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The ‘life course epidemiology’ (LCE) approach articulated by Ben-Shlomo, Cooper and Kuh (BCK) in their review (this special issue of IJE) is one among several substantive paradigms that are informed by the concept of ‘life course’. It is an important subfield of epidemiology that focuses on the processes (biological, behavioural, psychosocial) that link adult health and disease risks to early life events and exposures. In recent years, the key themes in this work have drawn the attention of numerous scholars from several different disciplines.1–7 Despite competing theories and nuanced differences in empirical findings, the literature consistently suggests that individual differences in adult health depend in important ways on early life experiences—increasingly referred to as ‘the long arm of childhood’.8–10 An emerging consensus concludes that the influences of such factors extend well into adulthood through exposure to conditions that differentially shape the risks of disease and access to protective factors over the entire life span. The increase in life course thinking has been remarkable in its abundance and diversity. This is supported by a recent review based on Web of Science data that provide trend lines for three fields: sociology, psychology and epidemiology since the early 1990s.11 These results make a persuasive case for the proliferation of the term ‘life course’ in research over the past two to three decades, especially in the field of epidemiology (see also Figure 2 in BCK). So pervasive has the life course concept become in recent years that it was recently adopted by the World Health Organization (WHO) as its approach to understanding the determinants of health in older age.12 These trends raise questions about the potential growth in understanding the dynamics of human lives. The intent of this commentary is to reinforce the broad appeal of the life course perspective, as advocated in the BCK article, and to suggest how their LCE approach fits nicely with (and benefits from) complementary approaches in related fields. To begin, I distinguish between the life course, on the one hand, and life course perspective(s) on the other, in order to reduce some confusion. The former consists of a complex set of interlocking trajectories, or pathways, over the life span of an individual (from conception to death), within and between several domains (e.g. residential, household, family, schooling, work, nutrition) that are marked by sequences of events, transitions and exposures (ETXs) across (and within) the biologically- and socially-defined life stages (or phases) that comprise the human life span. Traversing life stages and moving between them, while experiencing unique sets of ETXs at each stage, defines an individual's life course, theoretically producing differential outcomes. Early on, sociologists suggested that the study of the life course is the study of event trajectories, that is ‘the study of a sequence of events… a process which is both unintended and the result of intentionality in which earlier events condition later events’.13 This life course conception of an age-graded sequence of statuses and their carry-over effects implies the existence of resources and opportunities that inhere in making transitions; therefore social inequalities are central to the study of individual differences in health and other outcomes.14–15 Life course perspectives, on the other hand, are disciplinary or sub-disciplinary lenses on the life course, focusing on people's lives with a particular interest in: either one phase of life or the connection between two or more phases and/or outcomes within one particular domain (e.g. health); or in one set of outcomes (e.g. chronic disease in adulthood). There is more than one life course perspective, each attending to different aspects of the individual's life course; but in general, life course analysis focuses on the nature and determinants of particular transitions, their timing, their links to events and exposures in other life stages and their consequences for outcomes of human development. The life course concept emerged in sociology more than 50 years ago, beginning with Leonard Cain's chapter on ‘life course and social structure’ published in the 1964 Handbook of Modern Sociology.16 He defined the life course as ‘those successive statuses individuals are called upon to occupy in various cultures and walks of life as a result of aging’. It was not a new idea, as the age-graded nature of stages of human life has been recognized for millenia. For example, Cain noted the writings of Solon, the Athenian poet and lawmaker born in the seventh century BC, who suggested a 10-stage life course of 7 years each, beginning with ‘the boy as the unripe man’ and ending with ‘the time to depart on the ebb-tide of Death’. This age-graded perspective on the life course, specifically involving the critical importance of life stages, was reinforced in the early 1970s by others, focusing primarily (but not exclusively) on ageing.17,18 At about the same time, the work of both demographers and psychologists emphasized the importance of locating human development in historical time through the re-introduction of Mannheim's concept of ‘birth cohort’ or ‘generation’ into the social science toolkit.19–21 And soon thereafter Glen Elder, who perhaps more than anyone else has helped proliferate the concept of the life course within sociology and demography, published his early work on The Children of the Great Depression, emphasizing the interaction of historical and biographical factors in human development.23–28 Elder's work emphasized the importance of studying ‘sequences of life events and transitions’ as the essential meaning of the life course concept, a view that was compatible with the life span framework in studies of human development, as well as the ‘age stratification’ or ‘institutionalized life course’ perspective in sociology. Meanwhile, the life span developmental perspective was initiated by psychologists who were critical of the primacy being given to early childhood development, and who stressed the need to understand that human development is a