If you are convicted, then previous convictions will be considered for sentencing, but not for determination of guilt or innocence.
Many states now make an exception for sex crimes. If you are charged with a sex crime, the prosecution is usually allowed to base its case on a general character assassination by scrutinizing your sex life.
That seems wrong to me, and now the Iowa court agrees:
But in State v. Cox, decided April 30, the Iowa Supreme Court held that the admission of such propensity evidence violates the Iowa Constitution’s Due Process Clause. “The policy against admissibility of general propensity evidence stems from ‘a fundamental sense that no one should be convicted of a crime based on his or her previous misdeeds.’ ‘A concomitant of the presumption of innocence is that a defendant must be tried for what he did, not for who he is.’ This concept is ‘fundamental to American jurisprudence.’” In this, the court departed from the view of federal courts and most state courts, and adhered to the minority view, which until then apparently was followed only by the Missouri Supreme Court.