HOMEOSTASIS

In the field of demography, the term homeostasis refers to the way in which populations control their long-term growth. Drawing on concepts of self-regulating systems in biology, demographic homeostasis usually is regarded as a phenomenon in which interrelated social, cultural, and economic institutions ensure that the long-term rate of population growth is close to zero. During the demographic transition of the last 200 years, there has been a huge expansion in the human population. In contrast, the centuries that preceded the transition were characterized by much more modest population growth. Thus, the study of homeostasis focuses primarily on the pre-transitional era. However, with the end of the demographic transition in sight during the twenty-first century, it is likely that homeostasis will become a topic of research interest in the future.

Terminology

For almost two centuries those trying to explain the nature of equilibrating mechanisms in populations have looked to the economist T. R. Malthus (1766–1834) for inspiration. Malthus proposed that competition between individuals is the basis of the equilibrating mechanism and that population balance can be arrived at through two possible means: the positive check of worsening mortality and the preventive check of reduced fertility.

The first edition of Malthus's Essay (1798), written in the style of a polemical pamphlet, with a limited exposition of theory and little supporting evidence, expressed this contrast with particular starkness and emphasized the positive check of mortality. Later, better-documented editions tended to stress the role of the preventive check through the mechanism of delayed marriage. In either case the model remained austere, with total population size being the key regulating element. Thus, the Malthusian model often is referred to as being based on density dependence. Because it includes mechanisms by which countervailing forces work to keep a population in balance with its resource base, it sometimes is characterized as showing negative feedback. Both density dependence and negative feedback are features of population control systems that can be described as homeostatic. In fact, these terms are used so interchangeably in the context of population dynamics that they can be regarded as near synonyms.

There are two forms of evidence that suggest the existence of homeostatic mechanisms in human populations: the fact that observed growth has been near zero over most of human history and detailed assessments of the observed strength of homeostatic forces.

Slow Growth

Although the nineteenth and twentieth centuries saw a remarkable growth in human numbers (especially since around 1950), population growth over the greater part of human history must have been very close to zero. In light of the long duration of human prehistory, growth must have been close to zero during the Paleolithic era. Even the expansion of human numbers that took place with the development of agriculture, starting about 10,000 years ago, involved only marginally higher growth rates. As Joel E. Cohen puts it, "Before the several local inventions of agriculture, local human populations grew at long-term rates just above zero. Where agriculture was invented, local human populations grew ever so slightly faster" (Cohen, p. 32). However, events that occurred during this period cannot be quantitatively documented.

Although it most likely never will be possible to make reliable estimates of world population size before the last 200 years or so, there are some populations in which the ratio of facts to deductive reasoning is sufficiently favorable to make tolerably robust estimates of population growth for the last 2,000 years. The best documented of these cases are Europe and China, both of which show periods of growth, decline, and stagnation at various times during the last two millennia. Until the relatively recent past, however, even episodes of growth took place at modest rates compared with modern experience, and the growth rate over the very long run was extremely low. For example, the average annual growth rate in China from 2 to 1500 c.e. was in the neighborhood of 0.03 percent.

That long-term stability partly masks shorter-term fluctuations. In addition to explaining the factors leading to very slow growth, models of population change must consider the fluctuations. However, it is not easy to distinguish the cultural, economic, political, and environmental causes of such swings. Analyses of fluctuations in both European and Chinese history now generally emphasize nondemographic factors such as the degree of political stability and climatic change. In many cases periods of growth appear to be associated with eras of political stability, whereas times of generalized social unrest saw population declines. Possibly the most sophisticated analysis of this phenomenon is found in C. Y. Cyrus Chu and Ronald D. Lee (1994), who used data for China. Similarly, in Europe the demise of the Western Roman Empire appears to have been associated with a long-term decline in population size.

In contrast to the slow growth of settled populations, the potential for rapid growth clearly exists for all populations but seems to be realized only in some circumstances. In particular, rapid growth in pretransitional populations appears to be restricted to places and eras where a population was able to settle new territory that previously was either not settled or thinly settled. Such expansions often were associated with the spread of alternative forms of farming or other technological innovations that allowed more effective exploitation of natural resources. A variation on this situation occurred in areas where catastrophic mortality from diseases introduced by new settlers devastated the indigenous population, effectively rendering the land thinly settled (Crosby 1986).

Tests of Homeostasis

Malthusian ideas and terminology are so widely employed in describing both long-and short-term population movements that it is common to find them used routinely in both historical and contemporary discussions of economic-demographic relations. However, the strength of these simple density-dependent mechanisms rarely has been tested for human populations. With European historical data, researchers are in a position to do this. Using European data, Lee (1987) has concluded that only a very small proportion (0.5 percent) of year-to-year changes in settled agricultural populations can be attributed to simple density-dependent mechanisms.

On a short-term basis this means that homeostasis is overwhelmed by a multitude of other factors. However, as Lee goes on to note, "It is essential to realise that as long as there is any trace at all of density dependence, no matter how weak, this tug, by its systematic persistence, comes to dominate human population dynamics over the long run, if not the short" (Lee 1987, p. 452). Thus, homeostatic mechanisms can be seen to operate in a long-term perspective, but they do this through such complex and indirect pathways that their effects are difficult to detect.

