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  The number of live births produced by a woman. Fertility is generally distinguished from fecundity, a biological term for the ability to conceive. Fertility, mortality and migration are the three fundamental influences on the population geography of any area. Although they have recognized the importance of spatial variations in fertility, geographers have made relatively few contributions to research in this field. Demographers, on the other hand, have developed sophisticated measures of fertility and have made considerable progress towards establishing and explaining fertility trends in both developed and developing countries (see development). Fertility behaviour is still imperfectly understood, however, and projections are problematic.

Measures of fertility range from the very simple to the very complex. A general distinction is drawn between period fertility and cohort fertility. The former is the most straightforward and relates to the study of births occurring to all females in their reproductive period — i.e. to groups of females of given ages at a certain point in time or over a relatively short period. The latter is used to trace the reproductive history of a group of females born or married at the same time, so illuminating the ways in which families are built up through time, plus changes in completed family size and the spacing of births. The cohort approach requires long series of accurate vital statistics and so is most commonly applied to the populations of developed countries in the twentieth century.

The most simple and widely used fertility measure is the crude birth rate which expresses the number of live births in a given period as a ratio of the average total population alive during that period in parts per 1000; rates vary from about 10 to 55 per 1000, the latter being an estimate of the biological maximum. The crude birth rate has the advantage of being easy to calculate and is one element in the basic demographic equation of birth, death and migration for any area, but it may be an extremely misleading measure of underlying fertility patterns, because of variations in age structure in the base population (see age and sex structure). Other more sophisticated measures have therefore been devised. The simplest is the general fertility rate, or the number of births per 1000 women in the fecund ages (defined variously as 15-49 or 15-44). The marital fertility rate, on the other hand, expresses the number of legitimate live births per 1000 married women. These rates are thus useful in relating births to the actual section of the population responsible for them. A further refinement is the age-specific fertility rate, defined as the number of births to a specified age-group per 1000 women in those ages, usually taken in five-year periods. This allows more detailed analysis and comparisons of fertility experiences. The total fertility rate tells us how many children on average each 1000 women have while passing through their fecund years. In the developed world, a rate of 2.1 children per woman is necessary to ensure replacement of generations (see replacement rates). Replacement may also be measured by the gross reproduction rate, the average number of daughters produced by a woman during her reproductive lifetime, and the net reproduction rate, where mortality is taken into account. A net reproduction rate greater than 1.0 ensures replacement of one generation by the next. Of the cohort fertility measures, completed family size is the most useful, expressing the average number of children ever born to women currently aged 45 or over.

Factors affecting fertility levels have given rise to much discussion. The general ideas of the demographic transition have been only partially acceptable, given recent fertility behaviour in both developed and developing nations. Explanations have been sought in relation both to the long-term decline of fertility over the last two centuries in much of the developed world (see figure for Norway) and to cross-cultural variations in contemporary fertility patterns. Fertility is linked to other aspects of demographic behaviour, to mortality and particularly to marriage patterns, for age at marriage and overall nuptiality are important. Explanations of fertility decline in Europe, which began in France in the later eighteenth century, have been sought in the broad pattern of urbanization, industrialization and modernization, which changed attitudes to birth control, marriage and family formation and were associated with, for example, higher levels of education and changes in religious attitudes. Interestingly, other links, for example with issues raised in feminist geography, have attracted relatively little attention, though for a Marxist critique see Seccombe (1983) and for a view of the wider perspective being urged on demographers see Greenhalgh (1996).

Considerable geographical variations in fertility exist within countries today and these too have been related to a very broad series of factors including social and occupational status, rural or urban residence (see rural-urban continuum), education, religion and the changing role of women in society. Great care must be taken in analysing fertility trends in the Third World. Because of less complete sources, it is sometimes difficult to estimate accurately even recent trends and there is no guarantee that relationships postulated for developed countries hold true either to explain the past or to predict future trends. Certainly fertility levels, and their geographical variations, represent one of the most important fields for research in population studies. (PEO)

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Fertility Period and cohort fertility rates for Norway 1820 to 1973 (Rallu and Blum, 1991)

References Greenhalgh, S. 1996: The social construction of population science: an intellectual, institutional and political history of twentieth-century demography. Comparative Studies in Society and History 38: 26-66. Rallu, J.-L. and Blum, A., eds, 1991: European population. 1. Country analysis. London: John Libbey, 132. Seccombe, W. 1983: Marxism and demography. New Left Review 137: 22-47.

Suggested Reading Andorka, R. 1978: Determinants of fertility in advanced societies. London: Methuen; New York: Free Press. Coale, A.J. and Watkins, S.C., eds, 1986: The decline of fertility in Europe. Princeton, NJ: Princeton University Press. Daughtery, H.G. and Kammeyer, K.C.W. 1995: An introduction to population, 2nd edn. New York and London: Guilford Press, ch. 8. Davis, K., Bernstam, M.K., Ricardo-Campbell, R., eds, 1987: Below replacement fertility in industrial societies. Causes, consequences, policies. Population and Development Review. A supplement to Volume 12, 1986. New York: The Population Council; Day, L.H. 1992: The future of low-birthrate populations. London: Routledge. Gillis, J.R., Tilly, L.A. and Levine, D., eds, 1992: The European experience of declining fertility, 1850-1970. The quiet revolution. Cambridge, MA and Oxford: Blackwell. Gould, W.T.S. and Brown, M.S. 1996: Research review 2: fertility in sub-Saharan Africa. International Journal of Population Geography 2 1: 1-22. Greenhalgh, S., ed., 1995: Situating fertility. Anthropology and demographic enquiry. Cambridge: Cambridge University Press. Jones, H.R. 1990: Population geography, 2nd edn. London: Paul Chapman; Leete, R. and Alam, I., eds, 1993: The revolution in Asian fertility. Dimensions, causes and implications. Oxford: Clarendon Press; Lutz, W., ed., 1990: Future demographic trends in Europe and North America. London and New York: Academic Press, part 2. Tilly, C., ed., 1978: Historical studies of changing fertility. Princeton: Princeton University Press.



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