A baby sucks at a mother’s breast for comfort, and of course for milk. Milk is made within specialised cells of the mammary gland, and is normally stored inside those cells. For a baby to feed, the milk must be released from those cells; when it is released it gathers in a collecting chamber from where it can be extracted by sucking. Milk is released or “let down” as a reflex response to the sucking and kneading of the baby at the breast, and sometimes also in response to the sight and smell and sound of the baby. The reflex originates from stimuli that are sensed by nerve endings, and which are communicated to the brain through a chain of nerve cells, and the reflex is effected by the release into the circulation of a hormone, oxytocin, from the pituitary gland. The reflex, first described in the rat by Wakerley and Lincoln in 1973 (1), is a spectacle of power and economy. When hungry pups suckle at the nipples of their mother, oxytocin cells in the hypothalamus all discharge an intense burst of spikes every 5-10 min, resulting in an abrupt milk let-down. Each burst lasts for just 1-2 s, and every oxytocin cell will burst within about half a second of one another. These bursts cause oxytocin to be released not only into the blood, but also into the brain – from the dendrites of oxytocin cells, and this stimulates yet more oxytocin release (see 2-4). So much oxytocin is released from these dendrites (5) that it can reach sites throughout the forebrain, where it causes striking changes in behaviour, reinforcing the bond between the mother and her offspring (6). But oxytocin is also released during sexual activity, in both males and females, and experiments in the prairie vole showed that this release is instrumental in forming bonds between partners (7). The prairie vole is one of those unusual mammalian species, which, like humans, forms monogamous bonds, and oxytocin release in the female brain, stimulated by sexual activity, is essential for the formation of these bonds. Oxytocin is also released in the male brain, as is a closely related hormone-vasopressin, which in the male brain triggers the aggressive, territorial behaviour, akin to jealousy, that is a signature that a bond has been formed by the male. In humans, some studies indicate that intranasal application of oxytocin can induce trust, and there is evidence that links deficiencies in oxytocin with autism and related deficits in social behaviour (8). These have attracted intense interest in the search for novel therapies – and have also raised some difficult ethical questions.