The CSI Effect is the belief that TV crime shows are effecting the decisions of juries. Jurors have an expectation of what they will see brought into evidence because of the forensic evidence they see on CSI shows. In this series of posts, we will look at each type of evidence jurors expect to see. We will then look at issues associated with each type of evidence that you can add to your work to get your character off the hook. In this post, we will focus on blood pattern evidence.
Blood Pattern Evidence
Blood pattern analysis (BPA) seeks to answer three questions. Whose blood is this? How did it get where it is? What was the chain of events that created this blood pattern? More specifically, the analysis hopes to uncover the type of instrument used, who it was used on and from which direction the instrument was wielded. BPA also tries to recreate the position of the victim(s) and assailant(s) throughout the crime. It also helps law enforcement create more productive questions to ask witnesses and potential suspects as well as determine the truthfulness of any of their statements.
Stains vs. Spatters Blood Pattern Evidence
In the analysis of blood, there are stains and there are spatters. Stains occur as blood naturally flows in response to gravity. Blood stains can show how the victim moved as they bled because as the body changes position or orientation, blood will still flow toward the floor. Blood stains that break apart into smaller droplets because of an opposing force, other than gravity, are known as a spatter. If the spatter occurs from a moving force such a cough, or a wielded knife, it is considered projected blood spatter.
BPA Reliability Blood Pattern Evidence
The interpretation of blood pattern has an error rate of as much as 11% which may not seem like much until your child is one of the eleven of 100 wrongly imprisoned for life[i]. That error rate may be due to the fact that, in some areas, a mere forty-hour workshop can deem one able to interpret these patterns[ii]. And, as Professor Adrian Linacre, Chair in Forensic DNA Technology at Flinders University, Glasgow said, analysis of a blood pattern “is based upon the experience and knowledge of the examiner[iii].”
Blood Is Unpredictable Blood Pattern Evidence
Blood is not a predictable fluid like water. Water flows at the same rate regardless of how much is it stirred or shaken. Blood, however, is highly impacted by movement. The more it is moved, the thinner it is. That is why blood drips slow but sprays quickly[iv]. That is also why the higher the velocity of the spatter, the smaller the droplets.
Also, as soon as blood leaves the body it reacts to the ambient temperature. If the blood of your character lands on a cold wall, it will coagulate quickly and drip slower than it would on a warmer wall. Blood also reacts differently as it lands on different surfaces. If your character’s blood drops onto a smooth surface such as tile or glass, it will create a circular shape. If it strikes a surface that it not flat or it is not smooth, the droplet will break into smaller droplets.
If an assailant has cleaned an area of blood, its presence can still be detected with luminol even years after the blood has been present. Luminol is a powder that glows blue only when it contacts blood. The glow only lasts for about thirty seconds and can be captured with long-exposure photography. Luminol is used as a last resort as the same chemical reaction that detects blood also breaks the blood down.
Despite what you may see on screen, blue light does not detect the presence of blood. Blue light detects the presence of other body fluids such as urine and semen. Blood actually absorbs blue light and rather than glowing, shows up black[v]. So, when you see that in a movie, it is for theatrical purposes only.
In our next post, we will take a look at fingerprints. Until then, get blood on your pages!
[i] https://nij.ojp.gov/topics/articles/study-reports-error-rates-bloodstain-pattern-analysis
[ii] http://www.forensicpieces.com/lists/5-csi-and-recon-courses/items/450-basic-bloodstain-pattern-analysis
[iii] https://cosmosmagazine.com/science/bloodstain-pattern-analysis-explainer/
[iv] https://www.pbs.org/wgbh/nova/article/forensics-bloodstain-pattern-analysis/
[v] https://www.glogerm.com/pdf/Crime_blood.pdf
