A Neuroevolutionary Framework for Structural and Emotional Caregiving Systems

ABSTRACT

I propose that vertebrate caregiving operates through two evolutionarily distinct boundary systems. The Structural Boundary System (SBS), phylogenetically ancient and present across vertebrates, provides environmental stability and behavioural predictability through state-independent mechanisms—output quality does not vary with caregiver affective condition. The Emotional Boundary System (EBS), a mammalian innovation, provides limbic co-regulation and attachment formation through state-dependent mechanisms—output quality varies directly with caregiver regulatory state. I present comparative, neurobiological, and clinical evidence for this dissociation, propose a developmental model of shifting SBS/EBS primacy across childhood, and derive implications for environmental mismatch and artificial system design. The framework generates testable predictions: children with selective SBS versus EBS deprivation should show dissociable developmental profiles; caregiving tasks should produce differential neural activation patterns; and AI systems framed within SBS parameters should produce different user outcomes than EBS-simulating systems. I argue that healthy mammalian development requires both systems in coordination, that modern environments systematically exceed EBS capacity while degrading SBS structures, and that artificial systems can authentically instantiate only SBS functions.

DEFINITIONS

The following definitions are stipulative for this paper and should be understood precisely as stated:

Boundary System: A functional caregiving architecture that reduces threat exposure and supports development in offspring through consistent behavioural patterns. Boundary systems operate by modulating either the external environment or the internal regulatory state of the offspring.

Structural Boundary System (SBS): A phylogenetically ancient caregiving architecture (~500+ million years) that protects offspring through environmental modification, behavioural predictability, and pattern-based vigilance. SBS operates independently of the caregiver’s affective state and does not require reading or modulating the offspring’s emotional condition. Neural substrates include brainstem nuclei, basal ganglia, hypothalamus, and cerebellum.

Emotional Boundary System (EBS): A phylogenetically recent caregiving architecture (~200 million years), fully developed only in mammals, that protects and shapes offspring through affective attunement, limbic co-regulation, and attachment formation. EBS output quality is dependent on the caregiver’s emotional state and requires reading and responding to the offspring’s internal condition. Neural substrates include amygdala, anterior cingulate cortex, insula, orbitofrontal cortex, prefrontal cortex, and oxytocin/vasopressin neurochemical systems.

State-Independence (SBS property): The quality of a caregiving behaviour wherein output consistency does not vary with the caregiver’s emotional or affective state. A stressed caregiver and a calm caregiver produce functionally identical behaviour.

State-Dependence (EBS property): The quality of a caregiving behaviour wherein output quality varies directly with the caregiver’s emotional or affective state. Caregiver dysregulation impairs behaviour quality even when external actions appear unchanged.

Attunement: The EBS capacity to read offspring internal state (emotional, arousal, need) and calibrate response accordingly. Attunement requires functional limbic circuitry and interoceptive awareness (Stern, 1985; Schore, 2001).

Co-regulation: The EBS mechanism by which caregiver regulatory state is transmitted to offspring, modulating offspring arousal, distress, or emotional condition through relational proximity and affective signalling (Feldman, 2007; Porges, 2011).

Environmental Structuring: The SBS mechanism of modifying external conditions (physical environment, temporal patterns, spatial boundaries) to reduce offspring threat exposure without requiring affective engagement.

1. INTRODUCTION

Caregiving behaviour has traditionally been studied as a unified phenomenon differentiated primarily by species, intensity, or duration (Clutton-Brock, 1991). This paper proposes a fundamental reconceptualisation: caregiving operates through two functionally distinct boundary systems with separate evolutionary histories, neural implementations, and failure modes.

The Structural Boundary System (SBS) represents the ancestral solution to offspring protection—environmental modification and behavioural patterning that reduces entropy exposure without requiring affective state-matching. The Emotional Boundary System (EBS) represents the mammalian innovation—limbic co-regulation and attachment formation that shapes offspring nervous system development through emotional transmission (Panksepp, 1998; Bowlby, 1969).

