74 aging and the life course, theories of

Age, period, and cohort are core concepts in
the life course perspective. Briefly defined, age
refers to biographical time; period refers to
historical time; and cohort refers to a group
whose members experience a particular event
at the same time in their lives. Persons born at
the same time constitute a birth cohort. As
they age they come to encounter historical
events from a different social vantage point
than other birth cohorts. So, for example,
members of the US baby boom cohort, born
between 1946 and 1964, face historical events
such as the Vietnam War or the stock market
bubble of the 1990s, and experience them
differently from other birth cohorts because
of their age and life course statuses during
those events (Hughes & ORand 2004).
The life course framework is founded on
three general principles. The first is the age
stratification principle or the conceptualization
that age is an independent social basis for
differentiation and inequality across societies.
First, age is a gauge of human development,
marking some largely universal psychophysical
transitions in the aging process from birth to
death. Human development is a product of
the coevolution of the brain and its cognitive
capacity with a long life span, an extended
period of juvenile dependence on parents/
caretakers, and a complex familial organization
for provisioning offspring until they reach
adulthood (Kaplan et al. 2000). Hence, age
has an underlying biological component that
differentiates and stratifies developmental sta
tuses. Second, age is also a social construction,
defined by institutional arrangements that
allocate individuals into social statuses, such
as student, voter, and retiree, on the basis of
age. Social allocation on the basis of age dis
tributes resources and advantages unequally by
defining rights and obligations.
The second principle may be termed the
heterogeneity principle. This refers to processes
of social differentiation as increasing functions
of age. Birth cohorts may live through history
together, but they do not experience that his
tory similarly because of two sources of differ
entiation. First, birth cohorts are themselves
heterogeneous in socially meaningful ways
from the beginning. Gender, race, class, and
geographical locations are among the initial
differences within cohorts that anchor the
trajectories of lives and condition opportu
nities and actions over time. The baby boom
cohort is not a homogeneous group, but one
highly stratified by education, work history,
race/ethnicity, and other meaningful social
characteristics (Hughes & ORand 2004). Sec
ond, individual lives become increasingly dif
ferentiated within cohorts over time because
later life statuses (such as wealth status or
disability) are affected by social origins and
by highly variable and interdependent transi
tions that intervene across several domains of
life, including education, family, work and
health, from birth to death. Levels of educa
tional attainment, employment stability, mar
ital stability, and health maintenance, along
with personal responses to these life events
across the life course, interact in complex ways
to increase differentiation with age. These
diverse life trajectories can also be deflected by
historical events, which can have more severe
consequences for some members of a cohort
than for others. Glen Elders extensive studies
of the impacts of war, depression, and eco
nomic hardship repeatedly demonstrate the
diversity of experiences with history within
cohorts (e.g., Elder 1998; Conger & Elder
1995).
The demographic principle refers to changing
aggregate patterns of lives that are responses
to changing historical circumstances and stra
tified opportunities. These are the day to day
behavioral responses of individuals to their life
conditions that, in the aggregate, can exert
forces for social change. For example, the
baby boom was unexpected. Fertility behavior
in the century before the baby boom and
following it exhibited a long term trend of
declining fertility. However, the post World
War II economy and culture led to changes
in fertility behavior including earlier and lar
ger families. Since the post World War II
period, even more demographic changes have
occurred, including increased labor force par
ticipation among young mothers, delayed fer
tility until middle age, and rising divorce and
serial marriage, all of which are challenging
traditional institutions associated with the
family and the market.
This principle challenges the age stratifica
tion process that has differentiated the life
course along strict age criteria. Matilda White