The primatologists noted that because of their highly sociable nature the non-human primates have to maintain personal contact with the other members of his social group. The number of members with who a primate can maintain said contact seems to be restricted by the size of the neocortex, and that suggests that there is an index of group size relative to the specie. So in 1992 Dunbar used the correlation observed in primates to predict the size of the social group in humans. He predicted a size of 147.8 group members, usually represented as 150.
Dunbar compared this prediction with observable human groups and noted that the groups fall in three categories, of 30-50, 100-200, and 500-2500.The research of Dunbar on the size of tribes and villages seemed to corroborate his prediction: 150 was the estimated size of a village farmer of the Neolithic; 150, as the point of break and separation of a hutterite settlement and 150 as the basic size of a military unit in ancient Rome and in modern times since the 16th century.
Dunbar theorized that a group with a size of 150 persons needs to have a very high incentive to stay together, thus 150 members groups only occur due to an absolute necessity, for example, aggressive economic pressure. For a group this size to maintain such cohesion, he speculated was needed to dedicate a 42% of the time to socialization; a dispersed group would have fewer ties, meeting less frequently.
Dunbar proposed also that the language may have generated as an easy way to socialize, because without language the humans would have had to occupy almost half their time socializing; the language may have allowed societies to remain cohesive, reducing the need for physical and social intimacy.
It's considered that -according to Dunbar's theory- in small-scale (less tan 150 individuals) most social orders could function properly, but due to population growth, problems arise due to the difficulty of maintaining properly controlled relations between individuals. This theory also applies to problems of overpopulation, because the further away a group moves of the hypothetical limit of 150 in any delimited system, its more likely for conflict to grow.