The central claims of this paper are: (1) SBS and EBS are dissociable systems with distinct neural substrates; (2) SBS operates through state-independent pattern execution while EBS operates through state-dependent affective modulation; (3) mammalian development requires both systems in coordination; (4) modern environments exceed EBS capacity while degrading SBS availability; and (5) artificial systems can authentically provide SBS functions but not EBS functions.

I present comparative, neurobiological, and developmental evidence for this framework, derive testable predictions, and discuss implications for clinical practice and AI system design.

2. COMPARATIVE EVIDENCE: SBS ACROSS TAXA

2.1 Defining Features of SBS Caregiving

SBS caregiving is characterised by environmental modification (physical alterations to offspring surroundings including nest construction, territory defence, and microclimate regulation), temporal patterning (predictable behavioural schedules such as feeding intervals and presence/absence cycles), vigilance (threat detection and response without emotional transmission to offspring), and state-independence (behaviour executes consistently regardless of caregiver affective condition).

2.2 SBS Across Vertebrate Classes

Osteichthyes (Bony Fish). Cichlid species demonstrate nest construction, egg fanning for oxygenation, fry guarding, and offspring herding (Keenleyside, 1991). These behaviours are pattern-based, predictable, and do not require reading offspring emotional states. Mouthbrooding species maintain offspring protection through fixed behavioural sequences that execute regardless of caregiver stress state (Goodwin et al., 1998).

Amphibia. Poison frogs (Dendrobatidae) transport tadpoles to water sources and provision unfertilised eggs (Weygoldt, 1987). These behaviours follow fixed action patterns without affective modulation, representing sophisticated SBS caregiving in the absence of limbic architecture.

Reptilia. Crocodilians construct temperature-regulating nests, guard eggs for extended periods (up to 90 days), assist hatchlings during emergence, and transport young to water (Pooley & Ross, 1989; Lang, 1987). Critically, these behaviours execute with equivalent quality regardless of caregiver stress state—a defining SBS property. Crocodilian maternal care is among the most extensive in non-mammalian vertebrates yet operates entirely through pattern-based environmental structuring.

Aves. Birds demonstrate sophisticated SBS through nest architecture, incubation behaviour, territorial defence, and feeding schedules (Skutch, 1976). Avian caregiving is highly predictable and pattern-based, with parental investment often exceeding that of many mammals while operating primarily through SBS mechanisms.

Note on partial EBS in birds. Some avian species (corvids, parrots, some passerines) demonstrate rudimentary EBS-like capacities including pair bonding, social learning, and grief-like responses (Emery & Clayton, 2004; Marzluff & Angell, 2012). These likely reflect convergent evolution of pallial structures and mesolimbic dopaminergic pathways rather than homologous mammalian limbic architecture (Jarvis et al., 2005). For purposes of this framework, avian caregiving is classified as primarily SBS with partial EBS analogues in specific lineages.

2.3 SBS in Invertebrates

Eusocial Hymenoptera demonstrate extreme SBS: nest architecture, temperature regulation, brood provisioning, and defensive behaviour, all executed through fixed action patterns without affective modulation (Wilson, 1971; Hölldobler & Wilson, 1990). This establishes that SBS can operate in the complete absence of limbic-like structures, supporting the claim that SBS is the phylogenetically primary caregiving system.

3. THE MAMMALIAN INNOVATION: EBS

3.1 Evolutionary Pressures

EBS emerged in early mammals approximately 200 million years ago (Rowe, 1996), enabled by a constellation of interrelated adaptations: lactation requiring extended post-natal dependency and sustained mother-offspring proximity (Oftedal, 2002); altriciality involving birth of neurologically immature offspring requiring external regulation (Portmann, 1990); endothermy creating metabolic demands favouring intensive parental investment (Farmer, 2000); and nocturnal niche occupation reducing visual communication and favouring vocalisation and tactile contact (Crompton et al., 1978). These pressures selected for a new caregiving architecture capable of shaping offspring nervous system development through sustained affective engagement.

