Grant's distinction between "giver cultures" and "taker cultures" as two end-points of a scale, with "matcher cultures" in between. In evolutionary psychology Evolutionary psychologists have used the norm of reciprocity to explain altruism by emphasizing our expectations that “helping others will increase the likelihood that they will help us in the future.” The underlying justification lies in the human desire to reciprocate kindness and cooperate for survival value has enabled our continued existence in a hostile world. Thus, the norm of reciprocity ultimately has survival value. Furthermore, being as this sentiment is intrinsic to our evolutionary history and existence, adherence to the norm would constitute “natural” behavior whose neglect might necessarily cause a degree of dissonance in an individual who, among many other self-concepts, consciously labels himself a human being, perhaps leading to a reduction in self-esteem. The norm of reciprocity is reciprocal altruism, a concept coined by the evolutionary biologist Robert Trivers. However, the rise of sociobiology was not well received by mainstream psychologists. It is therefore not surprising that the stigma of the evolutionary theory led reciprocal altruism to be revitalised underneath the name "norm of reciprocity". The norm of reciprocity is arguably less scientifically advanced than reciprocal altruism, due to the degree of research underneath the name "reciprocal altruism" as opposed to the name "norm of reciprocity". In developmental psychology Developmental psychologists have studied the norm of reciprocity and the development of this norm in children. Psychologists have found that children begin to show the reciprocal behavior around the age of two, when they observe the behavior of others and begin to have their own relationships with peers. One way that psychologists have been able to study the norm of reciprocity in children is by observing and experimenting on their toy sharing behaviour. Kristina R. Olson and Elizabeth S. Spelke (2007) conducted an experiment in which they used dolls to represent family members and friends and gave the child various items to distribute to the dolls after a series of situations were explained to the child. These situations represented private and public reciprocity and gave the child the choice of which dolls to share the items with. An example of a situation involves one of the dolls sharing with the rest of them. Olson and Spelke found that children will give to family and friends more than strangers, repay those who shared with them, and reward those