[Solved] Variable costs per unit vary directly with levels of production. Fertile Females Theory: A hypothesis in evolutionary biology suggesting that certain behavioral or physical traits in females signal reproductive potential to males. Fraternal Birth-Order Effect: A phenomenon where the more older brothers a man has, the higher his likelihood of being homosexual. Genetic Basis: The underlying genetic makeup or heritable factors that contribute to the physical and behavioral characteristics of an organism. Erotic Plasticity: The degree to which an individual's sex drive or sexual interests can be shaped and altered by cultural, social, and situational factors.