Dynamics of Human Reproduction

Biology, Biometry, Demography

James W. Wood

• Publisher: Routledge
• Year: 2017
• ISBN:
  9781351521468
• Edition: 1
• Topic: Medicine
• Subtopic: Gynecology, Obstetrics & Midwifery

Awarded the W. W. Howells Award for the Outstanding Book in Biological Anthropology, this volume presents a comprehensive, integrated, and up-to-date overview of the major physiological and behavioral factors affecting human reproduction. In attempting to identify the most important causes of variation in fertility within and among human populations, Wood summarizes data from a wide range of societies. Trained as an anthropologist as well as a demographer, he devotes special attention to so-called ""natural fertility"" populations, in which modern contraceptives and induced abortion are not used to limit reproductive output. Such an emphasis enables him to study the interaction of biology and behavior with particular clarity.The volume weaves together the physiological, demographic, and biometric approaches to human fertility in a way that will encourage future interdisciplinary research. Instead of offering a general overview, the focus is to answer one question: Why does fertility and the number of live births vary from couple to couple within any particular population, and from population to population across the human species as a whole?Topics covered include ovarian function, conception and pregnancy, intrauterine mortality, reproductive maturation and senescence, coital frequency and the waiting time to conception, marriage patterns and the initiation of reproduction, the fertility-reducing effects of breastfeeding, the impact of maternal nutrition on reproduction, and reproductive seasonality. This unique combination of comprehensive subject matter and an integrated analytical approach makes the book ideally suited both as a graduate-level textbook and as a reference work.

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PART 1

Natural Fertility

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1

Introduction

This book is about the basic mechanisms underlying observed patterns of human reproduction. A single question is addressed throughout: Why do individuals (or couples) have one child, two children, eight children, or whatever, over the course of their lives? In other words, this book asks how the levels of fertility observed in human populations are achieved. Fertility is defined by demographers as the production of a live birth, that is, a child born alive (Pressat, 1985). As such, it is to be distinguished from fecundity, which is defined as the biological capacity to reproduce. Fertility has the advantage of being marked by a more or less unambiguous, countable event, a birth. This fact has made it the focus of study by demographers for many decades. Fecundity, in contrast, is a theoretical potential and therefore inherently more difficult to measure. Clearly, however, fecundity is crucial to fertility: fecundity interacts with various behavioral processes—at the very least, the frequency and timing of sexual intercourse, and in some cases, deliberate family planning—to determine the level of fertility. Unfortunately, the inherent “fuzziness” of the concept of fecundity has made it inaccessible to empirical analysis. One aim of this book is to decompose fecundity into its major physiological components, which are observable, at least in principle.

The primary focus of the book is fertility variation and its causes. Variation will be considered along three dimensions: within individuals over the reproductive life course, among individuals within the same population, and among different human populations. Chapter 2 presents some of the basic empirical findings about the extent of such variation; the rest of the book is devoted to accounting for those findings. Chapter 3 argues that the explanatory approach most appropriate for this task should satisfy two requirements: (1) it should be framed in terms of a small set of basic and universal mechanisms known in demography as the proximate determinants of fertility (Bongaarts, 1978) and (2) it should focus upon the timing of reproductive events across the individual life course. Temporal aspects of reproduction are of fundamental significance from several points of view. From a purely biological perspective, individual fitness is the summation of reproductive events over the life course as a whole, discounted for any prereproductive deaths among the resulting offspring. Thus, to understand how the human reproductive process evolved, we need to know how it operates in real time. Second, for the demographer, the most powerful models for investigating the effects of fertility on the behavior of populations, so-called stable population models, are themselves dynamic (Coale, 1972; Keyfitz, 1985). For investigators interested primarily in informed policy formulation, the timing of births forms the fundamental framework within which couples make decisions about future reproduction and family planning.

Paradoxically, the reproductive patterns characteristic of contemporary industrialized societies, about which we know a great deal, tell us remarkably little about how the reproductive system functions in time. This is true for a simple reason: the pace of childbearing in such societies is overwhelmingly dominated by modern contraception and induced abortion. For illustration, we can look to the contemporary United States. The average number of live births experienced by U.S. women during their reproductive lives has declined in recent years to about 1.9 (Mosher and Pratt, 1990b). Since this number is below the absolute minimum (2+) needed to replace each couple and thus maintain total population size, it cannot be a long-term characteristic of the human species as a whole. In fact, it is a quite recent achievement reflecting widespread and highly efficient use of modern contraceptives and induced abortion. And the trend toward lower fertility is expected to continue: approximately 80 percent of women in the United States today expect to have three or fewer children during their lifetimes, and almost 10 percent plan to have no children at all (Pratt et al., 1984).

Not only has the total number of offspring declined among U.S. women, there has been a dramatic shift in the timing of reproduction across the female reproductive span.¹ Increasingly, reproduction is being concentrated within a narrow “window,” reflecting an increase in the age at which childbearing begins and a simultaneous decrease in the age at which childbearing ends. Between 1970 and 1982, the percentage of first births occurring in women 25 or older nearly doubled, increasing from 19 percent to 36 percent (Baldwin and Nord, 1984). Consistent with that change, the mean age at first birth for U.S. women born in 1940 was 24.7 years; for women born in 1956, it is projected to be between 27.1 and 27.5 (Evans, 1986). The reproductive window has been closing from the other end as well: births to women 35 and above declined from 11 percent of all births in 1950 to 5 percent in 1980 (Baldwin and Nord, 1984). This change reflects the fact that U.S. women have become remarkably adept at terminating reproduction. At present, approximately 23 percent of all married women of childbearing age in the United States have been surgically sterilized, most of them through tubal ligation (Mosher and Pratt, 1990a).

