<p>The “twofold cost of sex” is one of the greatest mysteries in evolutionary biology. In anisogamy (fertilization between egg and sperm), since half the offspring produced by the mother are male, to compete with thelytoky—which produces only females—in terms of reproductive rate, the number of daughters × juvenile survival probability must be doubled. The genetic benefits of sex (e.g., Red Queen) are too slow to suppress quickly increasing thelytoky. Here, I propose the “Double-Income Anisogamy” hypothesis. Resource defense by males is observed across many taxa, from insects to humans. To attract many females, males defend patches containing the resources necessary for offspring development. Since males only allow access to resources to females that accept mating, thelytokous females are eliminated. Thus, thelytokous females incur the “cost of being single”. Anisogamous females already achieve the same egg number (although half of which are males) and survival rates as thelytokous females using their own resources alone (single-income situation). If anisogamous females use resources from males (double income) to increase not the number of eggs but the survival rate of existing offspring, the number of surviving daughters (fitness) doubles, offsetting the twofold cost without considering genetic benefits. Simulations confirmed the model predictions.</p>