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Diffusion Processes and Fertility Transition: Introduction

JOHN B.CASTERLINE

By the early decades of the twentieth century, it was apparent that a historic change had occurred in childbearing patterns in Western societies. Whereas in the past women had borne an average of five or more children by the end of their reproductive years, under the newly emerging fertility regime, the norm was roughly two children per woman (Livi-Bacci, 1999). The decline had occurred in the space of a generation or two, and was in no sense a silent revolution. Instead, this was a development noted by social scientists and laymen alike, and widely regarded as a fundamental departure from childbearing patterns in the past (Szreter, 1993). Among social scientists, the decline in fertility was generally viewed as but one element in a larger transformation in the family and its functions, with far-reaching implications for Western societies (Davis, 1945). The fertility decline in the West, largely completed prior to World War II, has been followed in the second half of the twentieth century by comparable fertility declines in Asia, Latin America, and, most recently, Africa (United Nations, 2000). By any criteria, the fertility decline of the late nineteenth and twentieth centuries must be ranked as among the more profound social changes of this era. Accordingly, it has begged for explanation, and social scientists have applied themselves to this task from the 1930s to the present.

Not surprisingly, the earliest efforts to explain the decline in fertility linked it with the other social and economic changes that were themselves

John B.Casterline is senior research associate at The Population Council, New York.



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Diffusion Processes and Fertility Transition: Selected Perspectives 1 Diffusion Processes and Fertility Transition: Introduction JOHN B.CASTERLINE By the early decades of the twentieth century, it was apparent that a historic change had occurred in childbearing patterns in Western societies. Whereas in the past women had borne an average of five or more children by the end of their reproductive years, under the newly emerging fertility regime, the norm was roughly two children per woman (Livi-Bacci, 1999). The decline had occurred in the space of a generation or two, and was in no sense a silent revolution. Instead, this was a development noted by social scientists and laymen alike, and widely regarded as a fundamental departure from childbearing patterns in the past (Szreter, 1993). Among social scientists, the decline in fertility was generally viewed as but one element in a larger transformation in the family and its functions, with far-reaching implications for Western societies (Davis, 1945). The fertility decline in the West, largely completed prior to World War II, has been followed in the second half of the twentieth century by comparable fertility declines in Asia, Latin America, and, most recently, Africa (United Nations, 2000). By any criteria, the fertility decline of the late nineteenth and twentieth centuries must be ranked as among the more profound social changes of this era. Accordingly, it has begged for explanation, and social scientists have applied themselves to this task from the 1930s to the present. Not surprisingly, the earliest efforts to explain the decline in fertility linked it with the other social and economic changes that were themselves John B.Casterline is senior research associate at The Population Council, New York.

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Diffusion Processes and Fertility Transition: Selected Perspectives a major preoccupation of social scientists, particularly the massive economic transformation labeled “industrialization,” and the concomitant shift in settlement patterns toward greater concentration of populations in towns and large urban centers. Industrialization and urbanization, it was argued, resulted in a substantial increase in the costs of rearing children and a decrease in the benefits children conferred on older generations (Thompson, 1929; Davis, 1945; Notestein, 1945, 1953). By mid-century, arguments along these lines held sway as the dominant explanation for fertility decline in the West. Some scholars also emphasized the importance of mortality decline as a precondition (Davis, 1963), and in the emerging field of social history there was an interest in the causal contribution of changing notions about family life (Aries, 1962, 1980; Caldwell, 1982) and declining adherence to long-dominant religious and ethical systems (akin to the secularization argument rearticulated by Lesthaeghe and collaborators in the 1980s [Lesthaeghe, 1983; Lesthaeghe and Surkyn, 1988]). By the time rigorous quantitative research was initiated on the European fertility decline and the emerging declines in Asia and Latin America, the presumption was that variables such as modes of production, urbanization, and levels of schooling, themselves indicators of basic economic and social structural changes that had taken place in these societies, would largely account for the decline in fertility. Hence it came as some surprise when researchers associated with the European Fertility Project at Princeton discovered that the empirical associations between the standard battery of economic and social indicators and fertility decline in fact were rather modest in strength (van de Walle and Knodel, 1967; Knodel and van de Walle, 1979; Watkins, 1986). In hindsight, this may have been a mistaken conclusion, drawn from aggregate-level studies that were incapable of detecting the many linkages at the household level between social and economic change and demographic change (see, e.g., Kertzer and Hogan, 1989). In any case, confronted with these findings from the Princeton project, scholars turned to other explanations to augment, or even to supplant, the dominant theoretical framework in which the primary causal forces underlying fertility decline were mortality decline and the paradigmatic economic and social changes that occurred in Europe in the nineteenth and early twentieth centuries. One set of alternative explanations that came to the fore has usually been collected under the label “diffusion.” As we shall note below, the arguments classified as “diffusion theories” vary somewhat in their emphasis, and particularly in what they regard as the unique causal contribution of diffusion theory. What unites them is an overarching model of social change in which attitudes and behaviors become more prevalent in a population through their spread from some individuals to others,

