Under the Federal Rule of Evidence 404, the rule is that the government cannot use propensity evidence of a defendant's past actions (i.e., just because he did something bad in his past, that means he did it again).
However, there are exceptions to this rule. One is FRE 414:
In a recent published opinion from the Eighth Circuit Court of Appeals, the court elaborated briefly on this rule:
United States v. Splettstoeszer, No. 19-1321, 2020 WL 1870252, at *1 (8th Cir. Apr. 15, 2020)
But the court also reminded us that FRE 403 must also be taken into account.