Introduction

This chapter describes a formal demographic perspective on human population dynamics. It first attempts to summarise the way in which human population dynamics are treated in the more technical and theoretical demographic literature. The next section considers some demographic fundamentals, including population structure (especially the age and sex composition) and the three components of population change: fertility, mortality and migration. The third part looks at some of the formal models which demographers have developed to help understand population change. These models make several assumptions in order to simplify a complex reality. One of these is that migration is zero: populations with zero migration are said to be closed. An attraction of this is that, if migration can be ignored, simple relationships exist between fertility, mortality, the population growth rate and the age structure.

In the fourth section population dynamics in the short to medium term are considered. The age and sex structure of a population is itself a dynamic feature, containing a record of the population's past fertility, mortality and migration. Moreover, the future age–sex structure is determined by past and current events. Discussion of these aspects of population dynamics leads naturally in the fifth section to a consideration of population momentum, or what is sometimes, inaccurately, called the ‘demographic time bomb’. The origins of population momentum are explained.

Finally, in the sixth and seventh sections long-run population dynamics are explored in the context of the demographic transitions in Europe and contemporary Africa.

Evolutionary biology became gradually transformed through the twentieth century from what was termed ‘typological thinking’ to ‘population thinking’. This happened from the increasing recognition that differences between individuals were as important as similarities. Variation contains the raw material of evolution and variation can exist only in populations. The transformation harmonised the relationship of evolutionary biology with other population based biologies: genetics, ecology and epidemiology, for example. More significantly, however, in humans it also established potential connections with some of the social sciences and especially sociology, social anthropology, human geography and social psychology, all of which are fundamentally population based areas of knowledge.

Population can thus be seen as a bridge between the ‘two cultures’ of natural sciences and the humanities. This was most effectively recognised by J.W.S. Pringle at Oxford, who was instrumental in establishing there the field of Human Sciences, essentially based on the analysis of human population structures from all perspectives. In the past twenty-five years the field has developed dramatically.

The concept of population is not, however, one without difficulties. Even the definition of a population can be a major problem. Human groups rarely exist as discrete, more or less uniform entities even at any one time, and they have never existed over time. Typically they intergrade, often very gradually. Then while one most frequently thinks of populations in some spatial context, they also exist in vertical dimensions such as social class and caste, and in ecological and economic terms reflecting natural environmental heterogeneity.

Introduction

The complex processes of human population dynamics work themselves out, not in an abstract limbo but, as for other species, in particular environmental settings. The specific physical and biotic characteristics of such settings may crucially affect these dynamics and vice versa. From their local environments, and/or from any others to which personal travel and exchange systems allow access, people must acquire the material resources, such as food and water, necessary for individual and population survival. Complicating factors peculiar to Homo sapiens have not abolished this fundamental dependence on our environments; but they have both changed the nature and time-scale of that dependence, and made its reality harder to see.

One of these complications, with a major effect on human population–environment relations, is our capacity to reach beyond the local environment to obtain resources. In a now globalised economy, access to other environments' resources is scarcely impeded by geographical barriers or distance, though very much impeded by poverty. Even in earlier times and more straitened circumstances, flows of material necessities have commonly extended well beyond the settings local to particular communities. The potential implications for population dynamics are huge.

Other complications apply in human societies too, even within local settings. People view landscapes and resources through culturally informed spectacles. They imbue them with symbolic meanings, religious beliefs, emotional attachments, proprietorial claims, and senses of individual and collective identity. They develop and communicate sophisticated understandings of geography, seasonality and human impacts.

Humans can be studied from a variety of academic perspectives. In some biological disciplines the focus may be on parts of individuals, perhaps even molecular parts, while in others humans are considered zoologically as an entire species, Homo sapiens, in the order Primatii. Some biologists are concerned with the physiologically functioning body of an individual, while others are interested in world-wide human diversity, distribution and population groupings. Among the social sciences, the focus is also frequently on the group, but here the emphasis tends to be upon the social and cultural factors which underpin the way such phenomena as societies, communities and ethnic groups are constructed, delimited and defined. There are also the behavioural sciences, which utilise both biological and social information in relation to the study of the group, the individual or elements in the behaviour of the individual. Recognition of the number of perspectives on these population variables emphasises the need to study material across the boundaries of traditional academic disciplines and several of these perspectives are introduced in this volume.

