Male primates often have larger and more projecting canine teeth than females. Species comparisons show an association between the level of canine sexual dimorphism and the intensity of male-male competition, suggesting that canines function as a weapon in inter-male conflicts. Compared with most non-human primates, humans have small and non-projecting canines. It has been shown that male canines were already reduced in Mio-Pliocene hominins, long before the emergence of tool making or large brain. In other words, humans were physically weak from the very beginning. Even though natural selection is likely to have played a part, it is unknown why small canines could be selectively favored over larger ones. My hypothesis supposes socially-induced selection for physical weakness. Specifically, I focus on the possibility that partner choice in coalition formation might have created a novel selection pressure. I further explore this possibility using an individual-based simulation model.