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Diffusion Processes and Fertility Transition: Selected Perspectives through informal face-to-face social interaction or at a distance through the mass media (Rogers, 1962; Brown, 1981; Valente and Rogers, 1995). Diffusion theory usually stresses the innovative nature of the attitudes or behaviors that spread—the common phrase is “innovation diffusion”— but for most scholars this is not an essential feature of this theoretical perspective. For example, Retherford and Palmore (1983), in one of the more extended reviews of the contribution of diffusion theory to research on fertility, distinguish “discontinuous” from “continuous” innovations, the former being entirely novel introductions into a population and the latter involving modification of something already present in the population (and thus not an innovation in the strictest sense of the term). Instead, what sets diffusion explanations apart from the mainstream theories that were formulated in the early and middle decades of the twentieth century is the assertion that fertility decline is not simply an adaptive response to changes in demographic, economic, and social structures; rather, it reflects the spread of certain key attitudes (e.g., about the costs and benefits of children) and behaviors (e.g., birth control technologies). For most scholars who have argued the case for diffusion theory, the crucial point is that the spread of attitudes and behaviors is not bound tightly to societal structural changes, rather, it has an independent dynamic of its own, and hence can account for a unique portion of the variation in the timing and pace of change (Bongaarts and Watkins, 1996). Some scholars have gone so far as to propose that diffusion theory can substitute for theories that feature economic and social structural changes (Cleland and Wilson, 1987; Watkins, 1991). The more common stance is that the two sets of explanations are complementary, not competing, with diffusion theory adding further independent factors to an enlarged theory of fertility decline (Retherford, 1985; Montgomery and Caster line, 1996). These are Cleland’s (this volume) “pure” and “blended” diffusion models, respectively. Early efforts to apply diffusion theory to fertility change were not submitted as challenges to the dominant social scientific theories of demographic transition; rather, they were directed to the more practical and programmatic goal of accelerating the adoption of contraception (Bogue, 1967; Palmore, 1967; Rogers, 1973). Although not recognized at the time, in hindsight the first articulation of a diffusionist argument that ran counter to, or added a significantly new element to, the demographic transition theory developed by Davis, Notestein, and others, was Coale’s address to the International Union for the Scientific Study of Population (IUSSP) General Conference in Liège (Coale, 1973). In this influential paper, Coale proposes that sustained marital fertility decline has three preconditions, one of which can be viewed as primarily a function of societal structural changes (birth control must be perceived as advanta-

