Turning the tables of sex distinction in craniofacial identification: Why females possess thicker facial soft tissues than males, not vice versa

Abstract

Males are universally reported to possess larger facial soft-tissue thickness (FSTT) than females, however, this observation oversimplifies the raw data yielding an underpowered assessment of FSTT sex-patterning where: differences are small (?2 < 5%) and inconsistent (females are routinely larger than males at the cheeks). Here we investigate body-size normalized data to assess whether more general and improved understanding of FSTT sex-variation in humans is possible. FSTTs were measured in 52 healthy living Australians aged 18 to 30 years using B-mode ultrasound. Participants' stature and body mass were also measured. Sex differences were calculated before and after normalization by the aforementioned body-composition variables. Methods were repeated in three other independent samples to evaluate reproducibility: 100 American Whites and 60 American Blacks measured by B-mode ultrasound; and 50 Turkish residents measured by regular supine CT. Compared to raw mean differences (F < M, by ?6%), females displayed much thicker FSTTs than males when normalized for body mass (F > M, by +16%). Consequently, while the sexes share similar raw values, females possess much larger FSTTs for their relatively lighter bodies. The relative FSTT difference was 2.7× larger than the raw mean difference. Sex differences in FSTT are of larger magnitude and reversed direction in mass normalized data. Contrary to popular thought, females possess much larger FSTTs than males owing to their generically lighter bodies (?18 kg). These data patterns help explain why the pooling of sex-categorized FSTT does not jeopardize the sex-difference—it is encoded more strongly in terms relative to body mass. © 2016 Wiley Periodicals, Inc.