Age-Period-Cohort Model

The age-period-cohort model is a theoretical model that aims to explain how society changes. In this model, variation over time is thought to occur because of the simultaneous operation of three factors: individual aging, period influences, and generational (or cohort) turnover.

Popular theories of social change rest on the idea that culture, social norms, and social behavior change through two main mechanisms: (1) through changes undergone by individuals, and (2) through the succession of generations (or cohorts). Thus, there is a linkage between individuals and social change. Several things connected to the lives of individuals have a bearing on how society changes. The goal of the age-period-cohort model is to understand the contribution of the effects of aging, time periods, and cohorts to any phenomenon that changes in the aggregate at the society level.

Changes to individuals that influence social change are normally thought to happen because of factors associated with two different phenomena. The first of these is aging. Simply put, people change as they get older. Aging is usually identified with differences among individuals that are linked to their getting older, becoming more mature as a function of having lived more of life, or because of physical or cognitive impairment. For example, the older people get, the more medications they take. America, as a whole, may be taking more medications because the population is getting older—an age effect. The second source of individual change comes about through people's responses to historical events and processes— sometimes called period effects. When the entire society gets caught up in and is affected by a set of historical events, such as a war, an economic depression, or a social movement, the widespread changes that occur are called period effects. The Civil Rights movement, for example, may have changed ideas about race for all ages of Americans, not just those birth cohorts growing up in the 1960s (if it affected primarily the young it would be called a cohort effect—see below). Similarly, not only were the youngest cohorts of women and men affected by the Feminist movement of the 1960s and 1970s, but the movement may have influenced the views of almost everyone living in the society at that time. In fact, it is impossible for most members of society to remain unaffected by some changes—such as the influence of computers on society.

The third source of change in society is cohort succession, which is the gradual replacement of earlier born cohorts by later ones. The terms "generation" and "cohort" are often used synonymously, and while in some cases this may be appropriate, we should clarify their meaning. The word generation is normally used in one of two different ways: first, as a kinship term, meaning a single stage in the succession of natural descent, referring to relationships between individuals who have a common ancestor; or second, as a group of people born at about the same time and therefore living in the same period of history who share an identity. Thus, within a given family, generations (in the first sense) are very clearly defined, and while generational replacement is a biological inevitability within families, the replacement of generations in this sense does not correspond in any neat manner to the historical process at the macro-social level because the temporal gap between generations is variable across families. For this reason researchers often prefer the term cohort for this second meaning of generation. A cohort is a group of people who have shared a critical experience at the same time. For example, people who enter college in a given year are referred to as an "entering college cohort" and those who graduate in the same year would be called a "graduating cohort." In each case, there is an event or experience in common that defines the cohort. When people talk about a group of people who share the same historical frame of reference during their youth, they often are using year of birth as the defining event (Ryder). Thus, the term "cohort" is often used as shorthand for "birth cohort," and is thought to index the unique historical period in which a group's common experiences are embedded. It is often correctly suggested that the term generation should not be used when we mean birth cohort (Kertzer). At the same time, we can tolerate use of the term generation in both ways, but reserve the term (in the second sense mentioned earlier) when the focus is on groups of people who share a distinctive culture and/or a self-conscious identity by virtue of their having experienced the same historical events at roughly the same time in their lives (Mannheim). In this sense generation is not the same as cohort—it implies much more. Thus, cohort differences (i.e. differences tied to year of birth) may be suggestive of generational differences. However, cohort differences may be necessary, but are not sufficient to say that generations truly exist in the sense of having a shared identity.

Earlier-born cohorts die off and are replaced by those born more recently. When the effects of historic events tied to particular eras mainly affect the young, the result is a cohort (or generation) effect. For example, it is sometimes suggested that civic engagement has declined in America overall, even though individual Americans have not necessarily become less civic minded. This may be because older, more publicly engaged citizens are dying off and being replaced by younger, more alienated Americans who are less tied to institutions such as a church, lodge, political party, or bowling league. As a cohort effect, this refers simply to the effects attributable to having been born in a particular historical period, but as a generation effect (in the sense of Mannheim) it would also imply a status that is recognized both from outside and from within the group. When, for example, people say there is a Depression generation that is particularly self-consciously thrifty, they imply that the experience of growing up under privation permanently changed this set of cohorts' economic style of life and their identities due to their formative years. As these members of society die off, they may leave behind a somewhat less frugal set of cohorts.

Cohort replacement explanations of social change make several critical assumptions: (1) that youth is an impressionable period of life in which individuals are maximally open to the socialization influences of the social environment; (2) that people acquire their world views (values, beliefs, and attitudes) during these impressionable years and largely maintain those views over most of their lives; (3) that unique cohort experiences are formed due to the distinctive influences of historical events and experiences, and that there are clear differences across birth cohorts in typical beliefs and attitudes, i.e. there are cohort effects; and (4) that public opinion and social norms change gradually in the direction of the more recent cohorts. If the cohort differences become ingrained in the identities of social actors who take them into account in their behavior, then it may be appropriate to refer to this as "generational" replacement.

One of the major questions in research on social change is: Do the unique formative experiences of different cohorts become distinctively imprinted onto their world views making them distinct "generations," or do people nevertheless adapt to change, remaining adaptable in their dispositions, identities, and beliefs throughout their lives? Unique events that happen during youth are no doubt powerful. Certainly, some eras and social movements (e.g., the women's movement, or the Civil Rights era) or some new ideologies (e.g. Roosevelt's New Deal), provide distinctive experiences for youth during particular times. As Ryder put it: "the potential for change is concentrated in the cohorts of young adults who are old enough to participate directly in the movements impelled by change, but not old enough to have become committed to an occupation, a residence, a family of procreation or a way of life" (Ryder, p.848)

How do age, period, and cohort factors combine to shape social change, and how can their influences be studied using empirical data? The age-period-cohort model recognizes that these are all important causal factors. Unfortunately the individual parts of this model—namely the effects of aging, cohorts, and time periods—are not easy to understand in isolation from one another, and there are serious problems with uniquely identifying their separate effects. It is thus sometimes difficult to place any one interpretation on observed data. Generally speaking, it is necessary to concede that social change could be due to the operation of all three of these factors at once.

The best research designs for the study of aging are longitudinal studies of the same people over time. Often such designs control for cohort differences, but while it is possible to study how individuals change using such designs, it is usually more difficult to understand why they change as they do. Often, the best designs for studying cohort effects are repeated cross-sectional surveys, which do not study the same people but the same cohorts. Such survey designs can study the same cohorts over time, and while less useful for studying how individuals change, they can provide estimates of cohort replacement. There are many potential pitfalls that await the age-period-cohort analyst, and many precautions must be taken to guard against potential fallacies and errors of inference.