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Diffusion Processes and Fertility Transition: Selected Perspectives geous) and the other two of which can be viewed as somewhat independent of changes in social and economic structures (birth control must be “within the calculus of conscious choice” and couples must have at their disposal the means of avoiding births). In the decade following this paper, scholars increasingly came to perceive that the latter two preconditions could change through diffusion processes, that is, the spread through a population of attitudes and behaviors (Knodel and van de Walle, 1979; Retherford and Palmore, 1983). As Lesthaeghe (this volume) reminds us, a great virtue of Coale’s framework is that it acknowledges the causal contribution of both the diffusion of innovative attitudes and behaviors and changes in economic and social structures (the latter affecting parental calculations about the costs and benefits of children). In Coale’s framework neither set of explanatory factors is neglected, in contrast to much of the literature of the past two decades that has touted the contribution of one set at the expense of the other. Lesthaeghe cogently argues that much needless dispute would disappear if the field returned to Coale’s full framework of three preconditions—each one necessary—for fertility decline. While scholars struggled with the intellectual challenge of isolating the causes of the fertility declines that occurred in Europe in the past and that were under way in developing countries in the present, public policy concerns about rapid population growth in developing countries provided an entirely separate motivation for examining the explanatory power of diffusion theories. If the main obstacle to fertility decline in developing countries was not that couples did not perceive birth control to be in their interests (the argument of Davis’s [1967]) famous dismissal of the potential returns from investments in family planning programs), but rather that birth control was regarded as either unacceptable or infeasible, then programs that attempted to inform couples about birth control, legitimize its practice, and make services and supplies more conveniently available and affordable would have the potential to accelerate the decline in fertility. Beginning in the 1960s, there was an enormous financial investment in “purposive diffusion” of birth control (Retherford and Palmore, 1983) through government and private family planning programs. Whether these programs were having a net impact on the timing and pace of fertility decline, and how substantial that impact was in relation to the financial cost of these programs, were research questions of utmost importance beginning in the 1970s and continuing into the 1990s (Bongaarts et al., 1990). Because these questions have been addressed in numerous other articles and volumes, they are not singled out for separate attention in this volume. However, it is worth noting that one very important issue, not considered at any length in this volume, is the extent to which social diffusion might condition the impact of formal programs,

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Diffusion Processes and Fertility Transition: Selected Perspectives either amplifying or dampening their impact (Montgomery and Casterline, 1998). Once the concept of innovation diffusion gained general currency in the field around 1980 (through, for example, widely read articles by Knodel [1977] and Knodel and van de Walle [1979]), the term “diffusion” appeared with increasing frequency in the literature on demographic change. Initially this reflected the frustration described above with the apparently poor performance of explanatory theories that ignore diffusion dynamics (Cleland and Wilson, 1987; Watkins, 1990; Bongaarts and Watkins, 1996; Kirk, 1996). Increasingly in the 1990s, the concept has been employed in fresh empirical studies that have yielded direct evidence supportive of one or more variants of diffusion theory (Montgomery and Casterline, 1993; Rosero-Bixby and Casterline, 1993; Entwisle et al., 1996; Kohler, 1997; Rutenberg and Watkins, 1997; Munshi and Myaux, 1998; Kohler, 2000). Diffusion explanations have been applied not only to fertility decline but also to mortality change (Montgomery, 2000), and to the experiences of both historical and contemporary populations. The main objective of the January 1998 workshop organized by the National Research Council (NRC) Committee on Population, from which the papers contained in this volume are drawn, was to assess the potential contribution to our understanding of fertility decline of explanations that invoke the concept of diffusion. The twofold question motivating this workshop was, “How might diffusion dynamics affect the timing and pace of fertility change, and how might the magnitude of those effects be ascertained through rigorous empirical research?” This question reflected the committee’s unease with the existing research literature. The committee noted several tendencies in the literature that undermined the contribution of innovation diffusion theory. The first was the tendency to construct arguments that relied heavily, or exclusively, on innovation diffusion theory or, worse, only portions of innovation diffusion theory (as described below). The committee felt the need for a more balanced treatment of diffusion in fertility theory and research, a treatment that allowed full play to the potentially powerful implications of the concept of innovation diffusion while not losing sight of the other determinants that together make up a comprehensive theory of fertility change. A second weakness in the literature was the common failure to employ the concept of diffusion in a manner that would permit rigorous assessment of causal contribution. That is, although it may be descriptively accurate to say that fertility decline “diffuses,” in the sense of reaching societies and subgroups at different times, the same descriptive generalization applies to virtually all social change. Surely if diffusion theory is to make a significant contribution to demography, it must be informative about the determination of the timing and pace of demographic change. It was with

