Comparison of Footprint Ridge Density between Two South African Ethnic Groups

Fingerprints and footprints are common forms of forensic evidence and can be used to aid identification of individuals or link them to scenes. Ridge density refers to the number of friction ridges in particular demarcated areas, typically on the hand or foot. Previous studies have suggested ridge density as a marker for sexual dimorphism, and to a lesser extent, for ethnicity, which both have forensic significance in terms of presumptive demographic profiling of marks left on crime scenes. Here we studied footprint ridge density in two South African ethnic groups and report that ridge density in the heels was significantly higher in South African white Afrikaners (n = 50) than in Cape Coloured individuals (n = 50); left heel: white Afrikaans 9.88 ± 1.87, Cape Coloured 8.84 ± 2.16 (p < 0.05); right heel: white Afrikaans 10.02 ± 2.17, Cape Coloured 8.94 ± 1.80 (p < 0.05) (13-72 years old; 64 females and 36 males). As shown, logistic regression analysis can be used to generate receiver operating characteristic curves for a rule for ethnicity approximation based on ridge density measurements, from which a likelihood ratio can be determined. The value of these probabilities within a forensic context requires consideration in relation to Bayes' theorem which links the likelihood ratio and prior odds to yield the overall posterior odds. Our results suggest that, based on a ridge density measurement, the prior odds could approximately be doubled, and that footprint ridge density could be used as an attribute for ethnicity approximation. Ridge density was studied for the first time in Cape Coloured and white Afrikaans individuals and even though these two ethnic groups share some ancestry, significant differences in ridge density were observed between the groups in the heel areas of the feet.