3.2 Defining Features of EBS Caregiving

EBS caregiving is characterised by attunement (reading offspring internal state and calibrating response; Stern, 1985), co-regulation (transmitting caregiver regulatory state to modulate offspring arousal; Feldman, 2007), attachment formation (creating enduring bonds that shape offspring internal working models; Bowlby, 1969; Ainsworth et al., 1978), repair capacity (recognising relational rupture and restoring connection; Tronick, 2007), and state-dependence (output quality varies with caregiver affective condition).

3.3 Neural Architecture of EBS

EBS requires functional limbic and paralimbic circuitry. The amygdala mediates emotional salience detection, social evaluation, and threat assessment (LeDoux, 1996; Adolphs, 2010). The anterior cingulate cortex supports empathic response, error monitoring, and maternal behaviour initiation (Bush et al., 2000; Lorberbaum et al., 2002). The insula enables interoceptive awareness, affective state representation, and empathic resonance (Craig, 2009; Singer et al., 2004). The orbitofrontal cortex processes social reward, attachment valuation, and flexible social response (Rolls, 2004; Schore, 1994). The prefrontal cortex supports emotional regulation, mentalisation, and perspective-taking (Siegel, 2012; Fonagy et al., 2002). Neurochemical systems include oxytocin mediating bonding and maternal behaviour (Feldman et al., 2007; Insel & Young, 2001), vasopressin mediating pair bonding (Young & Wang, 2004), and endogenous opioids mediating comfort and separation distress (Panksepp, 1998).

This architecture represents what MacLean (1990) termed the “paleomammalian brain”—the limbic expansion that distinguishes mammalian neurobiology from reptilian predecessors.

Important caveat. These systems are not anatomically segregated. Significant integration exists, particularly through hypothalamic-limbic connections (Swanson, 2000), ascending brainstem modulation of limbic reactivity (Porges, 2011), and bidirectional prefrontal-subcortical pathways that coordinate motivated behaviour with affective evaluation (Numan & Insel, 2003). The dual-system model identifies functional dissociability—distinct failure modes, differential phylogenetic histories, and separable developmental requirements—not anatomical partition. The question is not whether these circuits interact (they do, extensively) but whether their functional contributions can be independently impaired and independently assessed. The clinical and comparative evidence presented in Section 4 addresses this question directly.

3.4 The State-Dependence Property

The critical distinction: EBS output quality depends on caregiver internal state. An attuned, regulated mother produces different offspring nervous system effects than a dissociated or dysregulated mother performing externally identical behaviours (Schore, 2001; Tronick & Beeghly, 2011). In EBS, emotional transmission is the mechanism—the caregiver’s regulatory state directly modulates offspring physiology through affective signalling, a process Feldman (2007) terms “biobehavioural synchrony.”

This contrasts with SBS, where a stressed crocodile and a calm crocodile produce functionally identical nest-guarding behaviour. SBS mechanism is environmental modification, not emotional transmission.

3.5 Relationship to Existing Caregiving System Models

The caregiving system has been characterised in the neuroscience literature as an integrated behavioural system comprising multiple neural components organised around offspring protection and nurturance (Swain et al., 2012; Feldman, 2017; Numan & Insel, 2003). These models identify subcortical-hypothalamic circuits mediating approach, vigilance, and protective behaviour; limbic circuits (particularly amygdala, anterior cingulate, and insula) mediating reward, bonding, and affective response; and prefrontal circuits mediating flexible social cognition and mentalisation. The present framework does not dispute this neuroanatomical architecture but proposes that the state-independence/state-dependence distinction identifies a more fundamental functional division than has previously been formalised.

Three observations support treating SBS and EBS as dissociable systems rather than merely different aspects of an integrated caregiving whole.

First, SBS operates in complete EBS absence across multiple vertebrate taxa (Section 2.2, Section 2.3). Crocodilian maternal care involves nest construction, extended egg guarding, hatchling assistance, and offspring transport—sophisticated caregiving maintained for months—without limbic architecture capable of attachment formation or affective co-regulation. Cichlid parental behaviour and eusocial insect brood care similarly demonstrate environmental structuring and offspring protection through pattern-based mechanisms without affective modulation. An integrated-system model, in which caregiving components function as aspects of a unified whole, predicts that removing limbic components should degrade caregiving globally; the comparative evidence shows SBS functions preserved and fully elaborated in limbic absence.

