Shear thinning behavior, often observed in shear viscosity tests of IgG therapeutic molecules, could lead to significant disparities in the projections for the viscosity profile of a molecule. Despite its importance, molecular determinants of sheer thinning in protein suspensions are largely unknown. To better understand the factors influencing sheer thinning, viscosity profiles of IgG1 and IgG2 molecules were monitored over a wide range of bulk concentrations (0.007-70 mg/mL). The degree of shear-thinning of 70 and 0.007 mg/mL samples was minimal in comparison to the 0.7 mg/mL solution for both IgG molecules. These observations suggest that bulk concentration alone does not determine the degree of sheer thinning, and additional factors play a role. Additional data reveals, within a threshold range of concentrations, that a strong correlation exists between the degree of shear thinning and the surface area to volume (SA:V) ratio of an IgG sample exposed to the interface. The influence of the interface, however, diminishes when the bulk concentration falls outside this concentration window. Also revealed by interfacial oscillatory rheological testing, both IgG molecules showed solid-like behavior (G'i) at the air-water interface at 0.7 mg/mL, whereas liquid-like behavior (G"i) was dominant at 0.007 and 70 mg/mL concentrations. These observations imply that the lack of solid-like behavior was due to the absence of a network structure. Likewise the addition of polysorbate 20 (PS20) to the 0.7 mg/mL solutions decreased the degree of shear thinning by disrupting the network structure at the interface. Taken together, the results presented here suggest that, although shear thinning behavior is a manifestation of an interfacial, rather than a bulk, phenomenon, the extent of it depends on how susceptible the surface molecules are to the air-water interface, where the surface molecular structures are influenced by the bulk properties.