Originally published 2017-09-16.
The chemical structure of oxytocin: a chain of nine amino acids, with a sulphur bridge between its two cysteines closing part of the chain into a ring.
Oxytocin is often called the "cuddle chemical" or "love hormone", but that nickname tells only part of the story. It is a small hormone made in the brain that drives contractions during labour and the release of breast milk, and it also influences social behaviour. Research over the past two decades shows that its social effects are complicated: oxytocin can strengthen bonds with people we see as "our own" while doing little for, or even working against, outsiders. Many early claims about oxytocin and trust have also proved hard to replicate, and a large 2021 trial found that an oxytocin nasal spray did not improve social skills in autistic children.
What oxytocin is and where it comes from
Oxytocin is a peptide hormone made of nine amino acids. It is produced by nerve cells in two areas of the hypothalamus, the supraoptic and paraventricular nuclei, and stored in the posterior pituitary gland at the base of the brain, which releases it into the bloodstream. Nerve cells also release it directly inside the brain, where it acts as a signalling molecule.
Its history goes back more than a century. In 1906 the British physiologist Henry Dale found that an extract of the pituitary made the uterus contract, and the name oxytocin comes from Greek words meaning "quick birth". In 1953 the American chemist Vincent du Vigneaud worked out its structure and synthesised it, the first polypeptide hormone ever made in the laboratory, work that earned him the 1955 Nobel Prize in Chemistry.
Its well-established jobs: birth and breastfeeding
The best-understood effects of oxytocin are physical. During labour, pressure on the cervix triggers its release, oxytocin strengthens contractions, and stronger contractions trigger more release. This positive feedback loop helps push the baby out and, after birth, helps the uterus contract to limit bleeding.
During breastfeeding, a baby's suckling sends signals to the brain that cause bursts of oxytocin. The hormone makes tiny muscle cells around the milk glands squeeze, producing the "let-down" reflex that moves milk to the nipple. Stress and pain can block this reflex, which is one reason a calm setting helps feeding.
Synthetic oxytocin is one of the most widely used drugs in obstetrics. Doctors use it to start or speed up labour and to prevent or treat heavy bleeding after birth. These uses are carefully dosed and monitored, and anyone with questions about them should talk to a doctor or midwife.
Where the "cuddle chemical" label came from
The social reputation began with animal research. Studies of prairie voles in the 1990s found that these small rodents form lifelong pair bonds, and that blocking oxytocin in females prevented them from bonding with a partner. Work in sheep and rats linked the hormone to mothers' attachment to their young.
In 2005 a much-publicised study in Nature by Michael Kosfeld and colleagues reported that people given an oxytocin nasal spray were more willing to trust a stranger with their money in an investment game. Headlines about a "trust hormone" in a bottle followed, along with popular claims that hugging, sex or even friendly eye contact flood the body with oxytocin and make us kinder.
Touch and close contact do seem to be linked with oxytocin release, and warm social contact is genuinely good for well-being. The problem is the leap from that to the idea of a simple love chemical that makes everyone nicer to everyone.
The darker side: favouring the in-group
Later studies suggested that oxytocin's effects depend heavily on context and on who the other person is. In experiments led by the Dutch psychologist Carsten De Dreu, Dutch men given oxytocin showed stronger preference for their own national group, for example rating Dutch names more positively than Arab or German names, and in moral dilemmas became more willing to protect members of their own group at the expense of outsiders.
Other research found that oxytocin could increase dishonesty when lying benefited a person's group, and that people with difficult relationship histories did not necessarily become more trusting after a dose. This led researchers to describe oxytocin less as a love hormone and more as a social-salience signal: it may make social cues feel more important, which can push behaviour towards closeness or towards defensiveness depending on the situation.
That is also why some scientists caution against the idea that boosting oxytocin across a population would make society kinder. If its main effect is to sharpen loyalty to one's own group, it could reinforce the boundaries between groups as easily as it softens them.
Replication problems and the nasal-spray question
As more labs tested oxytocin, several early findings proved fragile. Attempts to repeat the famous trust result produced weaker or inconsistent effects, and reviews pointed to small sample sizes, publication bias and selective reporting of results across the field.
There is also a practical question about the sprays used in most human studies. Oxytocin is a relatively large molecule that crosses from the blood into the brain poorly. Studies measuring it in cerebrospinal fluid have found that levels rise only slowly after a nasal dose and do not track blood levels closely, so it is still debated how much of a sprayed dose reaches the brain areas thought to matter.
What the autism trials found
Because oxytocin seemed to influence social attention, researchers hoped it might help autistic people who wanted support with social communication. Small early trials reported some encouraging results, and the idea spread quickly. In a 2015 Nature feature on oxytocin research, the Toronto clinician-scientist Evdokia Anagnostou, who worked on several of those trials, reflected that the field had moved "a little bit too fast."
The largest trial so far gave a clearer answer. Led by Linmarie Sikich of Duke University, funded by the US National Institutes of Health and published in the New England Journal of Medicine in 2021, it gave children and teenagers with autism, aged 3 to 17, a daily oxytocin nasal spray or a placebo for 24 weeks. Of the 277 who completed the study, 139 received oxytocin and 138 the placebo. The two groups did not differ meaningfully, and the researchers concluded that oxytocin did not improve social interaction or other measures of social function.
Families should be cautious about any product or clinic promoting oxytocin as a treatment for autism or for relationship problems. Anyone considering it should talk to a doctor.
Common myths about oxytocin
- "It is the love hormone." It plays a part in bonding, but love and attachment involve many brain systems, including dopamine and vasopressin, as well as experience and choice.
- "More oxytocin makes people kinder to everyone." Evidence suggests effects depend on context and may favour the in-group.
- "Oxytocin sprays boost trust or confidence." Over-the-counter sprays sold online are not proven to do this, and the research base for such claims is weak.
- "Only mothers produce it." Men and women both make oxytocin, and it is active throughout life.
The more accurate picture is less catchy but more interesting: oxytocin is an ancient signalling molecule that helps the body give birth and feed babies and that tunes social behaviour in ways scientists are still working out.
Sources: Wikipedia: Oxytocin; NICHD (NIH): Oxytocin does not improve social functioning in children with autism spectrum disorder (13 October 2021); Helen Shen, Nature: Neuroscience: The hard science of oxytocin (24 June 2015); Psychology Today: Oxytocin basics.
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