There is a tendency among biologists studying social behavior to regard the adult sex ratio as an independent variable to which the species reacts with appropriate adaptations. D. Lack often interprets social behavior as an adaptation in part to an unbalanced (or balanced) sex ratio, and J. Verner has summarized other instances of this tendency. The only mechanism that will generate differential mortality independent of sexual differences clearly related to parental investment and sexual selection is the chromosomal mechanism, applied especially to humans and other mammals: the unguarded X chromosome of the male is presumed to predispose him to higher mortality. Each offspring can be viewed as an investment independent of other offspring, increasing investment in one offspring tending to decrease investment in others. Species can be classified according to the relative parental investment of the sexes in their young. In the vast majority of species, the male's only contribution to the survival of his offspring is his sex cells.