Infant attachment
Nursing requires proximity; proximity facilitates social interaction, and social interaction is essential for survival. Attachment exists to varying degrees among non-human mammals; it is greatly increased especially in humans. In hunter-gatherer and village-agricultural societies, the mother typically carried the baby or would leave it briefly with an allomother—someone with whom the child will have a life-long relationship such as a grandmother, aunt, or older sibling. Only with the rise of civilization and wealth, and especially within the last 60 years has this pattern of childraising, embedded in our mammalian biology, been significantly altered.