Oral intercourse or secondary transfer? A Bayesian approach of salivary amylase and foreign DNA findings

The Bayesian Approach allows forensic scientists to evaluate the significance of scientific evidence in light of two conflicting hypothesis. This aids the investigator to calculate a numerical value of the probability that the scientific findings support one hypothesis over conflicting opinions. In the case where oral intercourse is alleged, α-amylase, an indicator of saliva, is detected on penile swabs. The value of this finding is unknown as it may indicate the presence of saliva resulting from oral intercourse however it may also represent the presence of saliva due to innocent means such as background levels of salivary-α-amylase in the male population due to secondary transfer. Therefore, it is difficult to attach significance to this finding without background information and knowledge. A population study of the background levels of salivary-α-amylase was performed by analysing items of underwear worn under normal circumstances by 69 male volunteers. The Phadebas press test was used to screen the garments for amylase-containing stains and the positive areas were subjected to further confirmation of saliva by the RSID-Saliva kit. 44% of underwear screened had stains containing amylase. This study determined the background level of salivary-α-amylase and DNA on the inside front of male underwear which has potential implications on the interpretation of evidence in alleged oral intercourse.