The during childhood results in a reduction in density of nerve cells, thus increasing the brain's efficiency.

The synaptic pruning during childhood results in a reduction in density of nerve cells, thus increasing the brain's efficiency.
Synaptic pruning, which can include both axon and dendrite pruning, is the process of synapse elimination that occurs usually in early childhood. It is thought that this is the brain’s way of removing connections in the brain that are no longer needed and a way of maintaining more efficient brain function.