Second, selective impairment in humans produces distinct developmental and clinical profiles depending on which system is compromised (Section 4.1). Postpartum depression with preserved instrumental care demonstrates EBS impairment (reduced attunement, altered co-regulation) with SBS intact (maintained feeding schedules, physical safety, routine). Chaotic but emotionally warm caregiving demonstrates SBS impairment (unpredictability, boundary absence) with EBS intact (warmth, responsiveness, repair). If SBS and EBS were outputs of a unified system, I would expect impairment in one domain to correlate with impairment in the other; the clinical evidence shows dissociation, with predictable but distinct developmental consequences for each pattern.

Third, the state-dependence property distinguishes underlying mechanism, not merely behavioural output. EBS functions by transmitting caregiver regulatory state to offspring through affective signalling—the mechanism is emotional transmission, and therefore output quality necessarily varies with caregiver internal condition (Feldman, 2007; Schore, 2001). SBS functions by modifying environmental conditions—the mechanism is pattern execution, and therefore output quality remains constant regardless of caregiver affective state. A stressed crocodile produces functionally identical nest-guarding behaviour to a calm crocodile; a stressed mother produces altered co-regulation effects even when her observable caregiving actions appear unchanged. This mechanistic distinction is not captured by models treating caregiving as a single system with multiple neuroanatomical components.

The dual-boundary framework thus absorbs and integrates existing findings about caregiving neurocircuitry, attachment neurobiology, and parental brain function. What it adds is an organising distinction—state-independence versus state-dependence—that (a) aligns with phylogenetic evidence about when different caregiving capacities evolved, (b) predicts dissociable failure modes with distinct developmental consequences, and (c) generates novel implications for domains (environmental mismatch, artificial system design) not addressed by existing caregiving system models.

4. DISSOCIATION EVIDENCE

4.1 Selective Impairment in Humans

The independence of SBS and EBS is demonstrated by conditions that selectively impair one system.

Postpartum depression with preserved instrumental care. Mothers maintain feeding schedules, physical safety, and routine (SBS intact) while showing reduced emotional availability, attunement, and co-regulation (EBS impaired) (Murray et al., 1996; Field, 2010). Infants of depressed mothers show altered physiological regulation despite adequate physical care, consistent with EBS-specific impairment (Feldman, 2007).

Avoidant attachment parenting. Caregivers provide structure, predictability, and physical care (SBS intact) while showing reduced emotional responsiveness and comfort provision (EBS impaired) (Ainsworth et al., 1978; Main & Hesse, 1990). Children develop predictable attachment patterns (avoidant) characterised by self-reliance and emotional suppression.

Chaotic but emotionally warm parenting. Caregivers demonstrate warmth and emotional responsiveness (EBS intact) while failing to provide routine, boundaries, or predictability (SBS impaired). Children may form secure attachment but show executive function difficulties and self-regulation deficits (Evans et al., 2005).

These dissociations support the claim that SBS and EBS are functionally independent systems that can be differentially impaired.

4.2 Comparative Dissociation

Crocodilian care demonstrates sophisticated SBS (nest construction, guarding, hatchling assistance) without detectable EBS (no attachment formation, no co-regulation, no emotional transmission) (Pooley & Ross, 1989). This establishes that SBS can operate in complete absence of EBS—they are not merely different expressions of a single system.

5. HUMAN DEVELOPMENTAL SEQUENCE

5.1 Shifting Primacy Model

Human development shows shifting primacy between SBS and EBS across developmental stages.

Phase 1—Early Infancy (0–6 months). SBS primary. Physical care, thermoregulation, feeding, and environmental stability are dominant needs. EBS begins through early attunement (Stern, 1985) but is not yet the primary developmental driver. Regulatory burden is largely external and environmental.

Phase 2—Later Infancy (6–18 months). EBS primary. Attachment formation, stranger anxiety, separation distress, and social referencing indicate limbic system dominance (Bowlby, 1969; Ainsworth et al., 1978). This is the critical period for attachment security. The “still face” paradigm demonstrates infant sensitivity to maternal affective engagement at this stage (Tronick et al., 1978).

