A Procedure for Classifying Attachments and Affective Social Bonds in Human-Robot Dyads
Organizations: Department of Computer Science, University of Warwick, Coventry, CV4 7AL, United Kingdom. · Department of Psychology, University of Warwick, Coventry, CV4 7AL, United Kingdom.
Abstract
Human-robot interaction (HRI) claims that people form attachments and social bonds with artificial agents, yet the terms are often applied without the behavioural and physiological criteria that give them content in their source disciplines. Without this empirical grounding, studies deploy widely divergent methods, frequently producing expansive relational claims that far outstrip their underlying evidence. To address this, we propose a standardised four-question procedure, grounded in criteria established in the developmental, ethological, and neuroendocrine literatures, that classifies a given human-robot tie as an attachment, an affective social bond, or no relationship, with intermediate classifications when evidence is incomplete. We specify minimum evidential requirements for each question, and provide candidate HRI study designs, adapted from validated human-human, human-animal, and animal-animal paradigms. We then demonstrate the procedure by applying it to a representative set of published HRI studies, showing how often relational claims outstrip what the reported designs can establish. Finally, we discuss the ethical and regulatory burdens created when artificial agents engage human biobehavioural systems. By replacing the divergent operationalisations with a unified, criterion-based classification, this paper gives HRI practitioners a standardised basis for evaluating, classifying, and comparing human-robot relationships, and sets out the experimental rigour that each classification demands. We therefore call on researchers of human-robot relationships to adopt such rigour, or to consider alternative terminology in their descriptions of these ties.
Figures & tables
| Q1 | Q2 | Q3 | Q4 | Classification of the relationship |
| ✓ | ✓ | ✓ | Attachment. Both selectivity and security regulation requirements are met. Answer to Q3 not required. | |
| ✓ | ✓ | ✓ | — | Affective social bond. Both comparative requirements met, with the channel profile reported separately. |
| ✓ | ✓ | — | — | Affiliative preference. Ainsworth’s affectional-bond criteria are met. However, regulation criterion not tested. |
| — | ✓ | — | Non-specific regulation. Physiological changes are measured with respect to robot’s presence, without evidence that the state change is dependant on the specific agent | |
| ✓ | — | — | — | Affectional tie . Ainsworth’s affectional tie criteria met: Question 1 answered via observation. Selectivity of the agent and physiological regulation not tested. |
| — | No relationship to classify. The affectional criteria are not met, usually because the encounter was too brief for persistence to be observable. The interaction may still be engaging, trusted and valued. |
| Regulator | Physiological substrate | What an agent supplies | Observable proxy | Reference method | Practical proxy |
|---|---|---|---|---|---|
| Phasic (Reactive/Event-Driven) | |||||
| Stress buffering | HPA axis; cortisol response, CRF suppression ( Hostinar et al., 2014 ) | Contingent responsiveness | Cortisol reactivity and recovery (HPA); EDA and HRV (autonomic) | Trier Social Stress Test ( Allen et al., 2016 ; Heinrichs et al., 2003 ) ; salivary cortisol AUC i ( Pruessner et al., 2003 ) | Chest-strap ECG for HRV ( Schaffarczyk et al., 2022 ) ; wrist EDA ( Milstein and Gordon, 2020 ) |
| Pain and threat modulation | Endogenous -opioid signalling ( Machin and Dunbar, 2011 ; Løseth et al., 2025 ) | Presence; contact only if embodied | Pain threshold, tolerance and rating | Laser-evoked potentials and pain report ( von Mohr et al., 2018 ) ; threat-of-shock handholding ( Coan et al., 2017 ) | Contact against no-contact under thermal pain ( Geva et al., 2020 ; Nakae et al., 2025 ) |
| Tonic (Continuous/Baseline) | |||||
| Behavioural and physiological synchrony | Interpersonal cardiorespiratory coupling ( Goldstein et al., 2017 ) ; motor and speech timing ( Ramseyer and Tschacher, 2011 ) ; raised pain thresholds under synchrony ( Cohen et al., 2010 ) | Turn-taking, prosody and gesture mirroring, by fixed policy | Tonic autonomic state; movement, speech and cardiac cross-correlation | Motion Energy Analysis ( Ramseyer and Tschacher, 2011 ) ; cross-recurrence ( Coco and Dale, 2014 ) ; surrogate controls ( Palumbo et al., 2017 ) | Fixed-camera video; chest-strap ECG for cardiac coupling ( Schaffarczyk et al., 2022 ) ; the agent’s own turn logs |
| Thermal and circadian co-regulation | Homeostatic regulation carried by contact ( Hofer, 1994 ; Fotopoulou et al., 2022 ) | Requires contingent warmth and continuous presence | Skin temperature; sleep timing and continuity | Infrared thermography ( Nazzari et al., 2024 ; Filippini et al., 2020 ) | Consumer actigraphy, for sleep timing and continuity ( Yuan et al., 2024 ) |