Criteria for admissibility of character evidence in criminal trial

I’m pretty sure I once saw a true crime documentary where the prosecutor in a case was not allowed to bring up the defendant’s prior convictions for similar crimes. I may be wrong on the specifics but I do remember being stunned that evidence which seemed so obviously pertinent was labeled prejudicial and therefore inadmissible. Iin the Cosby trial, however, women were allowed to testify about acts the defendant allegedly committed that hadn’t even faced due process. Is the variability just the difference between states’ laws or is there some other criteria that is met in one case and not in the other?

I am not a criminal lawyer, and my recollection of the federal rules on this topic is hazy, but I believe the answer is that evidence of past bad acts by the accused is generally not admissible to show bad character but can be admissible to show an identifiable pattern or practice (e.g., drugging women with sedatives).

Oakminster or RNATB may be along to correct something I’ve misstated.

In general evidence of past convictions is not admissible because, as the OP so graphically illustrates, its prejudicial effect outweighs its probative value. The fact that two years ago I burgled 125 Acacia Avenue is not evidence that last month I burgled 316 Boronia Way; if it were then clearup rates would be much higher since all the state would have to do is charge someone with prior form and use that as evidence that he has committed a recent crime of the same kind. Prior convictions will only be admissible if there is something characteristic or distinctive about them that links them to the present charge - e.g. I am noted for committing burglaries while wearing a Donald Trump wig-and-mask set, and the offender in the present charge was similarly attired.

UDS has it. Prior bad acts can’t be offered up to prove that the accused acted the same way this time.

But they can be offered to prove absence of mistake, or as evidence of a common plan, scheme, or motive.

At common law, only time that previous criminal convictions or allegations of similar behavious could be brought up to the jury would be to rebut claims made by the accused.

In England this has changed a lot unfortunately since 2003.

But note that the sentencing judge does get to see the prior convictions.

After they’ve proved, by actual evidence, that it was you that did the crime, the judge gets to say that as a repeat offender, you’re going away.

Note that the same questions arise in scientific/medical research when you can’t do a randomized controlled trial. What is the meaning of prior data/opinion? It continues to be a difficult area for non-specialists to get their head around, and I’m not specialist enough to know if even specialist statisticians (now) agree.

In the US, can prior bad acts be introduced to demonstrate a striking similarity in the accused’s conduct, to show that the type of conduct was a “signature”? Is that how the other accusers were called in the Cosby trial?

There is, however, an exception to the rules regarding propensity evidence - sexual assault. Rules 413 (sex assault) and Rule 414 (child molestation) allows other people to come and testify to their incidents to prove that the defendant acted in conformity with in the present case. Although this evidence is still tempered by other rules of evidence (including the big one, Rule 403, which says that evidence is not admissible if its probative value is outweighed by the danger of unfair prejudice), it basically turns the rules of evidence on their head - while previously robbing one bank doesn’t mean that you robbed this bank, previous acts of sex assault can be used to show that you did this act. (Note, too, that it is not limited to convictions - I just had a trial where 2 victims’ stories were bolstered by the unrelated stories of 4 other people).

I should add that these are the Federal Rules of Evidence. States may vary (although they tend to track these rules, even down to the numbering).

Rule 403 seems to be a preservation of the common law rules of exclusion.