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Diffusion Processes and Fertility Transition: Selected Perspectives these concerns in mind and with the aim of contributing to a more balanced and insightful treatment of diffusion effects on reproductive change that the committee organized the Workshop on Social Processes Underlying Fertility Change in Developing Countries. This introductory chapter begins with a synopsis of the treatment of diffusion in the fertility literature, highlighting the inadequacies in this literature alluded to above. From this emerges a theoretical stance, labeled here “the social effects model,” that combines the causal effects of innovation diffusion and other determinants (such as demographic and socioeconomic factors). Variants of this model are the primary focus of the papers in this volume, although other diffusionist perspectives are also represented here. Key features of the social effects model are reviewed, with reference to the papers in this volume. The Introduction concludes with a discussion of theoretical developments and empirical investigations that promise to advance our understanding of the contribution of innovation diffusion to reproductive change. KEY ELEMENTS IN DIFFUSION THEORY The influential pieces in the literature on fertility decline that invoke the term “diffusion” are, with few exceptions, subscribing to the same theoretical propositions. The crux of innovation diffusion theory is an argument that has two closely linked, yet distinguishable, key elements that correspond to the two terms in the phrase “innovation diffusion”: fertility decline is the consequence of the increased prevalence of attitudes and behaviors that were previously very rare or absent in the population (i.e., they are innovative), and their increased prevalence is the consequence of the spread of these attitudes and behaviors from some segments in the population to others (i.e., a diffusion process). Although it is natural to join these two elements together—and, indeed, absent the other, the explanatory contribution of each element is severely weakened—a frustrating aspect of much of the pertinent research literature is the extent to which one element has been stressed to the neglect of the other. The consequence has been a diversity in theoretical emphasis that, because it has often gone unnoticed, has led to some confusion. We identify three distinct emphases in the research literature on fertility change that have been heavily influenced by innovation diffusion theory and concepts. In some portions of the literature, the emphasis is on the innovation component of “innovation diffusion.” This has yielded two distinct bodies of work: One body of work stresses the innovativeness of the exercise of deliberate fertility control. Modern fertility control as the diffusion

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Diffusion Processes and Fertility Transition: Selected Perspectives of a behavioral innovation (use of certain types of birth control techniques and technology, heretofore unknown or rare) is the main theme of this literature. A second body of work that focuses on the innovation half of the “innovation diffusion” concept stresses the spread of novel ideas. These are the so-called ideational theories of fertility transition. The basic argument is that declines in fertility occur because of the growing strength of certain knowledge, attitudes, and values. One might characterize these two literatures as concerned with the question “What diffuses?” Another set of papers, by focusing on the phenomenon of “diffusion,” draws somewhat different conclusions about the explanatory contribution of innovation diffusion theory: This third body of work focuses on the social dynamics of the spread of innovative information and behaviors, such as birth control practices. The fundamental premise motivating this literature is that changes in the attitudes and behaviors of some individuals can influence the likelihood that other individuals will change their attitudes and/or behaviors. This describes a social dynamic that in the aggregate over time results in a diffusion process, that is, the spread of attitudes and behaviors through the population. In theory, this dynamic can alter both the timing and the pace of fertility decline, and hence is properly classified as a causal factor. One might characterize this body of work as preoccupied with the question “How does diffusion occur?” By no means are these various literatures in contradiction with each other, nor are they even exclusive of each other. There is, in fact, considerable overlap among the three. One can view the differences as primarily, although not entirely, a matter of different weightings of the two key elements of innovation diffusion theory. However, this tendency to slight one or the other key element has seriously limited the contributions of innovation diffusion theory to the explanation of fertility change. This is apparent if each literature is subjected to a critical review. Behavioral Innovation This body of work stresses the innovativeness of the exercise of deliberate fertility control. Fertility decline is the consequence of the spread of innovative birth control techniques and technologies. The spread can be spontaneous or directed.

