Question: How do you know who the suspect is from using makeup as evidence?

Katy Bruce answered on 24 Jan 2023:

Ahhh that is the question!

The first thing to say is that we can never state with absolute certainty that a suspect matches with any kind of evidence, we can only give likelihood or probability.

The second thing to note is that even if some evidence did match with a suspect, we need to look at that evidence in context to work out whether it could have gotten there via innocent transfer, e.g. brushed against someone on a busy train.

Makeup as a type of trace evidence can provide a physical link between suspect and scene/object or victim and is sometimes called associative trace evidence, as it shows that contact has taken place.

The easiest way of comparing a makeup trace recovered from a crime scene with a reference sample, i.e. one taken from the victim, is to look for differences between them, rather than similarities. This is because although makeup is a very complex mixture of lots of components, many of them are common across different brands. As soon as you find differences between the samples, whether that be their Raman spectra, their microscopic appearance, their diffraction patterns, etc. you cannot say they are a match. This isn’t always clear cut though, as I’ve found that makeup changes composition over time as the components degrade so it’s a terribly complicated problem!! 😣

Makeup is unlikely to ever be used in isolation in a case, it would just provide another “piece of the jigsaw” in a complex case that also considers many other types of evidence, e.g. fingermarks, DNA, fibres, etc. But every little helps, right?!