The twentieth century began with observers of the human condition integrating, in what we now recognise to be a confused way, their ideas on the biological and social nature of humans. During the century the fragmentation of all disciplines grew, something exemplified most particularly in the divergence between biological and social pursuits of knowledge.

Introduction

The beginning of the twenty-first century is a most opportune time to look back fifty years to see what has happened to the spatial patterns of world population distribution, and to try to look forward over a similar period to see what is likely to happen, especially as we are in the midst of an almost inexorable process of massive concentration of world population. That concentration must be examined within the context of other enormous demographic changes that have taken place since 1950 and are likely to take place before 2050. Naturally, the reliability of the retrospective analysis is greater than that of the prospective analysis, although I am relieved to note that most of my speculations and predictions made in comparable papers about three decades ago (Clarke 1971, 1972) have not been far from the mark. As Hinde (Chapter 2) has mentioned, the perspectives of demographers and geographers overlap, but the latter pay great attention to spatial distribution and migration.

Rapid world population growth

The mid-twentieth century was a demographic threshold when world population growth accelerated. At that time, the world population was just over 2.5 billion; according to the United Nations Population Division it reached 6 billion in October 1999. It had taken roughly a century and a half for world population to multiply two and a half times from 1 billion to 2.5 billion, but in the second half of the twentieth century the world population grew so rapidly that 3.5 billion were added to the world total, one billion being added during the last 12 years (Table 3.1).

The Indian subcontinent, which contains nearly one-sixth of the world's population, has received migrants from many sources during its history. Its population is consequently diverse, both culturally and genetically (Nei and Roychoudhury 1982; Majumder and Mukherjee 1993; Majumder 1998). The patterns of this diversity can offer important clues to evolutionary history. In addition, unique sociocultural phenomena in India have influenced mating patterns for thousands of years. It is expected that these patterns have in turn affected the genetic structure of the population. For these reasons, the Indian population is an appealing subject for population genetic analysis. This chapter, while concentrating on some populations in southern India, shows how modern genetic techniques can play an important part in our understanding of human population dynamics.

Three major waves of immigration have influenced the genetic structure of India. Palaeolithic immigrants are thought to have originated from Africa, or possibly Australia, and may have contributed to the gene pool of present-day tribal populations (Maloney 1974; Chandler 1988; Majumder and Mukherjee 1993; Cavalli-Sforza et al. 1994). A second major wave of proto-Dravidian-speaking immigrants came from the Fertile Crescent area c. 10,000 years ago and populated most of the subcontinent (Cavalli-Sforza et al. 1994). The third major wave, emanating from west-central Asia about 3,500 years ago, consisted of Indo-European-speaking ‘Caucasoid’ populations. This immigration event is marked archaeologically by the appearance of Painted Grey Ware (Thapar 1980).

Introduction

This chapter compares causes and consequences of population growth in the Lua' and the Hmong highland minority groups in Southeast Asia. The study illustrates roles of cultural patterns in, and impacts of, modernisation on population dynamics. It exemplifies many of the processes introduced in earlier chapters in relation to demography, change in social and health conditions, and population genetics. It relates changes in mortality, fertility and migration, and in geographic and social boundaries, to socioeconomic modernisation, including public health programmes, education, development and the market economy. This condensation of more detailed papers by Kunstadter and colleagues (see References section) suggests the value of holistic and longitudinal approaches.

Data were collected over four decades by conventional anthropological participant observation, health examinations, unstructured and structured interviews, surveys and questionnaires, some modelled on those developed for international comparative studies, such as the Demographic and Health Survey, with appropriate modifications based on discussions with members of the populations. Native speakers conducted most surveys.

