*Sexual reproduction and variation.* Sexual reproduction is widespread across many eukaryotic lineages, while asexual and mixed reproductive strategies also occur. The evolutionary maintenance of sex remains an active research problem: recombination, parasite coevolution, mutation load, ecological conditions, and lineage-specific histories can all matter. Bdelloid rotifers, parthenogenetic vertebrates, and other asexual lineages require explanation, but they do not simply function as exceptions that confirm a single rule. Muller's ratchet and Red Queen models are influential proposals, not a settled proof that sex is the dominant solution to one universal variation-generation requirement. The book can use these literatures to motivate questions about variation and lineage continuity; it should not present them as confirmation of its modal-structural account.

*Affective registration.* Across mammalian and avian lineages, courtship behaviors involve sustained affective registration (Bekoff and Byers 1998; Bagemihl 1999). The neurochemistry of attraction (dopaminergic mesolimbic circuits, oxytocinergic bonding circuits) is shared across mammals and traces to the architecture [§8.1](<8.1 Drive.md>) names — the SEEKING and pair-bonding systems Panksepp (1998) identified as primary emotional circuits. Carter (1998) on oxytocin and pair-bonding extends the same picture into the social-affiliative register that adjoins sexual coupling.

*Identity construction.* Cross-cultural anthropology documents both the universality of gender as a structural feature of human social organization and the variability of its specific construction. Self-aware identity-positions in sexuality — orientation and gender — are documented across cultures with significant variability in how they are named and lived (Herdt 1994; Roughgarden 2004). The content of identity-positions is variable; that they are constructed at the depth [§8.5](<8.5 Identity.md>) names is universal. Same-sex sexual behavior is documented across hundreds of vertebrate species (Bagemihl 1999), foreclosing the assumption that orientation maps cleanly onto reproductive function. Asexuality — the stable absence of sexual attraction, distinguished in the empirical literature from low desire and from sexual dysfunction — is documented as a distinct orientation at roughly one percent of the population (Bogaert 2004, 2012): the predicted case in which the sexual-attraction register does not form while drive and, in many individuals, libido remain.

*Variation across the animal kingdom.* The substrate-variation pattern extends well beyond same-sex behavior, and the framework predicts it should. Sexual reproduction is universal across eukaryotes; the architecture supporting it is not. Sex determination has multiple structural implementations — XY in placental mammals, ZW in birds and many reptiles, haplodiploidy in hymenoptera, temperature-dependent in many reptiles, environmental in some fish. Sequential hermaphroditism is widespread: clownfish change from male to female (protandry), many wrasses and parrotfish change in the other direction (protogyny), with the transition triggered by social-structural cues. Simultaneous hermaphroditism is the norm across many invertebrate phyla and obtains in some fish; the chalk bass trades egg-laying and sperm-donating roles with a partner within a single spawning bout, switching roles as often as twenty times in an evening (Fischer 1984; Hart et al. 2016). Sustained same-sex pair-bonding, courtship, and coupling is documented in over 1,500 species (Bagemihl 1999; Bailey and Zuk 2009; Monk et al. 2019): roughly one-third of Laysan albatross pairs in some Hawaiian colonies are female-female (Young et al. 2008); bonobo female-female sexual contact is integral to social cohesion; bottlenose dolphins, Japanese macaques, and many other species show extensive same-sex coupling; sheep show stable male-male partner preference in approximately 8% of rams (Roselli et al. 2004) with corresponding hypothalamic correlates. Within-species role-morph variation appears wherever the framework predicts substrate can support multiple stable beings: side-blotched lizards have three male morphs running rock-paper-scissors reproductive strategies (Sinervo and Lively 1996); ruffs have three male morphs including female-mimicking faeders; bluegill sunfish have parental, sneaker, and satellite male types (Gross and Charnov 1980); Mozambique tilapia have three stable male morphs, one of which recruits a subordinate male into cooperative courtship of females (Roughgarden 2004, citing Oliveira and Almada 1998). Hyena females have a pseudo-penis and female-dominant social organization. What earlier observers read as a male-active, female-passive default has been extensively revised by a generation of evolutionary biologists working on female sexual selection, female-female competition, female promiscuity, and matriarchal social organization, with Cooke (2022) surveying the empirical reversal across taxa. Extended postreproductive female lifespan — menopause — has evolved independently in humans and in several toothed-whale lineages (orcas, belugas, narwhals, short-finned pilot whales), in each case coupled with grandmother-mediated kin support rather than continued reproduction (Ellis et al. 2024): a second architecture for allocating female reproductive investment across the lifespan, not a deviation from a single mammalian default. Across all of this, what the current human-centric framing calls “the standard arrangement” is one stable architecture among many — exactly what is predicted if sexuality is substrate-architecture-shaping-attraction-and-coupling rather than a single canonical type the lineage either instantiates or fails at.

