Sexual selection is frequently invoked to explain sexual dimorphism in characters such as the head size of lizards. For many species previously studied, it has been shown that male heads are larger than females heads at a given body size. This increased relative head size in males at sexual maturity is associated with advantages of large heads in intrasexual encounters. Males of the iguanid lizard Sceloporus undulatus have larger heads than females of similar size, but the difference is largely accounted for by a reduction in the rate of head growth relative to body-size growth in females rather than strictly by a relative increase in male head size. Presumably, females invest mostly in body growth and reproduction at the expense of an increase in head size once sexual maturity is reached. Consequently, we caution future investigators against attributing sexual differences in morphological characters to sexual selection unless independent data (e.g., behaviors) implicate morphological characters as determinants of differential reproductive success.