A second piece of evidence that social pain is real pain is the separation distress that mammalian infants show when separated from their primary caregivers. Observe the intense and relentless crying and distress that can occur when a mother leaves her child. Each of us is born with an attachment system. We all inherited an attachment system that lasts a lifetime, which means we never get past the pain of social rejection, just as we never get past the pain of hunger.

Newborn monkeys were raised apart from their mothers. Substituting for the mother were two surrogate “monkeys”. One surrogate was a wire-mesh frame that was constructed roughly in the shape of an adult monkey and that provided the milk the newborns needed for survival. The other surrogate was a wooden block that was covered in a layer of sponge rubber, with an outer surface of terry cloth, also roughly in the shape of an adult monkey. This cloth mother provided no milk.

Which surrogate did the infants became more attached to: the one associated with nourishment or the one that felt a little more like a real mother monkey? 

Soon after birth, infant monkeys were spending nearly 18 hours a day in contact with the cloth monkey and almost no time at all with the wire monkey that provided food. These monkeys were attached to the thing that felt most like a real monkey, regardless of the sustenance it provided. Given that all mammals are born incapable of caring for themselves, they all have a similar need to stay connected with a parent or caregiver.