The Dual-Layer Hypothesis of Breast Attractiveness: Evolutionary Baseline and Cultural Modulation

Background: Hypotheses on the evolutionary origin of human breast morphology have been proposed since Cant (1981); Caro's (1987) critical review concluded that existing hypotheses lacked sufficient empirical support, and Pawłowski and Żelaźniewicz (2021) have since proposed a by-product-then-co-option account of breast origin. On the attraction side, Stefańczyk et al. (2025) demonstrated through a cross-generational study of the Dani people that male attraction to female breasts persists independently of cultural exposure norms. Yet an integrative framework for the mechanisms governing variation in attraction intensity across contexts, cultures, and individuals remains undeveloped. Objective: This paper proposes the Dual-Layer Hypothesis: attractiveness responses to breast morphology emerge from the interaction of evolved baseline circuits and intensity parameters that are modulated by learning, culture, context, and individual differences. The baseline layer is stable at the population-mean level; the modulation layer manifests at the variance level and is therefore undetectable by mean-level measurement alone. Framework Components: In addition to the main hypothesis, five subsidiary hypotheses are formalized: (1) Variance Amplification, concealment cultures amplify variance in attractiveness ratings; (2) Context Code, contextual factors determine attraction intensity for identical stimuli; (3) Uncanny Bypass, two-dimensional abstraction bypasses the uncanny valley and directly activates mating circuits; (4) Arousal-Decision Decoupling, arousal and mate-choice decision circuits are partially independent; (5) Tactile Verification, breast contact functions as a multi-channel verification mechanism. Each hypothesis generates testable predictions (P1.1-P6.4). Implications: The framework does not refute Stefańczyk et al.'s results; rather, it reinterprets their intra-individual arousal-versus-importance asymmetry as direct support for the Arousal-Decision Decoupling hypothesis, and it formalizes the field's next-step questions. Variance-level re-analysis of cross-cultural data, controlled contextual experiments, and direct neuroimaging of contrastive (2D vs. 3D) breast stimuli constitute principal verification paths.