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Diffusion Processes and Fertility Transition: Selected Perspectives This is the issue that Carlsson (1966) isolates in his seminal piece on fertility decline in nineteenth-century Sweden. At the time of Carlsson’s article, it should be noted, there was already a relatively well-developed literature on the diffusion of new technologies, particularly agricultural technologies (Ryan and Gross, 1943; Beal and Bohlen, 1955; Griliches, 1957). Carlsson sets two arguments in opposition: fertility decline as an adjustment to changed social and economic circumstances that employed already known and accepted behaviors, against fertility decline as the consequence of the widespread adoption of birth control techniques that were for the most part unknown or unacceptable to previous generations. Carlsson concludes that the Swedish evidence on balance refutes the second argument. The most articulate and sustained effort to defend the behavioral innovation argument came from scholars involved in the Princeton European Fertility Project. A cardinal principle in this project was that in pretransition populations, family limitation was an alien concept, and effective techniques to avoid pregnancy were largely unknown. This is implicit in Coale’s (1973) three preconditions for marital fertility decline, one of which is that means of birth control must be known and available. This is the “able” condition in Lesthaeghe’s contribution to this volume. That birth control was innovative behavior in historical European (and selected non-European) populations has been argued persuasively by Knodel and van de Walle, both separately (Knodel, 1977; van de Walle, 1992) and jointly (Knodel and van de Walle, 1979). Watkins (1986) has adhered to this interpretation of the historical European evidence. However, most of the evidence that birth control was a behavioral innovation is indirect. During the past decade, further efforts have been made to uncover more direct evidence. Limited empirical materials from the European past can be brought to bear on this question; however, the gradual accumulation of evidence from literary and other sources raises doubts about the validity of the Princeton position in its simplest and purest versions (Szreter, 1993; Santow, 1995). More direct evidence can be gathered for contemporary non-European populations. In these populations, some researchers see clear evidence in pretransition societies of widespread awareness and acceptance of fertility regulation techniques, if not for limiting family size, then certainly for the purpose of spacing births (Bledsoe et al., 1994; Mason, 1997). By no means is there consensus on this issue, however. Cleland (this volume), updating an earlier review (Cleland and Wilson, 1987), concisely summarizes evidence for maintaining his position that pretransition populations are not familiar with birth control techniques, especially for the purpose of limiting the number of births, and therefore their widespread adoption during fertility transition should be regarded as truly innovative behavior.

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Diffusion Processes and Fertility Transition: Selected Perspectives Much dispute remains, therefore, on the relatively straightforward questions of whether the increasing prevalence of contraception as fertility transition progresses represents the spread through the population of innovative techniques and technologies and whether this spread is the critical catalyst to transition. For our purposes, an equally important question is: What might the answer to this question contribute to our understanding of the underlying determinants of fertility transition? If the answer is affirmative—that practice of birth control is the adoption of innovative behavior— how much has our understanding of the causes of fertility decline been advanced? The gain is far less than the intensity of the debate would imply, in our view, because this literature tends to neglect the related question of how the innovative birth control techniques and technologies reach individuals. The latter is a question about the diffusion process, and must be addressed if the argument is to provide any explanatory leverage. That certain behaviors were once alien and then become commonplace is itself a limited causal insight, more a description of social change than a theory for why and how the change came about. The arguments of most of the scholars named here are not as crude as the previous paragraphs might suggest. The proposition that birth control is innovative behavior typically is coupled with a recognition that childbearing motivations change over the course of transition, and that the reasons why these motivations change is itself a question that must be addressed. Cleland (this volume) proposes that mortality decline is the key factor motivating fertility decline in contemporary developing countries; and Lesthaeghe (this volume) emphasizes that for fertility to decline, all three of Coale’s preconditions must be present, including the perception that restricting fertility is advantageous (the “ready” condition). In short, whether or not the increased prevalence of birth control during the course of fertility transition constitutes behavioral innovation is undeniably a significant question. Were it to be answered in the negative, this would be a serious blow to innovation diffusion theory’s contribution to causal models of fertility transition. Yet by itself this question is insufficient, as becomes clear when we ask what an affirmative answer would contribute to theories of reproductive change. An emphasis on behavioral innovation must be complemented by theory and research on diffusion processes, that is, those processes through which behavioral innovations spread through a population. This is the emphasis of the second body of work reviewed below. Ideational Change A second body of work is closely related to the first because in both literatures the heart of the argument is the nature of the innovation rather