Determinants of primary population variables

In Lua' and Hmong populations almost all fertility is confined to marriage. Thus age at marriage, proportion who marry, and divorce rates are important determinants of fertility, as are use of deliberate methods to delay, space or end pregnancies (Davis and Blake 1956).

Important biological determinants of fertility include age of reproductive maturity and senescence. These are influenced by nutritional status and infections, especially sexually transmitted diseases. Breastfeeding suppresses ovulation and protects infants from infectious diseases.

The extent of human biological variation

Humans are biological organisms. After chapters on demographic and socioeconomic perspectives on human populations and their spatial distribution, it is now appropriate to focus on contemporary ways in which world-wide human biological diversity and dynamics can be studied through human population genetics in its development in the era of the Human Genome Project.

The description of human differences is ancient. There are references in antiquity to humans with different physical features, such as those described by Herodotus, but the confusion between biological features and those we would today consider cultural was common until the twentieth century and still persists in some material. Over the course of the last century, there have been continuous debates about human diversity, but these debates were dominated by descriptions of morphological characteristics, for example pigmentation, and body shape and size. These descriptions in turn gave rise to ‘racial’ classifications, which remain highly significant socially. Even with the increase in understanding of genetic processes, much of this body of work has done nothing to further the understanding of human diversity, because in most cases the analyses did not take into consideration two basic issues.

The first issue is the evolutionary perspective on a given pattern of diversity. Following a tradition in the description of nature itself, the description of human populations continued to take, even recently, a naturalistic perspective, more in accordance with a pre-Darwinian view of detailed description than with its meaning in biological terms.

It is clear that there are many perspectives on human populations and that these are studied within different disciplines, whose terminology and literature are becoming increasingly specialised. Furthermore, the features and conditions of the populations are always changing and the processes of this are viewed and reported upon from different angles. What is required, therefore, for a complete understanding of the complex dynamics of human populations is an appreciation of the work of specialists in different disciplines, something which this volume provides. It is anticipated that the book will be useful to all who consider human populations in their research or their studies, and particularly those who are undertaking degree courses in human sciences, anthropology, demography, human geography, ecology and human biology. The book will also have a wider appeal for the more general reader who wishes to advance their understanding of human population dynamics by adopting a wide-ranging disciplinary focus.

This volume arises from the fifteenth workshop of the Biosocial Society, which was held at the Pauling Human Sciences Centre, Oxford, in 2000. The Society thanks Ros Odling-Smee and the Centre for accommodating the conference. The help with arrangements for and at the workshop by the team of postgraduates from Oxford Brookes {University} is gratefully acknowledged, and the Society is thanked for providing finances for the workshop. The Editors also wish to thank Chris McDonaugh for reading the book and making helpful suggestions, Jennifer Jay for work on the glossary, index and referencing, and finally Geoff Harrison for writing the Foreword.

Introduction

Changes in child survival rates are very significant in the dynamics of human populations, and mortality is an important variable in studies of demography, genetics and anthropology. Furthermore, rates of child survival provide an important indication of the general biological welfare of a population.

Children are a particularly vulnerable group, both physically and socially, within human populations (Caldwell 1996). This vulnerability is reflected in mortality rates, which are highest during infancy, decrease from 1 to 4 years and reach a minimum from 5 to 9 years of age. The physical vulnerability of children stems from an immature immune system, rendering them particularly susceptible to infectious diseases. This is coupled with a high energy and nutrient requirement relative to body weight. These intakes are required to fuel the high energy demands of the human brain, as well as for somatic growth and development. One of the most critical periods for child survival is the weaning period, typically from six months onwards, when infants make the transition from dependence on mother's milk to reliance on the local diet (Martorell 1995). Children are also socially vulnerable, being entirely dependent on adult carers for the first year of life and remaining highly dependent up to the age of five. This social dependence means that children are susceptible to conscious or unconscious neglect (Caldwell 1996).

This chapter examines the sociocultural and biological correlates of child survival, contrasting populations with low and high survival rates.