A survey of 117 species of arthropods and 17 species of vertebrates showed a strong trend for male genitalia to have relatively low static allometric values. This trend contrasts with the allometry of other structures under sexual selection, which usually show steep allometric slopes. The trend to low allometric genital values is less consistent in mammals than in arthropods. Data not in accord with the previous the “one-size-fits-all” explanation for low allometric slopes in genitalia, which was based on sexual selection by female choice, suggest a more general version that includes both natural selection and sexual selection, and involves both mechanical fit and stimulation. Less-complete data on the female genitalia of arthropods suggest a trend to similar low allometric slopes, and may also be explained by mechanical fit and stimulatory one-size-fits-all arguments.