Oxytocin is known in popular culture by nicknames that are not necessarily scientifically accurate, such as love hormone hug chemical or even moral hormone. It is a small molecule with wide influence Oxytocin affects many central aspects of the reproductive system and human behavior. It serves the body in two roles both as a hormone a substance that delivers messages to distant tissues through the bloodstream and as a neurotransmitter a substance that affects the activity of nerve cells in the brain
Physiologically oxytocin’s main roles are to trigger contractions of the uterus that is labor contractions and contractions of breast tissue after birth for breastfeeding. In addition oxytocin has an important role in human behaviors such as romantic relationships sexual arousal parent child bonding trust empathy and more
Before birth the fetus’s body puts pressure on the cervix. This pressure causes nerves in the area to send a message to the hypothalamus which stimulates the pituitary gland which in turn releases oxytocin into the bloodstream. Oxytocin reaches the uterus and encourages contractions. The contractions continue to stimulate the gland to release more oxytocin increasing the intensity and frequency of contractions
After birth oxytocin encourages contraction of muscle cells in breast tissue causing the milk release reflex milk moves into the milk ducts and then to the nipples.
Suckling causes the pituitary gland to continue releasing oxytocin and when suckling ends milk secretion declines until the next feeding
In male animals oxytocin is involved in ejaculation the hormone contracts the vas deferens to push semen forward and also affects testosterone production the male sex hormone in the testes
These physiological effects are relatively easy to understand but when examining oxytocin’s effects on human behavior the picture is much more complex. Numerous studies have shown the hormone is involved in behavior and many theories have developed as a result. Even today research progress and new discoveries about oxytocin lead researchers to propose new hypotheses
Studies have shown oxytocin is released in response to touch and positive social interactions and promotes well being and calm not only emotionally but also physiologically it reduces activity of the sympathetic nervous system which is heightened in stressful situations and increases activity of the parasympathetic nervous system responsible for routine actions such as digestion. Through this mechanism oxytocin leads to lower levels of stress hormones such as cortisol reduces blood pressure and heart rate and thus lowers stress
Behaviorally oxytocin participates in creating bonds between parents and children and also affects romantic and social relationships across various animal species. In 2005 researchers proposed the calm and connection system model which includes psychological and physiological patterns related to feelings of well being and calm and positive social interactions. Researchers argued oxytocin plays a central role in this system mediating between physiological and psychological processes in responses of calm and connection
A 2011 review examined numerous studies linking oxytocin to well being. The review included studies showing oxytocin increases trust reduces fear by regulating brain regions responsible for stress and fear especially the amygdala; increases pro social behavior and social interactions in animals and humans; reduces anxiety and increases calm; plays a key role in sexual behavior; and oxytocin levels are higher in romantic love or mother infant bonding
The review also found in neuropsychiatric conditions such as chronic depression obsessive compulsive disorder and post traumatic stress disorder low oxytocin levels were observed. The review also addressed studies examining external oxytocin: it reduced fear and amygdala overactivation in people with schizophrenia reduced repetitive behavior and improved response to social cues in people with autism. The conclusion was oxytocin plays a vital role in promoting factors that improve well being
Another hypothesis regarding oxytocin’s behavioral effects is the pro social hypothesis which suggests oxytocin enhances socially positive behaviors. This hypothesis is based on studies showing oxytocin improves trust cooperation and generosity in social interactions promotes positive social behaviors and shifts decision making towards more pro social outcomes
One notable study published in 2005 in Nature examined oxytocin’s effect on trust game outcomes. In this game two participants start with a certain amount of money and each can give part to the other. If both transfer money the sum triples benefiting both. However giving money carries risk if one gives and the other acts stingily the stingy participant keeps all the money. Participants who received oxytocin nasally showed more trust towards the other participant compared to those given placebo
Another prominent study examined oxytocin’s effect on generosity in the ultimatum game. One participant receives money and decides how to split it with the other. The second can accept or reject the offer if rejected neither gets the money. Participants given oxytocin showed more generosity towards the other participant
