This study examined six musculoskeletal stress marker (MSM) scores that showed reverse sex differences in the previously published research. MSM scores often differ by sex within a population. Most often females have lower MSM scores than do males; however, reverse sex differences do occur. If reverse sex differences are culturally specific, then these MSM scores are more likely caused by activity patterns than biology. Yet, if the same MSM scores show reverse trends in multiple populations, then perhaps these reversals may be the result of biological sex differences, such as hormonal differences and body size, or methodological reasons, such as MSM location. In the current study, deltoideus, pronator quadratus and trapezius MSM scores were significantly more pronounced in males in the California Amerind population examined. Most MSM scores examined significantly correlated with body size and age. With articular size and age controls, only the pronator quadratus and trapezius continued to display significant sex differences. Males had greater MSM pronator quadratus and trapezius scores than did females. The lack of sex differences in four out of six MSM scores may relate to the confounding factor of body size. Thus, it is suggested that some reverse sex differences in the published literature may not relate to activity patterns whereas other MSM sites show more promise. Copyright © 2010 John Wiley & Sons, Ltd.