Diffusion in Population Theory

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Theorizing about the forces driving the transition from high to low fertility has been dominated by opposition between structural explanations and those referred to as diffusion-based explanations. The same opposition is ubiquitous, if less obvious, in the literature on health and mortality, international migration, and spatial and residential patterns. This contrast is not unique to population theory: A similar division is widely seen in the literature on the emergence of nation-states in political sociology, within the theory of social organizations, and in the study of the economic behavior of individuals and firms.

An important illustration of this opposition and one that has long been a problem in population theory relates to the explanation of the secular fertility decline in Western Europe that began around 1850. The phenomenon was initially thought to be a result of economic, social, and political transformations that altered the social context within which childbearing took place. These transformations are seen as structural changes, that is, changes emerging from larger societal transformations that alter the economics of fertility (for example, changes that affect the conditions of individuals' decision-making about childbearing). Some of these transformations are directly related to industrialization and the shifting division of labor and occupational structure. Others, such as the transition to a regime of low infant and child mortality, are thought to be secondary products of these large-scale transformations (and perhaps only loosely connected to them) but with a direct effect on fertility.

The empirical evidence from historical and contemporary societies stubbornly refuses to fit neatly into the patterns of conventional interpretations. Fertility declined in Western Europe in areas where such structural changes both did and did not occur. Onset was remarkably concentrated in time, much more so than the timing of onset of structural changes. In addition, the attainment of low fertility regimes follows a geographic contour that is more consistent with ethnic, cultural, linguistic, and political cleavages than anticipated by the structural explanation.

Similarly, what explanation exists for the more recent sharp fertility decline in Bangladesh, where economic development and large-scale industrialization are still elusive goals but where the total fertility rate dropped from 7 children per woman to nearly 3.5 children per woman in thirty years? Or for the statistics in Brazil where the total fertility rate is2.8 children per woman at the outset of the twenty-first century after topping 6.5 children per woman just forty years before? According to classic indicators of economic development and industrialization, even though Brazil has experienced deeper economic transformations than Bangladesh, its fertility levels are still not commensurate with the relatively limited influence of these changes. Further, the internal regional disparities in economic development are overshadowed by a remarkable homogeneity of fertility regimes.

The inconsistencies between fertility patterns predicted by structural explanations and those actually observed has led many researchers to abandon the structural perspective in favor of one that emphasizes the importance of dissemination of new ideas and the diffusion of modern fertility behaviors.

An analogous, and even older, controversy between structuralist and diffusionist theories exists over explanations of changes in mortality and health regimes. On one side are explanations that interpret the secular mortality decline as a result of economic transformations and improvements in standards of living. On the other side, mortality decline is seen as a result of the spread of medical technology and of the dissemination of advances in public health and individual hygiene that reduce exposure to infectious diseases.

Anatomy of the Theoretical Divide

What is the main distinction between a structuralist explanation and one based on diffusion? Structural explanations of behavioral changes seek the cause in the alteration of preferences and opportunities that result from either changes in the positions that individuals occupy (individual social mobility) or from the reshuffling of resources associated with a given social position (e.g., structural social mobility or redistribution of wealth). In contrast, diffusion explanations attempt to identify a mechanism that leads to cumulative adoption of behaviors by some individuals, even when their social position, or the resources associated with them, change only minimally or remain unaltered. In diffusion models, the behavior spreads and is adopted by individuals irrespective of their socioeconomic positions, including among those whose social or economic positions are such that a hypothetical cost–benefit calculation would not necessarily favor the new behavior. An individual adopts the new behavior as a result of a reevaluation of choices in light of the behavior of other people, not as a strategic response or accommodation to a realigment of resources associated with the individual's position in the social system. Diffusion models are built on the central idea that individuals transfer partial or total control of their own behavior to others. The implied decision process is at least as complicated as those normally associated with structural explanations.

Diffusion processes do not always involve adoption of new behaviors. In fact, they may include abandonment of a recently adopted behavior or resistance to change. The course of fertility decline in Europe revealed a marked tendency to proceed along, or be halted by, ethnic, linguistic, and religious boundaries. The resulting geographic clustering of fertility levels and patterns has been construed as evidence for the hypothesis that fertility changes were strongly driven by ideational or cultural changes, transmitted by diffusion mechanisms, rather than as supporting a structural explanation of fertility decline. The existence of a clustering of fertility changes along cultural lines could be evidence of diffusion of either a new behavior (such as adoption of contraception along with acceptance of a low-fertility norm) in areas were fertility declined below what would be expected based on existing levels of industrialization and urbanization (structural changes), or of resistance to the new behavior (rejection of birth control and adherence to a high-fertility norm) in areas where fertility remained higher than expected.