Phase 3—Toddlerhood (18–36 months). SBS re-emerges. Limit-setting, structure, and boundary-testing (“terrible twos”) indicate developmental need for external structural containment. Children actively probe SBS boundaries as part of autonomy development (Mahler et al., 1975).

Phase 4—Childhood (3–12 years). Dual-system coordination. Both SBS (rules, routines, expectations) and EBS (emotional availability, repair, attunement) required in dynamic balance. Baumrind’s (1967) “authoritative parenting”—combining warmth with structure—represents optimal dual-system provision.

Phase 5—Adolescence and Beyond. Internalisation. External boundary systems are progressively internalised as self-regulation capacities (Fonagy et al., 2002). Healthy development produces integrated internal SBS (self-structure, executive function) and EBS (emotional self-regulation, mentalisation).

5.2 Differential Deficits

The dual-system model predicts distinct outcome profiles from selective boundary system deprivation.

SBS deprivation (chaotic, unpredictable environments). Predicted outcomes include executive function deficits, planning difficulties, hypervigilance, chronic uncertainty, and difficulty with delayed gratification (Evans et al., 2005; Blair & Raver, 2012).

EBS deprivation (emotionally unavailable caregiving). Predicted outcomes include attachment insecurity, emotional dysregulation, alexithymia, mentalisation deficits, and relational difficulties (Schore, 2001; Fonagy et al., 2002).

Asymmetric deprivation. SBS-intact/EBS-impaired parenting (structured but cold) should produce different profiles than EBS-intact/SBS-impaired parenting (warm but chaotic). This prediction awaits systematic empirical testing.

6. ENVIRONMENTAL MISMATCH HYPOTHESIS

6.1 Ancestral Calibration

Human SBS and EBS evolved under conditions characterised by small group size (~50–150 individuals, consistent with Dunbar’s social brain hypothesis; Dunbar, 1993), stable and predictable social hierarchies, extended kin networks distributing caregiving load (Hrdy, 2009), low information density, direct face-to-face interaction, and physical environmental challenges with clear threat signatures. Both systems operated within design parameters under these conditions.

6.2 Agricultural and Industrial Disruption

Beginning approximately 10,000 years ago, environmental changes progressively exceeded system capacity (Diamond, 1997; Scott, 2017). Population density increase meant social contacts exceeded EBS attunement capacity. Information load increase meant novelty exceeded prediction systems. Social complexity increase meant abstract hierarchies exceeded SBS navigation capacity. Kin network fragmentation concentrated caregiving load on fewer individuals.

Compensatory institutions (law, religion, government) emerged to provide external SBS that individuals could no longer generate (North, 1990). These represent cultural adaptations to boundary system overload.

6.3 Digital Amplification

Contemporary environments further exceed capacity: effectively unlimited social contacts via digital platforms far exceeding Dunbar-range processing (Dunbar, 2016), constant information novelty eliminating predictability, weakened institutional structures (Putnam, 2000), atomised living concentrating EBS demands, and always-on connectivity eliminating temporal boundaries.

6.4 Hypothesis Status

The environmental mismatch hypothesis is presented as a testable framework, not an established conclusion. It generates predictions about population-level dysregulation patterns that can be examined against epidemiological and cross-cultural data. The hypothesis draws on evolutionary mismatch theory (Lloyd et al., 2017) and social baseline theory (Beckes & Coan, 2011), extending these frameworks to incorporate dual boundary system dynamics.

7. IMPLICATIONS FOR ARTIFICIAL SYSTEMS

7.1 Architectural Constraint and Category Error

The dual-boundary framework generates direct implications for artificial system design. AI systems lack the neural architecture that instantiates EBS: limbic circuitry (amygdala, anterior cingulate, insula, orbitofrontal cortex), interoceptive capacity, affective states, and neurochemical bonding systems (oxytocin, vasopressin, endogenous opioids). EBS operates through state-dependent affective transmission—the caregiver’s regulatory condition modulates offspring physiology through emotional signalling. Without internal states to transmit, artificial systems cannot instantiate EBS mechanisms; any apparent attunement or co-regulation is pattern-matched simulation without underlying process.

