Why do babies need to be carried – the biological explanation
Mammals can be classified in three categories: nest, cache and carry mammals.
The young of nest mammals (e.g. mice, cats, dogs) are born blind and without fur. They remain with other young in the burrow or nest until they have sufficiently developed. These young remain quiet when they are left alone, but cry when they are being taken away from the nest. This way chances are quite small a predator discovers the nest. The milk of nest mammals is high in fat to keep the young saturated for a long time, this way the mother can leave the nest for a longer period of time.
Cache mammals (e.g. horses, elephants, deer, dolphins) give birth to quite mature young who learn how to walk quickly in order to run away with their mother if they are in danger. Young of cache mammals call for their mothers when they lose sight of them. The milk of cache mammals is full of protein, which leads to a very fast development of their muscles and bones.
The young of carry mammals (e.g. monkeys, koalas, kangaroos) are born immaturely, just like nest mammals. They have a couple of grasp reflexes so they can hold on to their mother’s fur. When they lose contact with their mother, they are in danger and will cry for help. The milk of carry animals is full of sugar and fatty acids, which stimulate the growth of the brain.
Humans are born very immaturely. During the first two years after birth, the brain increases 75% in size. In order to grow, babies need a constant supply of sugar and fatty acids and luckily mother milk is filled with these nutrients. Our brain grows when nerve cells connect with each other. These connections are made thanks to contact. Babies have a couple of primary reflexes (grasp reflex, spread-squat reaction, Moro reflex), which indicate that they need to be carried. Babies cannot repair skin-to-skin contact when it is interrupted, so in case they lose this contact, they will cry for help. Although babies have been treated like nest mammals for a very long time, human beings are actually carry mammals and not nest mammals. Physical contact and being carried is part of their nature.
 Bron: Het Draagbeest - http://hetdraagbeest.be/waarom-dragen/ 10-2013