it harder to introduce such evidence. I agree with you. I believe the probative value of all character or pattern evidence is pretty low, and it should all get excluded if it implicates a Rule 403 issue such as sex, drugs or rock 'n' roll. The observation that sex crimes are being treated differently does mean that treatment is wrong. I would change Rule 404 to track the stricter language of 412 to make them consistent, rather than criticize Rule 412 as the one that is wrong. My criticism is addressed to Rules 413-415, which went the other direction and made it easier to introduce evidence of the defendant's character. The probative value is low when it's the defendant's pattern just as when it's the victim's pattern, and the prejudicial effect is just as high.
I'm happy to discuss this issue outside class with anyone especially interested in the development of rules of evidence in sex crimes cases or in the interaction between politics and the rules of evidence.