Socioeconomic Mechanisms

The potential for population growth must have been kept in check by powerful forces at most times. One potential force is, of course, high mortality and there is certainly evidence of spectacular mortality crises in the historical record. The return of bubonic plague to Europe in the middle of the fourteenth century, for example, is thought to have killed around a third of that continent's population. Even more dramatically, the arrival of Europeans in the Americas, Australia, and the Pacific islands led to the demographic collapse of indigenous populations. Such disasters may be attributed largely to the introduction of new diseases and the occurrence of so-called virgin-soil epidemics (Crosby 1986). However, they also may indicate the effects of the collapse of one form of social order and its replacement with another and even deliberate genocide.

In spite of the dramatic nature of such crises and their undoubted importance in the historical demography of the Americas, Australia, and the Pacific islands, it is not clear how significant events of this kind have been in regard to long-term demographic change in general. Historical evidence from Europe and Asia mostly indicates less extreme mortality and much less extreme fluctuations in population size, suggesting that control over fertility and opportunities for migration were more significant than was catastrophic mortality (Liu et al. 2001, Livi-Bacci 1993). The most persuasive interpretation of the data on long-term population growth, therefore, would seem to be that human societies developed regulatory mechanisms that kept long-run population growth rates close to zero.

The best-documented examples of these mechanisms come from Western Europe and East Asia. Although there was considerable local and regional variation in the nature of pretransitional demo-graphic regimes, the overall range of experience seems broadly similar in the Asian and European populations that have been studied to date. Total fertility generally was in the range of four to six children (i.e., well below any theoretical maximum), whereas life expectancy at birth ranged between 20 and 40 years.

Although the overall ranges of mortality and fertility in Asian and European populations had much in common, that cannot be said for their social, economic, and cultural contexts. Different patterns of social behavior, as well as differences in ecological factors, disease environments, and political stability, shaped the demographic equilibriums, and both fertility and mortality were subject to different sets of determinants. For example, Asian marriage patterns generally bore no relation to the so-called West European model, in which both men and women married late and many people remained single. Similarly, deliberate infanticide, which was almost unknown in Europe, played a visible role in some parts of Asia. Patterns of sex-specific mortality also differed markedly between Western Europe and parts of Asia.

In all cases, however, it is clear that extensive systems of demographic and economic interaction and elaborate social conventions shaped the process of reproduction. Moreover, in addition to social norms, individuals and couples thought about and took action that affected their childbearing. This seems to be especially evident in China, where there is a long history of collective intervention in demographic matters of various kinds, including fertility. Thus, Zhongwei Zhao notes, "As early as the Tang (618 to 907 AD) and Song (960 to 1279) periods, ideas and practice of limiting family size were already discussed and recorded in the works of some scholars" (Zhao, p. 214).

Origins

Consideration of these matters inevitably raises the question of how the institutions that underpin homeostasis arise and are sustained. Within demography there are two main schools of thought. One, which is found in the work of E. A. Wrigley (1978), stresses the role of unconscious rationality in generating homeostatic patterns. In this way of thinking, just as the "invisible hand" of classical economics guides markets even when individuals are unaware of its presence, analogous forces steer social arrangements toward homeostatic equilibriums. The second school, most succinctly presented by Ron Lesthaeghe (1980), gives greater emphasis to conscious "short-term goal setting" on the part of the elite groups that set the prevailing moral guidelines and controlled the economic bases of society. According to this line of reasoning, these institutions are best seen as instruments of social control, and the equilibrating mechanisms also are viewed as methods of social differentiation.

An approach to the question of origins that links individuals and populations and has been influential in anthropology in recent years is the Darwinian evolutionary perspective, as seen in the work of Laura Betzig and her colleagues (1988). This approach draws on modern evolutionary ideas of "inclusive fitness" and "kin-selection" to investigate the role of evolutionary forces in determining human fertility patterns. A problem with a Darwinian approach is the manifestly Lamarckian nature of much cultural transmission, in which acquired characteristics are transmitted just as easily as inherited characteristics are. An unanswered question is how social institutions that favored low growth or homeostasis could have evolved in the absence of "group selection." As James W. Wood states, "It is very unlikely that special behavioral and institutional mechanisms have evolved in order to restrain population growth or regulate population size. But that does not mean that factors do not exist that have that effect, even if it is not the reason for their existence"(Wood, p. 101).

Alternatives

In contrast to Malthus's ideas of negative density dependence, Ester Boserup (1965, 1981) proposed that increasing population density stimulated technological progress in pre-transitional populations, leading to positive density dependence. Her ideas have been synthesized with those of Malthus by Lee (1986), who shows that both can be accommodated within a wider model of the interaction between population and economic growth. Lee's ideas have been developed by Wood (1998), who generalizes the "Malthus and Boserup model" to deal with a broad definition of well-being and explicitly considers the question of variance and population heterogeneity.

See also: Demographic Transition, Evolutionary Demography; World Population Growth.

bibliography

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——. 1981. Population and Technological Change: A Study of Long-Term Trends. Chicago: University of Chicago Press.

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Chris Wilson