Thin mandibles and small incisors found in New World monkeys as compared with Old World monkeys suggest that there may be differences in craniofacial loading patterns between these two groups, particularly in levels of mandibular corpus twisting (Hylander, 1975, 1979a; Eaglen, 1984; Bouvier, 1986a,b). This study examined the hypothesis that changes in the relative force contributions of the masticatory muscles were responsible for lowering torsion on the mandibular corpus in New World monkeys. Muscle weight and physiological cross-sections were compared using data from the literature (Schumacher, 1960; Turnbull, 1970; Cachel, 1979) as well as new data on adult male Cebus apella and Macaca mulatta. Both age and sex had an effect on muscle ratios. Mixed samples such as those used by Schumacher and Turnbull probably are not appropriate for drawing conclusions concerning species or group differences in muscle ratios. In addition, biomechanical conclusions based on muscle weight ratios alone to estimate muscle force may be misleading because fiber length inversely affects the amount of force a muscle can exert. A comparison of ratios based on physiological cross-section as an estimator of muscle force in New and Old World monkeys does not support the hypothesis that alterations in force contribution by individual masticatory muscles are responsible for minimizing mandibular corpus twisting in New World monkeys. Therefore, if twisting has been minimized in New World monkeys as suggested by their thin corpora, other changes in the craniofacial musculoskeletal complex, such as different muscle recruitment or pinnation patterns, may be responsible.