What is Forensic Thinking?

Put plainly, forensic thinking is a way of considering complex problems in an honest and forthright fashion. Forensic thinking requires us to be explicit in our assumptions and only make definitive statements about facts that we can verify. Ideally, any good forensic scientist approaches their work in this manner.

When thinking about forensic analysis, the average person might understand the thought process to go something like this:

I extracted DNA from semen found inside a rape victim. The DNA profile from that semen matches a suspect. This DNA profile is so rare it is found in only one out of ten billion people, and therefore unlikely to be found in anyone else. Therefore, the chances against the suspect being innocent are approximately ten billion to one.

This sounds plausible, but is incorrect and not even remotely close to forensic thinking. A forensic scientist would instead present the question this way:

I extracted DNA from semen found inside a rape victim. The DNA profile from that semen matches a suspect. This DNA profile is so rare it is found in only one out of ten billion people, and therefore unlikely to be from anyone else. How relatively likely is it that I'd find this DNA profile when I compare two possibilities: The suspect contributed the semen found inside the rape victim versus some random, unrelated person contributed the semen?

The correct answer is approximately ten billion to one.

These two ways of thinking sound similar, but they are worlds apart. The first example mixes the analysis with the assumptions by leaping to the conclusion that the likely donor of the semen is the likely rapist. The suspect matching the DNA profile may be the victim's consensual partner and not a real assailant. At the same time the suspect could be both a frequent consensual partner of the victim AND the perpetrator of the rape. The DNA profile alone cannot distinguish between these two possibilities.

Making a definitive statement about events we were not around to observe is extremely perilous. The forensic scientist can only be certain of the evidence and analysis that he or she performed or examined directly. In this case, that would be the DNA profile resulting from the scientist's careful analysis.

One can only make informed statements about the probability of an event that has been observed or measured objectively. To do otherwise would be like trying to guess how likely a die roll will end up "6" without knowing how many sides the die has.