Propensity evidence is extremely prejudicial to defendants because it creates an easy and acceptable belief in the prosecution’s case – they acted bad before, so they acted bad again this time.
Proof that a defendant has been guilty of another crime equally heinous prompts to a ready acceptance of and belief in the prosecution's theory that he is guilty of the crime charged. Its effect is to predispose the mind of the juror to believe the prisoner guilty, and thus effectually to strip him of the presumption of innocence.
However, there are exceptions to this rule. If prior bad act evidence is allowed in, it should not be used to show that the person acted in conformity with their character, but it may be used to show motive, intent, absence of mistake or accident, common scheme or plan, or identity. This case has been codified into SCRE 404(b). Often one piece of evidence may sit in two boxes at once: it can be propensity and it can be used for another purpose. It is for the trial judge to determine the purpose of the evidence and if it passes SCRE 404(b) and 403.
In 2020, the South Carolina Supreme Court clarified that any Lyle evidence needs to pass the logical relevancy test. For many years, the courts had diluted this test to merely a “similarity” between the prior act and charged act. However, it is clear now that there needs to be more: “The State must show a logical connection between the other crime and the crime charged such that the evidence of other crimes ‘reasonably tends to prove a material fact in issue.’”