-- The Hormone That Has Guarded Humanity
The Discovery of Oxytocin
Oxytocin was first identified in 1906 by the British pharmacologist Sir Henry Hallett Dale. To put that in perspective, dopamine — a hormone most of us have heard of — wasn't discovered until the 1950s. Oxytocin was found remarkably early, in an era when scientific tools and knowledge were still limited. Dale had become interested in the idea that chemical substances might be involved in how brain cells communicate with each other — what we now call neurotransmitters. Working with pregnant cats, he extracted a substance from the posterior pituitary gland and showed that it caused the uterus to contract. He named it oxytocin, from the Greek for "swift birth." He then confirmed his findings by injecting the substance into the uteri of laboratory rats. For his work on oxytocin and his subsequent discovery of other neurotransmitters, including acetylcholine and histamine, Dale received the Nobel Prize in Physiology or Medicine in 1936.1
Nearly forty years later, the American biochemist Vincent du Vigneaud determined oxytocin's molecular structure and then successfully synthesized it in the laboratory. Oxytocin turned out to be a peptide hormone* made up of nine amino acids and a sulfur atom. For comparison, insulin — another peptide hormone — contains fifty-one amino acids. Because oxytocin is relatively small, it can be delivered today as a simple nasal spray. Du Vigneaud received the Nobel Prize in Chemistry the following year for synthesizing the first peptide hormone.1
[Hormones fall into two broad categories: peptide hormones and steroid hormones. Peptide hormones act through receptors on the cell surface; steroid hormones don't need surface receptors and act directly on the cell nucleus. For more detail, see the "Hormone 101" section.]
How does oxytocin work during childbirth? As delivery approaches, the posterior pituitary gland beneath the hypothalamus begins releasing more oxytocin. The hormone travels through the blood, binds to receptors, and tells the uterus to contract. Each contraction triggers another release of oxytocin, which causes an even stronger contraction. Through this feed-forward mechanism, the intervals between contractions get shorter and the contractions get stronger, until the mother's body pushes the baby out. At the peak of labor, a woman's oxytocin can reach ten to forty times its normal level.2
That same surge of oxytocin acts as a natural painkiller, easing the pain of delivery. It also helps heal the uterine lining and any injuries from the birth. Oxytocin intensifies the mother's bond with her newborn, driving her to hold the baby and breastfeed. In animal experiments, mothers deprived of oxytocin ignored their newborns after birth — and in some cases, killed them.
Oxytocin is also essential for breastfeeding. The more a baby suckles, the more oxytocin the mother produces. More oxytocin means more milk, and more milk means more vigorous suckling. Mother and baby end up raising each other's oxytocin in a virtuous cycle that benefits both. Remarkable, isn't it? Through this process, a new mother recovers faster after birth. Oxytocin plays a central role in both successful delivery and the nourishment that follows.2
The hormone also plays an important role in infant brain development. When oxytocin is released in a nursing baby's brain, it increases blood flow to the neocortex and cerebral cortex, supporting overall brain growth. Its effect on the neocortex is especially significant — this region makes up 80 percent of brain volume and handles abstract reasoning, inference, language, and mathematical thinking. The takeaway is clear: a mother's physical touch isn't just emotionally comforting for a newborn — it's essential for brain development. Whether breastfeeding or bottle-feeding, when an infant makes eye contact and has skin-to-skin contact with a parent during feeding, oxytocin levels rise even higher. Animal studies have found that pups separated from their mothers for just three hours a day grew into adults with lower sociability and weaker immune systems.3
Most of us know that breast milk boosts a baby's immune system. It contains a beneficial bacterium called Lactobacillus reuteri. But recent research found a troubling pattern: when scientists tested breast milk for Lactobacillus, about half of rural mothers carried it, while urban mothers had almost none. Given the well-documented decline in gut microbial diversity among city dwellers, breast milk probably used to contain plenty of Lactobacillus regardless of where the mother lived. Recent studies suggest this bacterium helps improve symptoms of autism and stimulates the body's own oxytocin production. Combined with the evidence that oxytocin helps heal wounds, breastfeeding turns out to be important not only for the mother's recovery but for the baby's brain development and long-term social development.4
Oxytocin: Commander of the Low-Birth-Rate Task Force
Oxytocin actually begins its work long before delivery. It starts from the moment two people meet, fall in love, and feel drawn to each other — shaping desire and conception along the way. Consider the roundworm Caenorhabditis elegans. It looks like a tiny earthworm, about one hundred to one thousand times smaller. Because of its genetic similarity to humans, it's widely used in laboratory research. When researchers disabled the oxytocin gene or its receptor gene in these worms, the males could barely find females. Those that did had trouble locating the female's reproductive opening. And even those that managed that step couldn't deposit their sperm properly. Reproduction essentially ground to a halt.5
What about humans? Before two people can become intimate, they have to recognize each other first — and then feel attracted. Does oxytocin play a role in both? The short answer: absolutely yes. Even in animals, when oxytocin doesn't work properly, it's as if they develop a kind of collective face-blindness — they can't recognize each other and can't mate. Rodents rely on smell rather than sight to identify each other, so when oxytocin is impaired, finding a mate takes much longer. Humans, of course, recognize each other primarily by face.
Research shows that oxytocin improves facial recognition. Multiple studies have found that inhaling oxytocin improves not just the ability to identify faces but also the ability to remember someone after a single meeting. In a 2013 study, researchers used fMRI to scan the brains of eighteen adult men while showing them photographs of various women. In one session the men inhaled oxytocin; in another, a placebo. When they'd inhaled real oxytocin, their attraction to the women in the photographs increased — not because of a dopamine boost, but because their negative emotions decreased. We'll explore the connections between oxytocin, trust, and altruism in more detail later in the book.6