Assume we're starting with a 1% prior of some person being the the perpetrator of a crime. Consider some completely static characteristic. If the suspect is th

Assume we're starting with a 1% prior of some person being the the perpetrator of a crime. Consider some completely static characteristic. If the suspect is the perpetrator then there's an 100% chance they still have this characteristic, whereas if the suspect is innocent then there's a 10% chance they have this characteristic just by coincidence. Therefore if the suspect turns out to have this characteristic, you should update your prior to (1*0.01)/0.1 = 10%. Compare to some dynamic characteristic. If the suspect is the perpetrator then there's an 20% chance they still have this characteristic, whereas if the suspect is innocent then there's a 10% chance they have this characteristic just by coincidence. Therefore if the suspect turns out to have this characteristic, you should update your prior to (0.2*0.01)/0.1 = 2%. At the extreme, a completely dynamic characteristic. If the suspect is the perpetrator then there's a 10% chance they still have this characteristic, whereas if the suspect is innocent then there's a 10% chance they have this characteristic just by coincidence. Clearly in this case you shouldn't update your prior at all.