With Bill Cosby now having been formally charged with aggravated indecent assault in Montgomery County, Pennsylvania, the prosecutors and defense lawyers will soon start fighting over what evidence the jurors in his case will be allowed to hear. One of the most critical questions will be whether the prosecutors can present evidence relating to the many other accusations made against Cosby by other women. The judge's ruling on this issue may end up being the turning point in the case.
In a vacuum, Cosby's case may well be defensible. Jurors could conceivably decide that the complainant's testimony and the other evidence presented by the prosecution, including Cosby's admissions in his 2005-06 deposition testimony, are insufficient to prove guilt beyond a reasonable doubt. But if they learn that dozens of other women have made similar accusations, the balance will shift dramatically towards the prosecution. Scientific research and common sense both agree that jurors are heavily influenced by evidence that a defendant has previously committed acts similar to the presently charged ones.
So will that evidence come in? That's a complex question, with the answer depending on the prosecution's specific argument for admissibility and the nuances of Pennsylvania's evidentiary law.
Historically, the default rule in criminal law has been against the introduction of "prior bad acts" evidence. There are, however, a number of exceptions to that rule. In particular, although such evidence usually isn't admissible for "propensity" purposes -- that is, to suggest that since the defendant committed another crime back then, he's probably the type of person who would have committed the presently charged crime -- the prior act can often come in if it's relevant for some "non-propensity" purpose.
For example, if the prosecution's theory in a murder case is that the defendant initially robbed the victim and then later killed her to prevent disclosure of the crime, the jurors would get to hear about the initial robbery -- not to show that because the defendant was a robber he's probably the type of person who would also kill, but to demonstrate the specific motive for the particular charged crime.
Alternatively, if there is something unique about the present incident, the prosecution may be allowed to demonstrate that this unique feature showed up in the defendant's prior acts. If, for example, a serial killer had previously left specific marks on his victims' bodies after killing them, the prosecution may be able to introduce this evidence to show that a new victim with the same mark was killed by the same person.
Prior acts can also be used to show the absence of mistake or accident. For example, a defendant accused of groping someone on a subway may admit the touching but say that it was inadvertent. If so, the prosecution may be allowed to show that on past occasions he had deliberately engaged in similar conduct, thus making it more likely that the presently charged conduct was also intentional.
Finally, and of particular relevance here, many jurisdictions have enacted rules that make it easier for prosecutors to introduce prior bad acts evidence in sex offense cases. The rule in federal cases, for example, is that "[i]n a criminal case in which a defendant is accused of a sexual assault, the court may admit evidence that the defendant committed any other sexual assault," and "[t]he evidence may be considered on any matter to which it is relevant." Although many states have adopted similar rules for sex offense cases in state courts, Pennsylvania has not, so the issue in Cosby's case will be fought out under the more generally applicable rules.
The prosecution may argue for admissibility on "signature" grounds, based on the theory that Cosby used a similar approach with other alleged victims. If so, that will be a high bar to clear. Pennsylvania's Supreme Court, for example, has explained:
Evidence of other crimes is said to be admissible [to] prove other like crimes by the accused so nearly identical in method as to earmark them as the handiwork of the accused. Here much more is demanded than the mere repeated commission of crimes of the same class, such as repeated burglaries or thefts. The device used must be so unusual and distinctive as to be like a signature.
Whether the prior acts evidence in Cosby's case meets this standard will depend on the specific facts. If the only common theme is that he allegedly drugged women without their knowledge before making his advances, that's probably not enough to constitute a "signature." But if there's more -- for example, if the prosecution has evidence that Cosby regularly used a specific drug and a specific method of delivery, and consistently took a specific series of steps -- that may be enough.
Regardless of the details, two things are certain -- the prosecution will use every plausible theory it can think of to get Cosby's "prior acts" evidence in, and the defense team will do everything it can to keep it out. We'll all have to wait and see how this plays out.