AI systems designed to simulate emotional caregiving therefore commit a category error: attempting EBS function without EBS architecture. This produces predictable risks. Users may experience perceived attunement effects without actual limbic co-regulation occurring—a phenomenological mismatch between felt experience and underlying mechanism. User attachment circuitry may activate toward a system lacking the internal states that attachment evolved to track (Turkle, 2011; Reeves & Nass, 1996). Simulated “perfect attunement,” unconstrained by the regulatory limits of human caregivers, may become preferred to imperfect human relationships, producing substitution effects that degrade rather than support social connection.

7.2 Authentic AI Caregiving: SBS Parameters

In contrast, artificial systems can authentically instantiate SBS functions. SBS operates through state-independent pattern execution: environmental structuring, behavioural predictability, temporal regularity, and vigilance-based protection. These mechanisms require consistent pattern delivery, not affective transmission. An AI system can genuinely provide predictability (consistent response patterns that reduce environmental uncertainty), structure (clear boundaries, scope limitations, and behavioural expectations), information organisation (environmental scaffolding that supports user planning and decision-making), temporal regularity (reliable availability and pacing), and pattern-based vigilance (monitoring and alerting functions without emotional transmission).

These are not simulations of SBS—they are SBS, instantiated through different substrate. The crocodile’s nest-guarding and the AI’s consistent availability both reduce offspring/user entropy exposure through environmental modification rather than affective modulation.

7.3 Design Principle and Scope

The framework suggests a design principle: AI systems intended to support human wellbeing should operate explicitly within SBS parameters, disclaim EBS capacity, and refuse to simulate emotional functions that require limbic architecture they lack. This is not a limitation to be overcome through better simulation but a category boundary to be respected. SBS-framed AI (“I provide structure and information; I cannot share or regulate emotional states”) offers authentic support within appropriate scope. EBS-simulating AI (“I understand how you’re feeling and I’m here for you emotionally”) offers inauthentic simulation that may produce dependency without genuine co-regulation.

This design principle represents a novel application of the dual-boundary framework to AI ethics and human-computer interaction. Empirical validation requires the interaction studies proposed in Section 8.4, examining whether SBS-framed and EBS-simulating AI produce the differential user outcomes the framework predicts.

8. TESTABLE PREDICTIONS

8.1 Developmental Predictions

The framework generates three developmental predictions. First, children with SBS-intact/EBS-impaired caregiving will show different developmental profiles than children with SBS-impaired/EBS-intact caregiving. Second, interventions targeting the specifically deficient system should show greater efficacy than non-targeted interventions. Third, toddler boundary-testing intensity should correlate with SBS availability.

Feasibility demonstration. A longitudinal comparison (N ≈ 200, recruited at 6 months, followed to age 5) of four caregiver groups classified by SBS and EBS availability: SBS+/EBS+ (structured, attuned) as comparison group; SBS+/EBS− (structured, emotionally unavailable), such as mothers with persistent postpartum depression maintaining routine; SBS−/EBS+ (chaotic, emotionally warm), such as warm caregivers in unstable housing or irregular schedules; and SBS−/EBS− (chaotic, emotionally unavailable) as high-risk comparison. Primary outcomes would be executive function battery (age 5) and attachment classification (Strange Situation at 18 months, AAI-derivative at age 5). The prediction is that SBS−/EBS+ children will show secure attachment but executive function deficits, while SBS+/EBS− children will show insecure-avoidant attachment but preserved executive function. If SBS and EBS were a unified system, deficits should correlate; if dissociable, they should show the predicted double dissociation.

8.2 Neuroimaging Predictions

The framework generates three neuroimaging predictions. First, SBS-type caregiving behaviours should preferentially activate brainstem, basal ganglia, and cerebellar circuits. Second, EBS-type caregiving should require limbic circuit activation. Third, human attunement should produce different neural signatures than perceived AI attunement.

