oxytocin

Oxytocin is a tiny chemical messenger, a string of just nine building blocks called a peptide, made by a cluster of cells deep in the brain in a region called the hypothalamus — the body's master control hub. From there it travels a short distance to the pituitary gland, a pea-sized gland hanging just below the brain, which squirts it into the bloodstream like a hose feeding the whole body. The same molecule also acts as a signal inside the brain itself, so it works in two ways at once: as a hormone carried by the blood, and as a brain messenger passed between nerve cells.

Out in the body, oxytocin is the muscle-squeezing signal of birth and nursing: it makes the womb contract to push a baby out during labour, and it triggers the breast to release milk when a baby suckles. Inside the brain, it gently tunes how we feel about each other — easing a mother's bond with her newborn, deepening the warmth between partners, and nudging up feelings of trust, calm, and closeness. This is why it is nicknamed the love or bonding hormone, though the real picture is subtler: rather than simply switching on good feelings, it seems to sharpen our attention to social cues, making the people who matter feel more important to us.