Maybe he'll find a more efficient way to kill someone or a quicker way to abduct a woman from a car. He'll start showing more control over the crime.

The MO is basically the way the predator commits the crime -- if he uses a gun rather than a knife or lures prospective victims by putting his arm in a fake sling.

While the MO tells us something about how he did it, the signature gives us some insight into why.

Unique to the person

The signature is a ritual, something the subject does intentionally for emotional satisfaction -- something that isn't necessary to perpetuate the crime. Evidence of torture is a signature. Posing -- displaying the victims in a certain way -- is a signature. Signature is a way of linking cases.

Like a real signature, it's a personal detail that's unique to the individual.

Sometimes there's a fine line between an MO and a signature.

I've found that signature is a more reliable guide to the behavior of serial offenders than an MO. That's because the MO evolves, while the emotional reasoning that triggers the signature doesn't. The method a killer uses to get women into his van may change, but the fact that he always tortures them once they're inside stays the same.

One subject covered the faces of his victims. That was his signature. So you look for that in other cases in order to tie them together. A bomber used to spray black paint over the components inside his bombs. It wasn't necessary -- it didn't make the bombs any better. I don't know what it meant, but he did it anyway. He felt the need to do it.

Scripting a rape victim

In a rape case, a signature could be the language the rapist uses in scripting the victim -- getting her to say what he wants her to say. It's something the rapist needs. He doesn't need it for the sex act, he needs it for himself.

Sometimes just the way a rapist wants his victims to react becomes his signature. Take the power-reassurance rapist. It's just like the name implies. Once he gains control over the victim, he's constantly asking for reassurance about his performance. You know, "Is this good for you? How do I stack up against your husband, your boyfriend?" He needs her to respond in a certain way.

Unfortunately, you often need more than one case to determine what the signature is. You don't know if there's a pattern by looking at a single case.