The evolutionary adaptation of body art: Tattooing as costly honest signaling of enhanced immune response in American Samoa

Background Tattooing has been practiced globally for thousands of years. From an evolutionary perspective, this tradition seems counterintuitive because it is a dermal injury that risks infection. Previous research indicates tattooing may habituate the immune system for subsequent stress, as with exercise or vaccination, an important benefit in high-risk areas. Visible injuries through tattooing may be a form of costly honest signaling-consciously or unconsciously drawing attention to immunological quality. Objectives We tested this habituation effect of tattooing in American Samoa, where its practice is common and extensive and infectious disease rates high. We hypothesized that people with more tattoo experience would have enhanced immune response related to the stress of being tattooed. We compared total and rate of tattoo experience to determine if tattooing is more analogous to exercise or vaccination. Methods We measured secretory immunoglobulin A (SIgA), cortisol, C-reactive protein (CRP), and tattoo experience in 25 adults receiving tattoos. We compared post-tattoo SIgA to total and rate of tattoo experience using analyses of covariance, controlling for pre-tattoo SIgA, tattoo duration, age, marital status, and stress and baseline health (cortisol, CRP, body mass index, and cigarette use). Results Post-tattoo SIgA positively correlated with total tattoo experience (P < .05). Furthermore, when dichotomized by experience, participants with low tattoo experience showed little to no stress-related immune change, whereas high-experience participants exhibited elevated SIgA, suggesting habituation to repeated tattooing. Conclusions The historical and cultural popularity of tattooing may be partly due to honest information tattoos convey about adaptive biology, similar to physical benefits of exercise.