Basically, California Evidence Code section 1108 allows the prosecution to introduce evidence a defendant has committed a prior sex crime during a trial for another sex crime.
You can read about it at
www.fredthia.com
It is a good law in general, but it can be abused.
If a man is charged with molesting a little girl and he has a prior conviction for indecent exposure, does the prior show he has a propensity to molest children?
What if the indecent exposure was exposing his penis to kindergardeners?
That might be relevant.
But, what if the prior was for running naked across the campus while pledging a fraternity at college?
Does that show a propensity to molest children?
Not everyone that has been convicted for a "sex crime" is a pervert.
In this case, Jeff was not allowed to explain the circumstances of his prior conviction.
The jurrors assumed his possession of the porn was for a sexual purpose and they convicted him.
This is a misuse of Evidence Code Section 1108.