lifelong process.29 Some authors suggested that ‘the distinction between life span and life course… is mainly a matter of scientific history’, and the terms are often used interchangeably.29,30 Others have argued that there are important differences between the two perspectives—one (life span psychology) seeing ‘development across the life span primarily as changes of genetically and organically based functional capacities and as behavioural adaptation’, whereas the other (life course sociology) views the development of lives ‘primarily as the outcome of institutional regulation and social structural forces’.31 By the early 1980s, the compatibility of the life course and life span developmental perspectives was widely recognized, and their potential for integration acknowledged. One view was that that the concept of life course was a key component of the life span developmental perspective, which posits that ‘developmental changes in human behaviour occur from conception to death, and arise from a matrix of biological, psychological, social, historical and evolutionary influences and from their timing across the lives of individuals’.32 Increasingly, however, the two perspectives incorporate aspects of the other domain, and there is considerable overlap between them.10 As noted in the BCK review, the life course perspectives of today transcend Cain's original notions of age-graded life stages. In addition to the ELC perspective, there are several recognizable and distinct life course paradigms that motivate the study of human lives. Each approach relies on its own concepts, which are often mistakenly applied interchangeably across fields, but each makes a distinctive contribution that deserves notice in mapping this domain.10,28 In the biologically based fields of anthropology and biodemography, the concept of life history is used to approach the study of the life course. ‘Life history’ is defined as the series of life forms exhibited by an organism in the course of its development from inception to death. Life history theory explains the broad features of a species-specific reproductive life cycle patterns, such as size at birth, growth patterns, age at maturity, size at maturity, number, size and sex-ratio of offspring, age-related reproductive investments, life span, age-specific mortality schedules and so forth.33–36 Substantial work is now being devoted to evolutionary theories of the nature and origin of life history traits of the human species. Human life histories, as compared with those of other primates and mammals, have several distinctive co-evolved characteristics: an extended period of juvenile dependence, an exceptionally long life span, support of reproduction by older post-reproductive individuals and support for provisioning of females and their offspring. Longevity (life span) and ageing are evolved life history characteristics, and some of the most profound questions being raised in these literatures concern the interconnection between the extended period of childhood dependency and older age. With regard to ageing and the life course, one of the recurrent themes in this literature involves the study of ‘life history strategies’, that is different strategies for resource allocation trade-offs across different life stages.35 Within the biological universals of a given species, cultures vary according to how they define the reproductive life cycle. The concept of ‘life cycle’ has a very precise meaning in the biological sciences—a fixed sequence of irreversible stages, tied specifically to sexual reproduction.36 In the social and behavioural sciences, the life cycle is more than a biological concept: referring to the socially defined, age-related sequence of stages individuals pass through beginning with birth and ending with death, linking individual ageing, the organization of roles in society and reproduction.37 According to this perspective, socialization processes transmit cultural schemas that define the meanings attached to stages of lives and the transitions between them. An appreciation of culture is essential to any understanding of the life course. As cultural schemas, life stages are organized bodies of knowledge held by members of the society/culture about a phase (or stage) of life, e.g. infancy, or old age, built up from experience and absorbed from the culture in which one lives. These stages through which lives are enacted are socially constructed, in the same sense that we construct other social schemas such as gender, class and race-ethnicity. Individuals negotiate the meaning of life stages as part of the process of constructing their lives within a given culture. Within the past 50 or more years, as noted earlier, the field of human development became aware of the importance of the ‘life span developmental’ perspective, which conceptualizes human development across the entire life span.29,38 New questions were raised about continuity and change in adult lives over time, because it was realized that the concepts and issues at stake with respect to children could not simply be extended to adults. The BCK review article importantly recognized the work of the late Paul Baltes and his colleagues, particularly the metatheory of selective optimization and compensation (SOC) underlying successful ageing. Baltes’ SOC framework has become important for the study of health and ageing because it highlights the determinants of some attributes of health and mortality linked to culture.39–40 These contributions were motivated by a set of conclusions regarding the evolution of human development, which Baltes articulated as follows: (i) evolutionary pressures have emphasized reproduction rather than longevity, and the benefits resulting from evolutionary selection do not favour chronological age, resulting in associated processes of ageing or loss;41 (ii) there is an age-related increase in the need or demand for culture, namely environmental resources that are needed to compensate for loss and to maintain levels of functioning; and (iii) ‘there is an age-related loss in the effectiveness or efficiency of cultural factors and resources’, largely conditioned by the ‘negative biological trajectory of the life course’.38 