Oxytocin: Bonding, Birth, and Trust

To understand the neuroscience of adult partnerships, researchers study prairie voles, a rare example of a species that mates for life. Several studies suggest that the hormone oxytocin cements the bonds that form between adult partners.

Photo by Todd Ahern, Larry Young.

This image shows the brains of monogamous prairie voles, with oxytocin receptors labeled in light blue, red, and yellow. When researchers used genetic techniques to increase oxytocin receptor levels in the brain (right column), they found female voles formed partner preferences faster.

Ross, et al. The Journal of Neuroscience, 2009.

The hormone oxytocin is produced in the hypothalamus — a brain structure that produces many different hormones and brain chemicals — and is released from nerve endings in the posterior pituitary (pituitary gland indicated in orange).

Courtesy, with Permission: Patrick J. Lynch.

When researchers administered oxytocin nasal spray to a group of healthy men, they found that men in monogamous relationships (solid gray line) preferred to keep a greater distance from an unknown attractive female as she moved toward (far) or away (close) from them than males that were single (dotted gray line). The effect of oxytocin on the monogamous men was the same regardless of whether the female experimenter maintained eye contact or averted her gaze.

Scheele, et al. The Journal of Neuroscience, 2012.

When researchers began searching for clues about how the brain signals to the body that the time has come to give birth, they had no way of knowing doing so would guide them to a hormone involved in social behaviors like attachment, trust, and empathy. Scientists are hopeful that studies of the brain chemical oxytocin will lead to a better understanding of disorders marked by social dysfunction, such as autism.

Identifying a safer way to induce labor led to understanding brain cell communication and influences on social behavior

From dictating body growth to regulating hunger and thirst, the brain plays an active role in numerous body functions. In the 20^th century, researchers studying cell communication identified the signals the brain sends the body when the time comes to deliver a baby. This discovery forever changed medicine and later had surprising implications for understanding the brain's role in social behavior.

Discovery of brain chemical improves labor outcomes

In the early 1900s, there were limited options for women whose pregnancies posed sudden risks to them or their unborn children. Existing methods of labor induction often led to infection, endangering the health of the mother and child. 

British physiologist and pharmacologist Sir Henry Dale studied chemicals that communicate throughout the body, which eventually led him to discover the neurotransmitter acetylcholine, for which he was awarded the Nobel Prize. But in 1906, he uncovered the body's natural signal for stimulating labor.

The posterior pituitary contains nerve endings from the hypothalamus — a brain structure that produces many different hormones and brain chemicals — which helps to store and release these hormones. When Dale injected the brain chemicals from the posterior pituitary into pregnant cats, dogs, and guinea pigs, he discovered he could stimulate uterine contractions without causing harmful side effects.

Dale later isolated the active brain chemical that was behind the contractions — a hormone he named oxytocin (derived from the ancient Greek words for “swift birth”). Additional studies soon revealed oxytocin was also responsible for the release of milk necessary for nursing.

Revolutionizing a medical procedure

As interest in using oxytocin to induce labor in women grew, so too did the desire to better understand the hormone. American biochemist Vincent du Vigneaud was studying the structure of proteins in the early 1950s when he became interested in oxytocin. After successfully determining its chemical structure, he found a way to manufacture synthetic versions of oxytocin in the laboratory.

Today, synthetic forms of oxytocin, like Pitocin, are used all over the world to facilitate labor and manage postpartum hemorrhage. For his work, du Vigneaud was awarded the 1955 Nobel Prize in Chemistry His research also laid the foundation for yet more discoveries about oxytocin's role in behavior.

Brain hormone's role in social behaviors

Knowing the role of oxytocin in birth and nursing led researchers of the 1970s and 1980s to begin asking questions about the brain hormone's involvement in the development of relationships that form between a mother and child.

Shaping maternal behavior

In general, virgin female rats do not like newborn pups; some prefer to avoid the pups, while others aggressively attack. Yet, as soon as a female mothers her own litter, this hostile behavior subsides.

To determine if oxytocin plays a role in this switch, scientists administered the hormone into the brains of virgin females before placing them in the presence of a newborn litter. Within hours, most of the virgins went from avoiding the pups to building a nest, grooming them, and carrying them around as if they were their own. In contrast, when researchers blocked oxytocin from binding to its receptors in the brains of new rat mothers, the animals were slower to respond to their young, and, in some cases, they ignored them all together. These and other studies support the role of oxytocin in maternal behavior.

A molecular matchmaker

In the 1990s, researchers became curious about what role, if any, oxytocin plays in the bonds that form between couples. To test this, they turned to the prairie vole — a rodent that, unlike most, mates for life. In voles, sex triggers the release of oxytocin, which appears to cement bonds between adults. In one study, scientists discovered that unlike untreated voles, voles that received oxytocin preferred spending time with voles of the opposite sex that were familiar rather than novel.

These and other animal studies led scientists to explore the role oxytocin may play in monogamous human relationships. In one such study, researchers administered oxytocin nasal spray to a group of healthy men before introducing them to an attractive female. They found that those in monogamous relationships chose to stand farther from the woman than did single men who received the hormone and those who received a placebo. These findings suggest that oxytocin may help promote fidelity within monogamous relationships.

New clues about social disorders

Double-edged actions

As evidence for oxytocin’s role in establishing and maintaining relationships grew, new studies focused on the effects of the hormone on other social behaviors.

Neuroeconomic studies revealed that investors given oxytocin are more willing to loan small amounts of money to strangers. While such studies suggest that oxytocin may be involved with promoting trust, more recent studies paint a far less rosy picture. For instance, while people given oxytocin may be more empathetic to people they consider like them, they fail to display the same level of empathy toward outsiders. In fact, one study found that participants given oxytocin express stronger biases against people outside their group than those who did not receive the hormone.

Potential role in developmental disorders

Due to oxytocin's nuanced role in social behaviors, scientists have started to look at the role of the hormone in disorders characterized by abnormal social interactions.

One such disorder is Williams syndrome, a rare condition that, in addition to cognitive deficits, causes people to be overly trusting and friendly. A recent preliminary study in a small group of participants showed those with the disorder had three times more oxytocin circulating in their blood than did other study participants, suggesting  the social aspects of Williams syndrome may be due to dysfunction of the oxytocin system.

In contrast to Williams syndrome, autism spectrum disorders are marked by social deficits. Scientists recently traced differences in the gene that makes oxytocin receptors to autism risk. While several studies have found that a single dose of oxytocin given to children and adults with autism results in an increased willingness to interact socially, much remains unknown about the long-term effects of chronic oxytocin administration, particularly in the developing brain.

The story of the surprising discovery that a hormone first identified for its effect on birth plays an important role in social behavior and neurodevelopmental disorders demonstrates promising new avenues for progress on disorders that cause untold family hardship and significant health care costs. It also highlights the value of cross-disciplinary research and continued investments in basic science research in fields as varied as chemistry, biology, physiology, economics, and sociology.

About the Author

Caitlin Kirkwood

Caitlin Kirkwood is a freelance science writer and PhD candidate in neurobiology at the University of Pittsburgh. Her translational research focuses on molecular mechanisms of Alzheimer's disease and psychosis. In addition to contributing to BrainFacts.org, she has written for Scientific American and Being Human.