Thus there is a contrast between an explanation that infers an expected behavior from a reading of individual socioeconomic positions (the structuralist explanation), and an alternative explanation that infers a pattern of expected behavior from the likely adherence of actors to ethnic, religious, or cultural prescriptions or beliefs shared by others in the same community, including individuals belonging to different social classes or occupying different socioeconomic positions. In the latter case, the likelihood of adherence to prescriptions increases as a function of others' adherence to them (or others' resistance to the novel behavior). Who is included in the group referred to as "others" is a key ingredient of the theory, as is the identification of the exact mechanisms that secure adherence to prescriptions and beliefs.

Both the structuralist and the diffusionist explanations rest on the idea that individuals are decision makers, acting in uncertain environments, sorting through limited information on prices, utilities, constraints, and potential outcomes of alternative behaviors, elucidating their own preferences, and ultimately taking some course of action. Whereas investigators are normally careful to produce a thorough definition of the decision-making process associated with the structuralist explanation, they all too often fail to specify the decision-making process associated with diffusion–to the point that the process appears, in many instances, to consist of passive contagion and the irrational or arational adoption of a behavior. Lack of theoretical specificity leads to accepting diffusion explanations as concession to failure, a fall-back position taken when one cannot confirm a structuralist explanation.

In both sociology and demography, most of the evidence adduced to distinguish between the two contending explanations of behavioral change is derived from aggregate, not individual, data. Since the individual adoption process is not well specified, it is unclear what type of aggregate evidence would be determinative. This leads to the common but methodologically flawed practice of inferring the validity of a diffusion explanation from the failure to support the validity of a structural one. A central problem in sociology and demography, and in economics as well, is the inability to identify key processes from observables.

The Identification Problem or How to Falsify Diffusion Theories with Observables

The only way to convincingly choose between a diffusionist or a structuralist theory is by observing patterns of behavior under conditions that hold the distribution of individuals by social positions and the distribution of resources associated with those positions constant, while allowing variations in conditions that trigger the spread of the behavior (participation in social networks, etc.). If the prevalence of the behavior grows, it cannot possibly be due to structural factors (they are constant) but to diffusion. The difficulty, however, is that at least one of the three mechanisms of diffusion identified above mimics the effects of structural changes: namely, when social positions or resources associated with them change as a result of the process of diffusion itself. Put another way, if we are to identify diffusion effects, the ideal experiment cannot allow the diffusion feedback mechanism to operate and simultaneously maintain invariance in individual characteristics. Thus, even under ideal conditions, it is difficult to sort out precisely how much of the ultimate change in behavior is due to all diffusion mechanisms, and how much to secondary changes in the social structure induced by diffusion itself. In the cases of interest here, including the study of fertility, the conditions are far from ideal; hence it is virtually impossible to make the necessary distinctions. This limitation is irrelevant when the feedback mechanism is weak or if its operation requires long time lags.

Needless to say, there are few ideal experiments. With few exceptions, the evidence marshaled in favor or against diffusion either coarsely identifies the processes of diffusion that the theorist postulates as empirically relevant or, worse, is unspecific, simply referring to what is left over after accounting for measurable conditions associated with individual positions and resources.

A behavior model must be a representation of individuals choosing among a set of alternatives, under a set of constraints. The model must seek to account for the persistence (or abandonment) of a behavior over time. This can be done most efficiently by imagining that individuals may occupy two states, one representing adoption of the target behavior and the other adoption of a different behavior (or refusal to adopt the behavior). Transitions between these two states are a function of the individuals' characteristics associated with social and economic conditions (costs and utilities), external characteristics acting as constraints or facilitators, the influence of external sources of ideas, and the influence of each individual's linkages to social networks. To the extent that individuals' transition rates are dependent on factors affecting the stock of external sources of ideas or their interaction with social networks, a diffusion explanation acquires greater credibility.

Advances in the empirical identification of diffusion as a feasible mechanism that triggers large-scale behavioral transformations depends on the ability of the scientific community (a) to precisely define the empirical processes through which external sources of influence and those associated with social networks alter the willingness, the ability, and/or the readiness of individuals to adopt or resist behavioral change, and (b) to gather empirical information on the mechanisms through which external sources and social networks operate to produce change.

Conclusions: Where Do We Go From Here?

The theoretical divide described above is still a reality in various areas of demography and continues to be a subject of theoretical debate in sociology and economics. Its nature, however, has been redefined and enriched. Testing theories that pose diffusion as a plausible phenomenon no longer rests on dubious arguments about residual explanations without identifying the precise mechanisms involved. Instead, there have been significant theoretical improvements, drawing elements from social network and social learning theories and applying ideas from economics on relations between individual behavior and aggregate properties of a system. These developments offer the prospect of progress toward the objective described in (a) above.

In addition to advances in theory and modeling, there is a need for more and richer information on aggregate and individual patterns of change. Efforts to identify social networks through both large-scale longitudinal surveys and ethnographically based research are promising developments addressing the issue raised in (b) above.

See also: Action Theory in Population Research; Culture and Population; Demographic Transition; Fertility Transition, Socioeconomic Determinants of; Mass Media and Demographic Behavior; Social Networks; Values and Demographic Behavior.


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Alberto Palloni