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Diffusion Processes and Fertility Transition: Selected Perspectives than how the diffusion conies about. While the first body of work emphasizes behavioral innovation, the second emphasizes the spread of innovative ideas. Since Cleland and Wilson’s (1987) influential piece, it has been common to speak of ideational theories of fertility transition. The basic argument is that fertility declines because of the presence of certain knowledge, attitudes, and values that either were not present previously or that grow significantly in strength. Although the innovative character of the ideas figures into the argument—and ultimately makes this argument difficult to distinguish from the behavioral innovation argument just discussed—this literature sets itself apart by its determination to contrast ideas from material conditions as possible causes of fertility change. Note that material conditions can include technological innovations, such as methods of contraception. Among the ideas identified in this literature are the notion of family limitation, knowledge/attitudes/values about modern contraception, and ideas about family behavior (the roles of women and children). Falling under this label are major contributions to the literature on fertility transition that differ significantly among themselves in the substance of their arguments. Caldwell’s theoretical pieces on fertility transition written in the late 1970s and collected in his 1982 book (Caldwell, 1982) attribute considerable causal power to the spread of Western ideas about family life, through schools and through the mass media. But the foundation of his theory is an argument that changes in modes of production—material conditions—alter the costs and benefits of children, and in this respect his theory is much larger than the ideational change argument (and, indeed, is on the whole compatible with the conventional demographic transition theory formulated several decades earlier). Similarly, Freedman (1979) suggests that new consumer aspirations, diffused through international networks of communication and transportation, constitute one of the powerful motivations to reduce family size. In doing so he adds content and specificity to a theory that at its core emphasizes the determining power of changes in social and economic conditions. Perhaps the most sophisticated contribution that might be classified in this body of work is Lesthaeghe’s research on fertility change in Europe (Lesthaeghe, 1983; Lesthaeghe and Surkyn, 1988). Lesthaeghe argues that secularization and the emergence of an emphasis on individual autonomy and self-actualization together explain important features of fertility trends and differentials in Europe in the twentieth century. But like Caldwell, Lesthaeghe also attributes considerable causal power to changing economic structures. In contrast, Cleland and Wilson (1987) explicitly reject the notion that fertility decline can be explained by changes in structural conditions. Instead, they argue, the relatively autonomous spread of information and

