Life-Course Theories

General Life-Course Concepts

Glen Elder has used the concept of interconnected trajectories to describe how people are linked together over time. Trajectories are different paths that people follow as they progress through life; there can be an educational trajectory, a family trajectory, or an employment trajectory. Interconnected trajectories describe a process in which events in one's life affect both one's own trajectory and the trajectories of others. Consider how families are affected by the criminal justice system. Experiences such as arrest or incarceration affect not only those incarcerated but also their spouses, children, and friends. Bruce Western and his colleagues have shown that incarceration has strong negative effects on both individual career earnings and family characteristics (Western and McLahanan 2000). The trajectories of both the person who has been incarcerated and his or her family are interconnected through the incarceration.

Life-course events embedded within trajectories are often called turning points or transitions. These events may alter the trajectories of individuals. Examples of turning points include divorce, marriage, and the death of a sibling. Sampson and Laub (1993) use the transitions of marriage and employment to understand how individuals alter a trajectory of behavior and move away from crime.

In a classic exposition of life-course concepts, Norman Ryder (1965) explicitly differentiated between age, period, and cohort effects. Ryder defined a cohort as an aggregate of individuals, within some population definition, who experience the same event at the same time. Under this definition, a cohort effect has specific consequences for a given population of individuals but different or no effects for individuals outside the population definition. A period effect is less specific than a cohort effect, because its scope is larger. Its effects are not limited to a certain group of people but affect most people in a point in time. Age effects speak to the differential malleability of young people. Relative to their elders, youth are more susceptible to change.

As an example of this distinction, consider the Great Depression. As a period effect, the depression had powerful effects on working patterns for large segments of the population, regardless of social location. A cohort effect would show differential effects of the depression on work patterns for those who were in early childhood as opposed to late adolescence during this period of economic strain. Last, age effects are operative in all historical eras structuring the relationship between employment and life-course stages.

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