The pattern of reproduction now typical of the United States and other industrialized nations stands in marked contrast to that observed in much of the rural developing world, especially where effective family planning programs have not been implemented. For example, rural women in the Indian state of Punjab, studied by Harvard epidemiologists from 1953 to 1960, could expect to have six or seven children over their reproductive spans, if they were fortunate enough to survive to menopause (Wyon and Gordon, 1971). The average age at which these women experienced their first births was approximately 17 years; thereafter they could expect subsequent births to follow at roughly 32-month intervals up to about age 40 or so. Fewer than 2 percent of Punjabi women attained age 45 without having had at least one child (Wyon and Gordon, 1971).

As subsequent chapters will show, not all women in all preindustrial societies have had reproductive patterns identical to those observed in the Punjab study. Nonetheless, rural Punjabi women are far more typical of the patterns that must have prevailed over vast stretches of human history and prehistory than are contemporary U.S. women. We stand to learn more about how the human reproductive system is “designed” to function in time by studying Punjabi women—and women in other preindustrial societies—than by studying modern American women. Consequently, this book will have little more to say about reproduction in populations where modern methods of birth control are widely available, except when data from such populations shed light upon the reproductive process as it operates in preindustrial settings.

The focus on preindustrial societies can be justified on several grounds. Generalizations about the level of fertility in such societies have played an important role in the development of theoretical models in demography and anthropology, and more recently in reproductive biology. In classic demographic transition theory, for example, it was often assumed that “pre-transition” societies were characterized by uniformly high fertility rates, which provided the starting point for the recent secular decline in fertility (Coale, 1986). Several ecological anthropologists, in contrast, have suggested that preindustrial societies, especially unacculturated hunter-gatherers, tend to regulate their reproductive output at a relatively low level (Birdsell, 1968; Dumond, 1975; Harris and Ross, 1987; Cohen, 1989). It has even been claimed that an earlier, “stone-age” demographic transition toward higher birth and death rates was associated with the emergence of agriculture and settled village life during the early Neolithic period (Handwerker, 1983; Roth, 1985). Recently, several reproductive physiologists have suggested that birth-spacing patterns in preindustrial societies may shed light on the biology of reproduction in the human species as a whole (Short, 1976; Howie and McNeilly, 1982), a prospect first raised by Louis Henry 40 years ago (Henry, 1953a). Although all these theoretical concerns are relevant to the present book, this last theme will be explored in the greatest detail.

Natural Fertility

Since the remainder of this book will be concerned primarily with what demographers call natural fertility, it is important to examine this concept in detail at the outset. Although the phrase “natural fertility” was used by Raymond Pearl at least as early as 1939, its technical definition was supplied by Louis Henry (1953b, 1961). According to Henry, natural fertility is

fertility which exists or has existed in the absence of deliberate birth control. The adjective “natural” is admittedly not ideal but we prefer it to “physiological” since the factors affecting natural fertility are not solely physiological. Social factors may also play a part—sexual taboos, for example, during lactation. Some of these factors may result in a reduction of fertility but this cannot be considered a form of birth control. Control may be said to exist when the behavior of the couple is bound to the number of children already born and is modified when this number reaches the maximum which the couple does not want to exceed. It is not the case [that control exists] for a taboo concerning lactation, which is independent of the number of children already born (Henry, 1961:81; emphasis added).

Several aspects of this definition are worthy of note. First, natural fertility implies the absence of deliberate control. Individuals and couples do any number of things that limit their reproductive output without intending to do so, and such behaviors are not inconsistent with natural fertility. Control must also be parity-dependent—that is, it must be “bound to the number of children already born.” (In demography, parity refers to the number of live-born offspring an individual has produced by a given time, usually the time of interview.) Thus, for control to exist, couples must have a prior notion of how many children represent a desirable family size, and they must modify their reproductive behavior as they approach or attain that number. For demographers, the parity dependence of fertility control is its salient characteristic.

As Henry emphasizes, natural fertility is not determined solely by physiological factors. Behavior, customary practices, and social institutions still have an influence on natural fertility—they simply do not act in parity-dependent fashion, nor are they generally intended to limit fertility. To illustrate this point, Henry uses the example of what is now called postpartum abstinence, a cultural proscription against sexual intercourse before the most recently bom child has been weaned. Rules of postpartum abstinence are quite widespread among natural-fertility populations, especially in parts of West Africa (Lesthaeghe et al., 1981). (How often these rules are obeyed is a separate issue.) Although such rules undoubtedly act to limit fertility in at least some settings, two considerations suggest that their presence is generally consistent with natural fertility. First, they do not seem to be practiced with the intention of limiting fertility; rather, the preponderance of ethnographic evidence suggests that they are practiced to safeguard the health...