The scientific sleuths form the past and present must be rolling over in their condominiums. See Richardson. The transfer of physical or biological material between two separate sources (in biology think spitting on your hands before swinging at a baseball pitch) is a pivotal area of crime scene investigation. Crime Scene 101 curriculua always makes this the “holy grail” of CSI paradigms. In reality and fiction (as seen on ABC) forensic investigation, Locard’s principle holds that the perpetrator of a crime will bring something into the crime scene and leave with something from it, and that both can be used as forensic evidence.
This snippet of knowledge is not the entire story. There are various scenarios of transfer “science.”
1) Mutual transfer: Two contacting surfaces, objects, or substrates (anything that can collect or retain the material) leave particulates (i.e., paint chips, skin cells, saliva from mutual kissing). This is the “exchange” aspect.
2) Unilateral exchange: Only one-way transfer occurs. In “trace evidence” parlance, the “source” (actually a person of interest or objects in his possession) suspected of a crime is compared to the particulate material recovered from the crime scene.
3) Multiple unilateral exchange: This is could be both hair and DNA from semen, sweat from skin, blood or saliva left at the crime scene or victim which is recovered. Each item is separately analysed for its DNA content.
4) All of the above scenarios can be made more challenging if analytical results reveal more than one source (i.e. a perpetrator’s profile) being recovered from the crime scene indicating violent contact with a victim. This would be multiple DNA profiles from the same physical locations (i.e. blood mixtures) on the victims body.
In the best scenario, DNA evidence would be all that could be necessary to solve the case. Chaos ensues when all the testing (DNA, toolmark, bitemark, hair comparison, paint and fiber evidence derived from Locard’s historical observations conflict. Recall that Locard was silent regarding the power and reliability of analyses brought to bear on the “exchanged” or transferred material. He was only talking about the collection part of the puzzle.
The use aspect of conflicting forensic evidence in the Richardson exoneration litigation happening in New Jersey is testing the court’s scientific sophistication and critical thinking. In the lawyer arena, the New Jersey DA is claiming the mixture of DNA (from an unknown male) recovered from a bite injury (claimed to be the defendant by a dentist) is explainable and still favors a new trial for crimes against the victim.
In simple terms, the crime scene perpetrator (Richardson) had an accomplice who spit on the victim’s body, either BEFORE or AFTER being bitten by Richarson. It also assumes the biter (Richardson) could bite and someone else’s saliva is all that remains. Readers of crime fiction would read this blockbuster plotline and just laugh. A lawyer’s s response to a scientific conflict is to create a “phantom” criminal at the end of the story. Really?
Mssr. Locard would be disappointed.