Sex-role evolution theory attempts to explain the origin and direction of male–femaledifferences. A fundamental question is why anisogamy, the difference in gamete size thatdefines the sexes, has repeatedly led to large differences in subsequent parental care. Herewe construct models to confirm predictions that individuals benefit less from caring whenthey face stronger sexual selection and/or lower certainty of parentage. However, weoverturn the widely cited claim that a negative feedback between the operational sex ratioand the opportunity cost of care selects for egalitarian sex roles. We further argue that ourmodel does not predict any effect of the adult sex ratio (ASR) that is independent of thesource of ASR variation. Finally, to increase realism and unify earlier models, we allow forcoevolution between parental investment and investment in sexually selected traits.Our model confirms that small initial differences in parental investment tend to increase dueto positive evolutionary feedback, formally supporting long-standing, but unsubstantiated,verbal arguments.
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