Studies have shown oxytocin improves trust cooperation and generosity in social interactions
These studies painted a very positive picture of oxytocin which also spread to the public. Researchers concluded the hormone leads to well being helps build trust and social bonds and increases generosity towards others. More oxytocin seems better for everyone
However the picture is not so simple. In 1992 a study found injecting oxytocin into the amygdala of female rodents after birth increased aggression toward male intruders. So is it still correct to say oxytocin promotes well being and social behavior
A 2010 study also examined oxytocin’s effect on social behavior dividing participants into groups. Researchers examined in group love how much participants gave for their group and out group hate how much they wanted to harm a competing group under oxytocin or placebo. Each participant received ten euros and decided to keep it contribute to in group pool or inter group pool. Each euro kept was one for them each euro in in group pool added half euro to each group member measuring in group love each euro to inter group pool added half euro to in group member and subtracted half from out group measuring out group hate. Results showed under oxytocin participants contributed more to in group pool showing more love but only within the group. It did not affect behavior towards other groups
In a follow up experiment researchers examined oxytocin on cooperation between groups under varying threat levels. Threat level was defined by loss magnitude: low threat small loss for in group and high threat large loss. Results showed when threat was low oxytocin had no effect on cooperation. When threat was high participants receiving oxytocin cooperated less with external group than placebo. Researchers concluded oxytocin may reduce cooperation with external groups when they threaten in group
A 2012 review on oxytocin’s effect on intergroup relations concluded oxytocin regulates fear and anxiety and allows trust development especially with in group members. It drives empathy concern and cooperation within the group but also non cooperation with potentially threatening external groups. That is oxytocin may not always promote social behavior but regulates cooperation and conflicts based on who is in front of us and whether they belong to our group
Accumulating evidence that oxytocin effects depend on context and individual traits led researchers to propose a hypothesis focusing on oxytocin’s role and its interaction with the dopamine system involved in many mental processes including reward learning and attention
According to the hypothesis combining social behavior and intergroup dynamics oxytocin can regulate social information processing and increase attention to certain social cues for example competition versus cooperative environment influencing various social behaviors and interactions. For example in money distribution games oxytocin increases sensitivity to situations where another person earns more or less amplifying envy or schadenfreude
Studies also showed oxytocin improves face memory and ability to recognize emotions from facial expressions and increases eye contact. The hypothesis emphasizes oxytocin dopamine interactions and plays a key role in attention regulation as a mechanism by which oxytocin affects how we perceive importance of social stimuli directing attention to the most relevant social cues. Effects of oxytocin depend on context and individual differences such as gender and personality which should be considered when assessing social function
Many hypotheses have been proposed to describe oxytocin’s effect on human behavior but what if a unified theory explaining all its effects is impossible. Is it even possible to have one theory including all oxytocin roles
Oxytocin is one neurotransmitter among many in the brain. It does not work alone in isolation so focusing only on it yields limited results. Studies from recent years show in 2023 a study challenged the view that oxytocin is responsible for monogamous behavior in voles showing even voles without oxytocin receptors still exhibited monogamy. Possibly vasopressin replaces oxytocin in voles lacking receptors
Another study showed oxytocin and dopamine work together to create motivation for social behavior and investment in pair bonding and care of others, enabling certain animals to maintain monogamy
Some hypotheses propose models including interaction with other neurotransmitters such as dopamine. Even these cannot reflect the full picture: oxytocin interacts with many neurotransmitters including acetylcholine glutamate GABA and serotonin and with hormones such as estrogen testosterone and cortisol
Oxytocin does not behave identically in all humans: studies show effects vary based on gender personality attachment styles and psychopathology
Another major issue is low reproducibility in oxytocin research many studies failed to replicate previous findings. For example attempts to replicate oxytocin effects on trust game outcomes failed in six different studies
Thus existing knowledge on oxytocin roles in human behavior should be examined carefully and until understanding all its functions if ever more research is needed