In 2009, the National Academy of Sciences (NAS) issued a blockbuster report challenging the scientific underpinnings of many fields of forensic evidence. The report found that such evidence is rarely supported by rigorous study. Moreover, the analyses are often performed unscientifically, and analysts often overstate the scientific rigor of their evidence during testimony.
This is crucially important. Numerous people have been exonerated after it was shown that the forensic evidence used to convict them was false or exaggerated. According to the National Registry of Exonerations, false or misleading forensic science is a factor in 23% of exonerations.
The NAS report specifically noted blood pattern analysis (BPA) as a technique needing scientific improvement, and several new papers reiterate that conclusion. Researchers at Washington State University, the Netherlands Forensics Institute, the University of Illinois at Chicago and Iowa State University have recently done work on the physics of blood patterns and found that most forensic scientists may be making serious mistakes in their analyses.
Traditional BPA doesn’t take gravity or drag into account
Although BPA analysts generally are trained, there are no standards of accreditation in the field. Analysts are typically not taught the physics of blood spatter but are merely instructed how to perform the analysis without a full understanding of the physics.
Most BPA analysts are taught to draw straight-line trajectories from the blood spots to the impact angle. However, blood does not fly in straight lines. A viscous mixture of fluid and solids, blood spatter actually travels in the shape of a parabola. A straight-line trajectory will miscalculate the impact source as higher than it actually was.
Just as a straight-line trajectory fails to account for the effect of gravity, most BPA also fails to account for the drag of air on the drops of blood. Like the straight-line trajectory, this tends to place the impact source in a higher position than it actually would be.
The researchers involved in these papers have been working to apply formal theories of fluid dynamics to blood spatter, and they have shown that there are much more accurate ways to perform BPA. In fact, one researcher has been analyzing how the shape of the bullet can affect the spatter pattern.
If you’re interested in the details, we recommend reading this account of the research on physicsworld.com. The bottom line, however, is that BPA is usually done by people with little understanding of fluid dynamics using techniques that are known to produce inaccurate results.
That could change if new standards and techniques are created based on this or similar research. For now, however, criminal defense attorneys must carefully question BPA analysts to understand whether their analyses suffer from these inaccuracies.