Uncovering mixture inversion: The prevalence of non-handlers as major contributors in forensic DNA evidence

This study presents the first systematic review examining the prevalence of DNA mixture inversion, defined as the occurrence of a major contributor to a DNA profile generated through indirect transfer via an intermediary individual. Following PRISMA guidelines, 45 empirical studies published between 1997 and 2022 were identified as containing relevant data to assess the prevalence of mixture inversion. Analysis of these studies, encompassing 3851 samples, showed that mixture inversion occurred in 1.69% of samples, consistent with previous reports. No significant differences were observed across substrate types. However, handlers classified as low shedders exhibited a higher frequency of mixture inversion compared to intermediate or high shedders. Mixture inversion was more likely following brief handling durations (<30 s) and when handlers had not recently washed their hands. These findings demonstrate that although mixture inversion is relatively uncommon, it may have important implications for interpreting DNA evidence at the activity level.