Differences in Wealth and Altruistic Behaviour: A Model

In an economy with N individuals each individual is concerned, by assumption, with his own felicity as well as that of all or some others. If the society is composed of very few individuals with a high level of wealth and many with a low one and if altruistic behaviour of any two individuals toward each other decreases with their wealth difference, the largest contribution to the felicity of other individuals is made by those whose wealth is neither too high nor too low (middleclass).