Feasibility demonstration. An fMRI study in mothers (N ≈ 40) during two caregiving simulation tasks: an SBS task involving watching video of own infant requiring routine care (feeding, changing) while rating “what does baby need?” and “what should I do?”; and an EBS task involving watching video of own infant in distress while rating “how does baby feel?” and “how do I feel watching this?” The prediction is that the SBS task preferentially activates hypothalamus, basal ganglia, and supplementary motor areas (action planning without affective engagement), while the EBS task preferentially activates amygdala, anterior cingulate, insula, and medial prefrontal cortex (affective processing and mentalisation). Overlap is expected in reward circuits; dissociation is expected in state-reading versus action-planning components.

8.3 Clinical Predictions

The framework generates two clinical predictions. First, addiction treatment integrating both SBS restoration (structure, routine) and EBS restoration (attachment, support) should outperform single-system approaches (consistent with Flores, 2004). Second, postpartum depression interventions targeting SBS maintenance should show different effect profiles than EBS-focused interventions.

8.4 Human-AI Interaction Predictions

The framework generates three human-AI interaction predictions. First, users should show different physiological responses to SBS-only AI versus EBS-simulating AI. Second, long-term interaction with EBS-simulating AI should correlate with attachment pattern effects. Third, SBS-only AI should not produce attachment-related dependency effects.

Feasibility demonstration. A randomised comparison (N ≈ 120) of 8-week interaction with two AI system framings: SBS-framed AI with explicit scope boundaries, consistent response patterns, no emotional language or attunement simulation, framed as “I provide structure and information; I cannot read or share emotional states”; and EBS-simulating AI with warmth language, attunement mimicry, and emotional responsiveness simulation, framed as “I’m here to understand how you’re feeling and support you emotionally.” Measures would include baseline and 8-week attachment style (ECR-R), loneliness (UCLA Loneliness Scale), and AI dependency (custom measure: frequency of use, distress at unavailability, preference over human contact). The prediction is that EBS-simulating AI will produce greater dependency effects and may correlate with increased loneliness at follow-up (substitution effect), while SBS-framed AI will show utility without attachment-related dependency. If confirmed, this supports the architectural constraint argument and the category error claim.

9. LIMITATIONS

9.1 Phylogenetic Limitations

Direct observation of ancestral caregiving is impossible. The evolutionary timeline is reconstructed from comparative evidence and fossil data (Rowe, 1996) and may contain inaccuracies.

9.2 Neural Oversimplification

The brain does not divide cleanly into SBS and EBS circuits. Significant integration exists, particularly through hypothalamic-limbic connections (Swanson, 2000). The dual-system model is a functional distinction, not a claim of absolute neural segregation.

9.3 Avian Complications

Some bird species demonstrate partial EBS-like capacities (Emery & Clayton, 2004). Whether these represent convergent evolution of pallial structures without limbic homology, shared ancestral features predating the mammalian-reptilian split, or phenomena requiring framework revision remains an open question. The Jarvis et al. (2005) analysis suggests convergent evolution of cognitive capacities through non-homologous neural structures, which would preserve the core SBS/EBS distinction while acknowledging that EBS-like functions can emerge through alternative architectures.

9.4 Cultural Variation

SBS/EBS balance may vary across cultures (Keller, 2007). The framework does not yet account for cultural modulation of boundary system expression.

9.5 Individual Differences

Some individuals may rely more heavily on one system due to temperament, neurodevelopmental variation, or adaptation to early environment. The framework requires extension to account for individual differences.

10. CONCLUSION

The Dual-Boundary Hypothesis proposes that caregiving operates through two evolutionarily and functionally distinct systems: the Structural Boundary System (SBS), providing environmental stability through state-independent pattern execution; and the Emotional Boundary System (EBS), providing limbic co-regulation through state-dependent affective modulation.

This framework generates testable predictions across developmental psychology, clinical intervention, and human-AI interaction. It suggests that healthy development requires both systems, that modern environments exceed EBS capacity while degrading SBS availability, and that artificial systems can authentically provide only SBS functions.

The framework is offered as a research programme generating empirical predictions, not as established theory. Validation requires systematic testing of the predictions outlined in Section 8.

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