The darwinian theory of evolution has been used to justify the statement that breast-milk is ideal for infants. However, a broader interpretation of the theory suggests that the mother-child dyad is the evolutionary unit. For the survival of the species, both mother and child should benefit. Simple arithmetic shows that where there is a conflict of interests, the welfare of the mother outweighs that of the infant. The dyad hypothesis suggests that the maximum evolutionary gain is obtained when protein and energy levels in breast-milk are just high enough to prevent prohibitive infant mortality rates, but low enough to spare the mother. The anti-infective constituents of breast-milk are very small in bulk, so they place a minimum metabolic load on the mother but have a large benefit for the child. The contraceptive effect of breast feeding is pronounced only when the mother is malnourished. A separation of the nutritive from the anti-infective properties of breast-milk suggests that a rational method of infant-feeding for many mothers is to breast-feed for several months to reduce the morbidity rate in the infant, but to give supplementary milks, formulae, and foods when needed to promote the baby's nutritional status.