Fig. 1.

Schematic illustration of fetal needs and maternal capacity to invest. TOP PANEL: X-axis describes Darwinian fitness. Y-axis on the left (orange) indicates level of resource need for fetuses F, which varies across pregnancies. Needs of fetuses F₁ decline with increased fitness. Y-axis on the right (blue dash) indicates maternal capacity to invest in children M₁; maternal capacity varies positively with maternal fitness. Selection in utero occurs when F₁ > M₁. Vertical dashed line indicates point at which F₁ = M₁. BOTTOM PANEL: To the left of this dashed line, the “left tail” of the frequency distribution of fetuses (shaded) undergoes selection in utero. Theory suggests that fewer high need infants will be born when the environment weakens women of reproductive age (i.e., down-shift of M₁ to M₂ while F₁ remains fixed). Selection in utero, moreover, would increase when maternal resources remain fixed but needs of the fetus shift upward (i.e., F₁ to F_2, with M₁ fixed). Perturbations of maternal capacity and fetal need may also occur simultaneously and interact according to parental-offspring conflict theory (Haig, 1993).