When success stops producing more descendants: Behavioral return, fitness return, and the parity-specific decoupling behind below-replacement fertility

Evolutionary Behavioral Sciences. 2026.

Gallyamova A., Grigoryev D.

Across many post-transitional societies, fertility has fallen below replacement even while reported family-size ideals often remain near or above two children. Because stated ideals are imperfect indicators of reproductive preferences and are declining in some populations, this pattern is not universal; the present framework addresses settings in which reported aspirations continue to exceed achieved fertility. We connect this demographic issue to a longstanding problem in human sociobiology: why success in socially valued domains no longer maps reliably onto reproductive and descendant success. We distinguish fitness return on investment (fROI), the long-horizon conversion of a practice into descendant output and lineage persistence, from behavioral return on investment (bROI), the behavior-regulating payoff structure that governs whether a practice is adopted, maintained, delayed, or abandoned in a given social ecology. We argue that post-transitional environments can produce partial, sex-specific, and parity-specific decoupling between these returns: practices that yield security, status, risk reduction, or social approval may remain behaviorally advantageous while delaying or suppressing reproduction. The reproductive sequence includes a context-dependent pre-parity partnership gate that can constrain entry into parenthood. The four principal pathways—preparation, postponement, support, and quality intensification—alter the probabilities of transitions from 0→1, 1→2, and 2→3+ children, with preparation also shaping access to the gate itself. Because information about proximate returns is visible and culturally transmissible whereas descendant consequences are delayed, high-bROI practices can persist after their coupling to long-run fROI has weakened. This framework explains how a culturally salient family-size ideal can coexist with a parity structure that does not sustain replacement, and yields testable propositions for evolutionary demography, human behavioral ecology, and cultural evolution.

Keywords: parity progression ratio (PPR), cultural evolution, parity progressions, life history theory, below-replacement fertility, parity progression, quantity-quality trade-off, evolutionary demography

Publication based on the results of: Independent creative action of different categories of the population in various spheres of activity: universal and specific characteristics (2025)