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Diffusion Processes and Fertility Transition: Selected Perspectives values about fertility regulation has been the primary stimulant of contemporary fertility transitions. Cleland repeats, and updates, this crucial point in his contribution to this volume. After reviewing the accumulated empirical evidence up to the present, he maintains tine position articulated in his widely cited piece with Wilson, namely that “the case that the idea of marital fertility regulation was a true innovation both in Europe and elsewhere remains robust despite widespread skepticism” [our italics]. There are many disturbing aspects of the baldest statements of the ideational change argument. Several of these have already been identified by Mason (1992) and Burch (1996). For one thing, the favoring in ideational explanations of ideas about contraception is arbitrary and unnecessary. There is every reason to give equal or greater weight to ideas that influence the demand for children, namely, ideas about the costs and benefits of children, the roles of women and children, and so forth. The more fundamental problem with this literature, however, is its implicit behavioral model. It is common in this literature to perceive ideas and material conditions as alternative, even competing, causes of fertility change, a definition of the terms of the debate that demands that ideas can be separated from material conditions. Most social science theory does not accommodate such a relationship between ideas and material conditions: Palloni (this volume) makes this point by drawing on mainstream sociological theory, and Carter (this volume) recounts the rejection by anthropology early in the twentieth century of the notion of autonomous cultural diffusion, supplanted by functionalist theory that emphasized the capacity of societies to invent their own idiosyncratic solutions to common human problems. Most social scientists recognize that ideas are grounded in social and economic institutions (see review in Hechter, 1993). This insight is valid at any level of aggregation, from the individual to the society. From this perspective, the disjuncture between material conditions and ideas found in some of the pieces in this body of work is a fiction. This key point is underscored by empirical work carried out in developing countries during the past two decades, some of it inspired by the ideational argument, that plainly reveals that the ideas that bear on fertility and family planning decisions more often than not are ideas about material conditions—changing labor and commodity markets, new economic opportunities, and so forth (Casterline, 1999b). An opposition between material conditions and ideas simply does not fit such empirical evidence, and this should come as no surprise: cultural systems cannot be detached from social and economic systems to the degree that some of this literature presumes. A rejection of the causal contribution of changes in material conditions does not necessarily follow from an emphasis on ideational change.

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Diffusion Processes and Fertility Transition: Selected Perspectives and family planning workers, school teachers); and mass media exposure. Measurement of individuals’ perceptions of the attitudes and behaviors of other persons. Prospective data collection, so that social exposure and perceptions at earlier times can be related to later attitudinal and behavioral transitions. Longitudinal designs, achieved through prospective data collection, are especially critical for obtaining valid estimates of causal effects, such as those specified in equation (1) (Palloni, this volume). In principle, social effects should be considered at all levels and via all mechanisms: personal social networks, local social organizations, influential elites, the mass media, and program personnel (health workers, school teachers, and so forth). In practice, simplification is unavoidable: no one data collection exercise can afford to obtain data that permits estimation of social effects at all these levels. In any case, if the social effects model is specified in full generality, it admits too many possibilities and lacks sufficient structure for these effects to be precisely and confidently identified (Montgomery and Casterline, 1998). Researchers have no recourse but to engage in some simplification, primarily through deletion, in their investigation of social effects. This requires an informed and in-depth understanding of the structure of social relations in the society where the investigation is occurring, as proposed by Carter (this volume). A good rule of thumb is that researchers’ protocols for sampling social networks (informal, formal, and long-distance) should to the extent possible mimic the sampling habits of the actors under investigation (Palloni, this volume). All this seems a daunting undertaking. And yet data collection carried out in the 1970s (as reviewed in Retherford and Palmore, 1983, and Valente, 1995) and during the past decade (Kincaid et al., 1996; Valente et al., 1997; Entwisle and Godley, 1998; Boulay, 2000; Casterline et al., 2000; Kohler et al., 2001) demonstrates that it is feasible to design projects that conform to the principles just enunciated. Despite the recent progress, it remains the case that the more imposing barrier to research on innovation diffusion and reproductive behavior is not the underdevelopment of theory but rather the lack of data that will support rigorous empirical testing of theory already in place. A number of the papers in this volume nicely demonstrate that a rich collection of concepts and theories are awaiting empirical investigation (Cleland, Palloni, Carter, Carley).

