How and why do humans grow thin skulls? A test of the systemic robusticity hypothesis

Two hypotheses for variation in skeletal robusticity (strength relative to size) receive wide support. Wolff ‘s Law posits that activity produces mechanical stimulus inducing osteogenesis, and increased weight bearing will increase the production of bone in areas of high stress to minimize bone strain. Recently, Lieberman (Am J Phys Anthropol 101: 217–36, 1996) suggested that increased cortical bone thickness is caused by the release of growth hormone (GH) during exercise rather than via mechanical loading per se . This research tested Lieberman's systemic robusticity hypothesis by comparing measures of robusticity of the humerus, femur and cranium in 25 individuals from the Terry Collection, curated at the National Museum of Natural History in Washington, DC. Results provide intriguing, but equivocal support for the systemic robusticity hypothesis. Ordinary least squares regression analyses fail to support a relationship between cranial and postcranial robusticity within the small sample. However, several pair‐wise comparisons, calculated by Spearman ρ rank correlation coefficients, support the systemic robusticity hypothesis, as do Kendall's W coefficients of concordance. The main problem influencing this and other studies is a lack of agreement on how to quantify cranial robusticity. Funding from an NSF Graduate Research Fellowship and a Smithsonian Institution 10‐week Graduate Student Research Fellowship.