*Neural correlates of orientation and identity.* The substrate-architecture prediction extends to the brain-level findings on human sexual orientation and gender identity. Orientation: twin studies show meaningful but partial heritability (Bailey and Pillard 1991; Bailey et al. 2000); a 477,000-person GWAS found polygenic contributions distributed across the genome with no single locus dominating (Ganna et al. 2019); prenatal androgen exposure shapes orientation outcomes (Hines 2011 on congenital adrenal hyperplasia; Kung, Louie, Spencer, and Hines 2024 meta-analysis of 50+ years of CAH studies); the fraternal birth order effect is consistently replicated and currently best explained by maternal-immune mechanisms (Blanchard 2001; Bogaert et al. 2018 on the maternal-immune mechanism via NLGN4Y); INAH-3 size differences between gay and straight men have been reported and partially replicated (LeVay 1991; Byne et al. 2001). Identity: the central subdivision of the bed nucleus of the stria terminalis (BSTc) in trans women resembles the cis-female pattern rather than the cis-male pattern (Zhou et al. 1995; Kruijver et al. 2000), with the developmental window predating adult hormone treatment; pre-treatment white-matter microstructure in trans individuals shows patterns closer to their identified gender than their natal sex (Rametti et al. 2011a, 2011b), with replication-focused studies converging on the putamen as the most robust structural correlate of gender identity (Mueller et al. 2021). The pattern across these findings matches what the framework predicts: orientation- and identity-architecture is shaped at the substrate level by genetic, developmental, and hormonal factors before the [§8.5](<8.5 Identity.md>) self-model has constructed anything around them, and the construction stabilizes around what the substrate already supports. These findings illustrate the substrate variability the framework predicts; they do not carry the structural claim. The identification of orientation and identity as self-model constructions over a variable substrate holds by fit with the overall pattern, and no single result — BSTc, INAH-3, the fraternal-birth-order effect, or any one GWAS or neuroimaging finding — is load-bearing: were a specific correlate revised or fail to replicate, the structural claim would stand, since identity and standing are fixed at the [§8.5](<8.5 Identity.md>) and [§8.12](<8.12 Morality.md>) registers rather than by any neural marker.

*Sexual ethics.* Cross-cultural moral codes invariably regulate sexuality, and the structural features the codes track — consent, fidelity, harm, the production and care of children — are everywhere recognized as morally weighted, even when specific rules differ. Brown's (1991) human universals catalog includes multiple sexuality-related entries; the universality of regulation is itself the empirical face of the [§8.12](<8.12 Morality.md>) prediction that intersubjective coupling at this density requires structural maintenance.

*Reproduction-choice.* In cultures with reliable contraception, reproduction has shifted from default to chosen, and the consequences for individual life-course planning, gender relations, and demographic structure are now the subject of substantial empirical literature. The shift confirms the prediction that, at sufficient self-modeling depth and given the technological conditions, the agentive field expands to include reproduction itself.


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← [8.6.3 What is Predicted](<8.6.3 What is Predicted.md>) | [8.6.5 What is Concluded](<8.6.5 What is Concluded.md>) →
