" But the physical health of these young monkeys hid a devastating
sickness: they had been wrecked by loneliness. Their short
lives had been defined by total isolation, and they proved incapable
of even the most basic social interactions. They would maniacally
rock back and forth in their metal cages, sucking on
their thumbs until they bled. When they encountered other monkeys,
they would shriek in fear, run to the corners of their cages,
and stare at the floor. If they felt threatened, they would lash out
in vicious acts of violence. Sometimes these violent tendencies
were turned inward. One monkey ripped out its fur in bloody
clumps. Another gnawed off its own hand. Because of their early
deprivation, these babies had to be isolated for the rest of their
lives. "

― Jonah Lehrer , How We Decide