Understanding the factors that contribute to the curly morphology of human hair is important for anthropological and physiological studies. In the present study, the possible role of mechanical stress on the formation and selection of such a curly pattern of hair is explored. Based on the model of hair curling upon being squeezed through pinched nails, it is found that different levels and configurations of external forces could lead to the variation of residual strains, which in turn, is related to the two-dimensional (2D) and three-dimensional (3D) spatial hair curvatures. The mechanical principles are readily applicable to hair curling geometry due to the growth force exerted by hair follicles. The key mechanical parameters controlling the formation of a curly pattern of hair, as well as the key geometrical shape factors characterizing the hair morphology, are identified and correlated. The results obtained from the mechanical model are qualitatively consistent with those of previous experiments and observations.