By Masooma Hyder Baig
Introduction
The phrase ‘we were girls together’ can be roughly traced from a novel titled ‘Sula’ by Toni Morrison. In recent culture, it has been popularised by a viral picture of a memorial bench in Central Park, New York, dedicated to a woman named Judy from her friend Janice. What causes us to care so deeply for one another? How do we process such grand feelings within community? This article explores social bonding through a neurobiological lens primarily focusing on social bonding on a wider scale– beyond parent-child relationships, kinship and romantic partners.
From an evolutionary perspective
The practice of social bonding is thought to predate the modern-day human, with evidence of forming alliances and caring for individuals amongst groups existing as adaptations due to the complexity of different social environments. It is thought that bonding behaviours have existed at least 50 million years ago due the presence of these behaviours in primate species (1). The hormones involved in influencing human social bonding (oxytocin and vasopressin) are thought to have existed in many mammalian species implying that these neurochemicals have existed and regulated social bonding for millions of years with homologs of both hormones existing in insects, hydra and vertabrates for approximately 700 million years (2).
But why are these behaviours necessary? What makes bonding so important? Survival. Social bonding can arguably be considered the main factor in the formation of groups which aided in providing protection, cooperation and improved life outcomes through reducing stress and group hunting. These functions of social bonding are not exclusive to humans and existing amongst modern day primates where grooming and affiliation serve hygienic and social purposes (3).
The evolution of human social bonding can be understood through: aggregation to group formation, cognitive and emotional complexities and cultural practice. Here, primates are documented to initially loosely aggregate amongst one another and transition into stable, bonded social structures where the idea of forming coalitions and sharing resources to improve survival is observed (1). Overtime, as these social group developed and became more complicated, the cognitive and emotional demands of these individuals increased. This can be related to the social brain hypothesis which proposes that larger group sizes spearheaded the adaptation of larger brains and more sophisticated, finuted emotional processing in primates and humans (4). Finally, the formation of cultures within human societicies helped to reinforce social bonds further through rituals, norms and language which help to maintain cohesion and identity (5).
Oxytocin
Oxytoxin is a neuropeptide hormone which is produced in the hypothalamus and released by the posterior pituitary gland. It fundamental in childbirth and lactation and is acknowledged for its affects on social behavours like empathy and social bonding, often called the ‘bonding hormone’. It can be released in response to physical touch, sexual activity, childbirth and breast feeding and postivie social interactions (which may include shared laughter, emotional conversations engaging eye contact and open body laguage.) These effects are exerted when oxytocin binds to oxytocin receptors (OXTR) which are found in different areas of the brain which are involved in emotion and social behavour (amgydala, nucleus accumbens and the prefrontal cortex). Oxytocin works on neural circuits in these regions to enhance the reward value of social interactions, reduce social anxiety and encourage emotional memory with social partners (6). It is important to consider the exploration of sex differences between females and males when looking at oxytocin signalling in rodent studies. Females generally demonstrate greater OXTR density in certain brain regions making them more receptive to its effects which may be partly due to interactions with oestrogen (7). This prompts further research into potentially differences in humans amongst sexes.
Vasopressin
Vasopressin (AVP) is also neuropeptide which is produced by the hypothalamus and released by the posterior pituitary gland (8). It acts peripherally (around the body) to regulate blood pressure, fluid homeostatis and body temperature and centrally (a neuropeptide within the brain) modulating social communication, aggression, pair bonding and social investigation mostly in males (9). These effects are enabled via three main receptor types: V1aR (influencing social behavour, aggression, pair bonding and social communication), V1bR (social memory) and V2 (renal fluid regulation, not related to social bonding). Vasopressin acts on neural circuits via central projections from hyopthalamic nuclei (para ventricular nucleus (PVN) and supraoptic nucleus (SON) ). These hypothalamaic nuclei contain AVP-expressing neurons which singal to various brain regions associated with social cognitication and emotional regulation like the amygdala, lateral septum (LS), thalamus, hippocampus and cortex (10). Social memory is influenced by AVP via the CA2 region of the hippocampus which densely consists of V1bR (11). The effects of vasopressin are also sex-dependent, where research has highlighted how the AVP neurons in the bed nucleus of the stria terminalis (BNST) are sexually dimorphic, meaning they are crucial particularly in male social investigation behaviours (12).
Mirror Neuron System (MNS)
The MNS is a neural network which is activated when individuals perform actions and when they observe other individuals performing the same or similar actions (13) . Various brain regions are involved in the MNS including the ventral premotor cortex (PMv), inferior frontal gyrus (IFG), inferior parietal lobule (IPL) and superior temporal sulcus (STS) (13) . Regarding social bonding, the MNS facilitates interpersonal connection by allowing for empathy, imitation, shared emotions and action understanding. Emotional mirroring particularly when individuals experience and/or observe pain or disgust, activate th anterior insula and rACC (rostral anterior congulate cortex) which implies a shared neural foundation for ones self and other emotions, underpinning empathy (14). Processing empathy may stem from the MNS interacting with the Theory of Mind (ToM) where individuals can infer and feel the feelings of others – allowing for increased social closeness and bonding (15). Finally, studies highlight that the MNS is more sensitive to actions of in-group members compared to out-group members, reinforcing its role in social bonding, empathy and group understanding (16).
