The Physiological Basis of Oxytocin
Oxytocin is a neuropeptide hormone of nine amino acids, synthesised in the paraventricular and supraoptic nuclei of the hypothalamus and secreted into the bloodstream from the posterior pituitary. It was originally known as the hormone that promotes uterine contraction during childbirth and the milk ejection reflex during breastfeeding, but research from the 1990s onwards revealed that it exerts a broad influence over social behaviour in general. Oxytocin functions not only as a peripheral hormone but also as a neurotransmitter within the central nervous system, acting on brain regions involved in social cognition such as the amygdala, the prefrontal cortex and the nucleus accumbens. The density with which oxytocin receptors are distributed varies between individuals, and this is thought to contribute to individual differences in how readily attachment behaviour and social bonds form. Secretion of oxytocin is promoted by social interaction, including physical contact such as hugging, kissing and sexual activity, as well as eye contact and warm conversation.
Oxytocin in the Formation of Attachment and Trust
Oxytocin is popularly called the attachment hormone or the bonding hormone, yet its action is not a simple matter of promoting affection. Oxytocin is thought to heighten attention to social cues, strengthen social memory and promote trust towards the in-group. In an experiment by Kosfeld and colleagues (2005), intranasal administration of oxytocin was reported to increase trusting behaviour in an economic game. Later replication attempts, however, have not reproduced that effect reliably, and describe it as small at best. Subsequent research has also made clear that the effects of oxytocin are context dependent: while it strengthens bonds towards those with whom a trusting relationship already exists, it can also strengthen defensive responses towards out-group members and towards those perceived as threatening. In romantic relationships, oxytocin is involved not only in the formation of an initial bond but also in the maintenance of a long-term one. Physical contact and positive interaction with a partner promote oxytocin secretion, which in turn heightens intimacy further, forming a positive feedback loop.
Oxytocin and the Biological Basis of Compatibility
Individual differences in the oxytocin system have attracted attention as one biological basis of compatibility. Polymorphisms of the oxytocin receptor gene (OXTR) have been reported to be associated with empathy, sociability and attachment style. The GG genotype of the rs53576 polymorphism of OXTR, for example, has been linked in some studies to higher empathy and to secure attachment. Since genetic effects emerge through interaction with the environment, however, it is impossible to predict compatibility from any single genetic polymorphism. From a practical standpoint, incorporating into daily life the behaviours that promote oxytocin secretion, such as physical contact, shared activity, expressions of gratitude and eye contact, contributes to maintaining the quality of a relationship. What the oxytocin literature offers is not material for reading compatibility as a constitutional matter settled at birth. Knowing about receptor distribution or genetic polymorphisms settles nothing about how many words you exchanged with this particular person today, or how close to them you stayed. It is on that second side that the strength of a bond moves, and unlike constitution, that side is within your own reach.