A generation is "all of the people born and living at about the same time, regarded collectively". It can also be described as, "the average period, generally considered to be about thirty years, during which children are born and grow up, become adults, and begin to have children of their own". In kinship terminology, it is a structural term designating the parent-child relationship. It is also known as biogenesis, reproduction, or procreation in the biological sciences.
Sociologist Karl Mannheim was a seminal figure in the study of generations. He elaborated a theory of generations in his 1923 essay The Problem of Generations. He suggested that there had been a division into two primary schools of study of generations until that time. Firstly, positivists such as Comte measured social change in designated life spans. Mannheim argued that this reduced history to "a chronological table". The other school, the "romantic-historical" was represented by Dilthey and Martin Heidegger. This school focused on the individual qualitative experience at the expense of social context. Mannheim emphasised that the rapidity of social change in youth was crucial to the formation of generations, and that not every generation would come to see itself as distinct. In periods of rapid social change a generation would be much more likely to develop a cohesive character. He also believed that a number of distinct sub-generations could exist.
Norman Ryder, writing in American Sociological Review in 1965, shed light on the sociology of the discord between generations by suggesting that society "persists despite the mortality of its individual members, through processes of demographic metabolism and particularly the annual infusion of birth cohorts". He argued that generations may sometimes be a "threat to stability" but at the same time they represent "the opportunity for social transformation". Ryder attempted to understand the dynamics at play between generations.
Karl Mannheim, in his 1952 book Essays on the Sociology of Knowledge asserted the belief that people are shaped through lived experiences as a result of social change. Howe and Strauss also have written on the similarities of people within a generation being attributed to social change. Based on the way these lived experiences shape a generation in regard to values, the result is that the new generation will challenge the older generation's values, resulting in tension. This challenge between generations and the tension that arises is a defining point for understanding generations and what separates them.