Conclusion
Social bonding in humans is deeply rooted in biological mechanisms which predate our species and are found amongst many animal taxa. Hormones like oxytocin and vasopressin are vital in modulating affliative behavours, social memory, emotions and social motivation where they effects may differ based on sex and context. The MNS provide an neural basis for empathy and individuals understanding actions and emotions which enables social learning and community. Together, these systems create a cohesive neurobioloigcal framework which is at the forefront of fostering social cohesion, group dynamics and relationships.
References
1.Shultz S, Opie C, Atkinson QD. Stepwise evolution of stable sociality in primates. Nature [Internet]. 2011 Nov [cited 2025 May 17];479(7372):219–22. Available from: https://www.nature.com/articles/nature10601?error=cookies_not_supported&code=7af719c7-ba18-41e5-b3a1-d7bdef809b1b
2.Donaldson ZR, Young LJ. Oxytocin, Vasopressin, and the Neurogenetics of Sociality. Science. 2008 Nov 7;322(5903):900–4.
3.Swedell L. Primate Sociality and Social Systems [Internet]. Nature.com. 2012 [cited 2025 May 17]. Available from: https://www.nature.com/scitable/knowledge/library/primate-sociality-and-social-systems-58068905/
4.Dunbar RIM, Shultz S. Evolution in the Social Brain. Science [Internet]. 2007 Sep 7 [cited 2025 May 17];317(5843):1344–7. Available from: https://pubmed.ncbi.nlm.nih.gov/17823343/
5.Boyd R, Richerson PJ. The origin and evolution of cultures. Oxford: Oxford Univ. Press, Cop; 2005.
6.Menon R, Neumann ID. Detection, processing and reinforcement of social cues: regulation by the oxytocin system. Nature Reviews Neuroscience [Internet]. 2023 Dec 1 [cited 2025 May 17];24(12):761–77. Available from: https://www.nature.com/articles/s41583-023-00759-w
7.Sofer Y, Noga Zilkha, Gimpel E, Wagner S, Chuartzman SG, Tali Kimchi. Sexually dimorphic oxytocin circuits drive intragroup social conflict and aggression in wild house mice. Nature neuroscience [Internet]. 2024 Jul 5 [cited 2025 May 17]; Available from: https://www.nature.com/articles/s41593-024-01685-5
8.Bilezikjian LM, Vale WW. Regulation of ACTH Secretion from Corticotrophs: The Interaction of Vasopressin and CRFa. Annals of the New York Academy of Sciences [Internet]. 1987 Dec 1 [cited 2025 May 17];512(1):85–96. Available from: https://nyaspubs.onlinelibrary.wiley.com/doi/abs/10.1111/j.1749-6632.1987.tb24952.x?sid=nlm%3Apubmed
9.Walum H, Young LJ. The neural mechanisms and circuitry of the pair bond. Nature Reviews Neuroscience. 2018 Oct 9;19(11):643–54.
10.Rigney N, de Vries GJ, Petrulis A, Young LJ. Oxytocin, Vasopressin, and Social Behavior: From Neural Circuits to Clinical Opportunities. Endocrinology. 2022 Jul 21;163(9).
11.Zhang L, Hernández VS. Synaptic innervation to rat hippocampus by vasopressin-immuno-positive fibres from the hypothalamic supraoptic and paraventricular nuclei. Neuroscience [Internet]. 2013 Jan [cited 2025 May 17];228:139–62. Available from: https://pubmed.ncbi.nlm.nih.gov/23085097/
12.Rigney N, Whylings J, Mieda M, de Vries GJ, Petrulis A. Sexually Dimorphic Vasopressin Cells Modulate Social Investigation and Communication in Sex-Specific Ways. eneuro [Internet]. 2019 Jan [cited 2025 May 17];6(1):ENEURO.0415-18.2019. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC6348451/
13.Jeon H, Lee SH. From Neurons to Social Beings: Short Review of the Mirror Neuron System Research and Its Socio-Psychological and Psychiatric Implications. Clinical Psychopharmacology and Neuroscience [Internet]. 2018 Feb 1 [cited 2025 May 17];16(1):18–31. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5810456/
14.Wicker B, Keysers C, Plailly J, Royet JP, Gallese V, Rizzolatti G. Both of Us Disgusted in My Insula: The Common Neural Basis of Seeing and Feeling Disgust. Neuron [Internet]. 2003 Oct [cited 2025 May 18];40(3):655–64. Available from: https://www.sciencedirect.com/science/article/pii/S0896627303006792
15.Carr L, Iacoboni M, Dubeau MC ., Mazziotta JC, Lenzi GL. Neural mechanisms of empathy in humans: A relay from neural systems for imitation to limbic areas. Proceedings of the National Academy of Sciences [Internet]. 2003 Apr 7 [cited 2025 May 18];100(9):5497–502. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC154373/pdf/pq0903005497.pdf
16.Cheng Y, Yang CY, Lin CP, Lee PL, Decety J. The perception of pain in others suppresses somatosensory oscillations: A magnetoencephalography study. NeuroImage. 2008 May;40(4):1833–40.
Astra Stem 
Leave a comment