Research with animals has permitted a more direct study of the relationship. 20 % of macaque mothers exhibit very supportive care of their infants. Process of breeding with the intention to create offspring that share the traits of both parent lineages. Increases the likelihood that a highly reactive monkey will be born. Social and physical environments shape the networks and patterns of the brain. Modification of neural pathways as a function of experience. Allow the nervous system to connect to and control other systems of the body. Starts prenatally and extends well into childhood years. Stability, strength of synapses depend on effective firing of connections. Neural development determined by speed that neurons conduct nerve impulses. A fatty substance that wraps itself around nerve axons. Rate and extent of myelin affected by experience and nutrition. The process of eliminating used neurons and neural synapse sites. Adults had a 41% in the number of neurons compared to newborns.