These ideas have led to the development of the concept ‘successful ageing,’ which has become ingrained in our language and our research. ‘Successful ageing’ (or its cousin concept of ‘healthy ageing’) is a perspective which is increasingly popular for framing issues about health and ageing.42,43 In sociology, the life course perspective assumes that the social pathways followed by individuals are embedded in social institutions, which are subject to historical variation and change. A key element offered by this perspective, is its emphasis on institutionalized age-graded transitions—referred to as the institutionalized life course—which shapes sequences of roles, activities and social pathways across the life span.44 This focus on social pathways allows research to concentrate on certain transition points between life cycle stages, such as the transition to school, the transition to adulthood or the transition to retirement in older age.45–48 Even though institutionalized norms governing the age-graded life course help define the transition from one life stage to the next, there is heterogeneity in the pathways actually followed. Much of what sociologists have written in the past 30 or more years has to do with accounting for this variation in and how different life paths lead to different outcomes at the individual level. Elder and his colleagues have defined four key principles of the life course perspective on human development—context, timing, interdependency and agency.49 The sociological life course perspective assumes that these developmental transitions and processes occur across the entire life span, that they are embedded in social institutions and are subject to historical variation and change. Perhaps the most important element in this sociological perspective is that lives happen in social-historical contexts, defined by time and place, which have indelible effects on cohorts of young people. The principle of timing emphasizes that the developmental antecedents and consequences of life transitions, events and exposures vary according to their timing in a person's life. The same events or experiences may affect individuals in different ways depending on when they occur across the life span. This principle also invokes the idea that cultural content is not constant over time. The emphasis on interdependency recognizes that people do not go through life by themselves. The life course perspective assumes that people's lives are linked both inter-generationally and intra-generationally, and social network phenomena are captured within the life course perspective, viz. the concept of ‘linked lives’ which emphasizes the fact that lives are lived interdependently. Finally, the manner by which life stages are negotiated involves the human capacity for agency as well. The idea of finding one's own life pathways, due in part to human agency, has been an important part of sociological discourse on life span development. Agency is an inherent feature in the construction of lives, and despite our conceptions of the biological ontogenesis of human development, the cultural definitions of life cycles (or stages of life) and the institutionalization of life transitions, it is important to recognize that humans employ complex repertoires of interaction skills that permit them to control their own pathways across a given stage of life, or between connecting stages in the life cycle.50,51 The subfield of demography, which cross-cuts the disciplines of biology, anthropology, sociology and economics, brings several conceptual and methodological contributions to the study of the life course. The development of the life course perspective in demography in many ways parallels the similar development within the field of epidemiology, in that both specifically focus on the linkages between childhood experiences and adult health. Demography is an event science, and historically demographers have studied some of the events in human lives around which lives are organized, namely births, deaths and migrations. These life course events were of interest because they impact directly on the size, composition and change of geographically-defined populations.52 Recognizing the complexity of human behaviour, in the modern era demographers have enlarged their territory, and at least by 1991 the life course was a major focus of demographers.53 Social demographers had by then been studying the precursors of fertility, mortality and migration, specifically focusing on life transitions and trajectories. The life course perspective assumes that people's lives are uniquely shaped by the timing and sequencing of life events and exposures. Life course research has made major inroads in the understanding the connections between lives, time and place, and how to handle these complexities in theory and research. Established in the mid 1990s, the subfield of life course epidemiology has made substantial progress over the past few decades. It has broadened its conception of the life course, and earlier models have been modified to capture the complexity of taking a broader compass on the life course. This special issue provides testimony for the progress that has been made by the LCE approach in this regard and in capturing the attention of researchers focusing on chronic disease and other health-related outcomes. LCE has relied (and continues to rely) on developments in other fields, and this is a mark of its flexibility and adaptability. The resulting cross-fertilization of ideas about life course processes is essential to understanding the dynamic processes in which the ontogeny of development interacts with the social environment. The theoretical richness offered by these diverse literatures is important, and although it is uncommon for researchers to look across disciplinary boundaries, the various life course perspectives emerging in recent years can foster improvements by building upon the work of life course researchers in other fields, and hence, it takes more than one to tango. The author wishes to thank Linda Wray for her generous comments on this paper while it was being drafted. Conflict of interest: None declared.
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