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Diffusion Processes and Fertility Transition: Selected Perspectives THE PAPERS IN THIS VOLUME Despite an interest in innovation diffusion theory among demographers that extends back at least to the 1960s, and the publication more than a decade ago of widely read pieces that argued vigorously that research on the determinants of fertility change should give far more attention to diffusion dynamics (Knodel and van de Walle, 1979; Watkins, 1986; Cleland and Wilson, 1987), research on the contribution of diffusion remains undeveloped. One reason for this, just noted, is the scarcity of data that will support the estimation of the models implied by innovation diffusion theory, including the basic model we have termed the “social effects model.” A more fundamental explanation for this state of the field, however, is that many of the efforts to date have employed incomplete or imbalanced conceptualizations of diffusion effects. The aim of this collection of papers is to fill in some of the existing gaps and achieve a better balance than has characterized the literature to date. A deliberate effort has been made to represent the various social science disciplines that have given systematic attention to diffusion processes (either recently or in the past)—sociology (Palloni), anthropology (Carter), social and cognitive psychology (Carley), and communication sciences (Hornik and McAnany). (Economics is the major oversight; an economic analysis was presented at the 1998 workshop [Durlauf and Walker, this volume].) As indicated above, the existing literature on fertility transition that was influenced by innovation diffusion theory tends to focus either on innovation—What are the innovative attitudes or behaviors that diffuse?—or on diffusion—By what process do attitudes and behaviors spread through the population? The latter has been given far less attention than the former, and hence this is the emphasis of the majority of the papers in this volume. Palloni reviews the evolution of theory and models of diffusion in sociology, and then presents and critiques a more complicated version of the social effects model of this introduction. Carley provides a concise yet comprehensive overview of research findings from social and cognitive psychology that speak to the general question of how individuals learn from, or are influenced by, other persons. All models implicitly, if not explicitly, make assumptions about the nature of interpersonal learning and influence. For social effects models of fertility change to become more powerful and precise, they must be informed by the behavioral research that Carley summarizes. Hornik and McAnany tackle the important problem of social effects through the mass media (with particular reference to effects on reproductive behavior). It is clear that in the contemporary world, this is an important channel for innovation diffusion, and that to ignore this channel is to run the risk of obtain-

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Diffusion Processes and Fertility Transition: Selected Perspectives ing a biased impression of the impact of diffusion dynamics on reproductive decision making. In the three other papers in this volume, far more attention is given to the issue of the content of diffusion processes—What are the innovative attitudes and behaviors that diffuse, and to what extent does this explain fertility change? Cleland reviews the fertility literature of the past four decades, for both historical Europe and the contemporary developing countries. From this he concludes that the diffusion of knowledge, acceptance, and technologies of birth control provides a parsimonious and compelling explanation for the onset of fertility decline in historical Europe and, while not as decisive for declines outside Europe in the second half of the twentieth century, it nevertheless stands as one of the primary underlying causal forces. Lesthaeghe revisits Coale’s (1973) three preconditions of sustained marital fertility decline, which as noted earlier was an early theoretical statement that can be viewed as (implicitly) arguing for a central causal role for innovation diffusion. Lesthaeghe argues, illustrating his point through analysis of recent Demographic and Health Surveys data, that in positing a causal role for innovation diffusion one need not deny the central causal contributions of changes in the demand for children, itself a response to societal structural changes (demographic, social, economic). Finally, Carter observes that anthropology early in the twentieth century accorded substantial causal power to cultural diffusion, only to conclude that this was an inadequate explanation for much of the meaningful variation among societies. Carter’s chapter serves as a caution against excessive enthusiasm for innovation diffusion theory. As noted earlier, the research literature on fertility transition contains examples of such excess enthusiasm. One of the conclusions that it is hoped the reader will take away from this collection of papers is that innovation diffusion is but one component in a more elaborate causal process that also involves factors such as mortality decline and economic transformation, and that the most revealing models will take due account of all these causal forces. REFERENCES Adongo, P.B., J.F.Phillips, B.Kajihara, C.Fayorsey, C.Debpuur, and F.N.Binka 1997 Cultural factors constraining the introduction of family planning among the Kassena-Nankana of Northern Ghana. Social Science and Medicine 45(12):1789– 1804. Akerlof, G.A. 1997 Social distance and social decisions. Econometrica 65(5):1005–1027. Aries, P. 1962 Centuries of Childhood: A Social History of Family Life. New York: Knopf.

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