2 How has the human population grown?
For most of human history there have been relatively few people in the world. Figure 2 shows that only over the last 50 years have numbers really shot up, and that, at the turn of the century, the population reached over six billion.
The size of a population depends fundamentally on just two factors: the birth rate and the death rate. When the birth rate exceeds the death rate, there will be net population growth; when the death rate exceeds the birth rate, there will be net shrinkage. When the two rates are balanced, the population remains stable. Common sense dictates that we should be aiming for a stable population which can be sustained by the Earth's limited resources.
Q Assume that in an ideal world every adult is in a relationship with just one other person. How many children would each couple have in order for the population to be stable?
A If each couple had two surviving children, they would effectively replace themselves in the population, and overall numbers would remain stable.
Of course, the situation in the real world is a long way removed from this hypothetical ideal. Some people have no offspring; others have one, or two, or several. The United Nations suggests that, based on current trends, the world population will reach between 10.4 billion and 14 billion before levelling off, and eventually declining, due to famine and disease. To put this into a more understandable context, 10 billion people are twice the number who were alive in 1987.
What factors affect the death rate of a population? Broadly speaking, the main factors are the biblical favourites war, disease and famine. These also affect the birth rate: disease and famine generally prevent successful pregnancies (see Section 4), whereas after a war there is generally an increase in birth rate. (This is allegedly due to an increased rate of sexual intercourse by couples who have been separated for long periods.) We in the developed world have been relatively uninfuenced by these factors in recent times. The wars our countries engage in are usually fought at some distance from us; most of us have more than adequate nutrition (at least quantitatively, but see below3), and modern medical science has significantly reduced the impact of disease for most of us. Yet a glance at the fertility rates of various countries shows that the rate is actually lowest among the developed countries (Table 1).
Table 1, Some population statistics. This table shows various figures about the populations of some developed and developing countries. Fertility rate is an estimated measure of the rate of fertilizations, and includes numbers of live births and numbers of reported abortions; infant mortality rate is infant deaths per 11000 live births; – means data unavailable.
|Population estimate mid-1993 (millions)||Total fertility rate||Infant mortality rate||Life expectan cy at birth (years)||Married women using contraceptio n (%)||People under 15 (%)||Literacy 1990 (male/ female) (%)||GNP per capita 1991 (US)|
Q Which other country shows a low birth rate?
A China, where there is a state policy allowing only one child per family.
In view of the generally high standard of health care and nutrition in the developed countries, one might expect the birth rates to be much higher than they are. Assuming that there is no problem of widespread infertility (that is, the biological inability to produce successful pregnancies), a reasonable explanation is that people are choosing to limit the size of their families, and have the education and means to be able to select both the number of their pregnancies and the intervals between them. A woman is fertile for around 40 years, and could in theory have one pregnancy each year. Yet it is unlikely that many women would bear 40 children, even in the absence of contraception, nor would many want to.
Nevertheless, in the developing world it is common for women to have many pregnancies, giving rise to the rapid and unsustainable increase in global population. Large families are highly desirable, and sons are particularly welcome as they will be able to work to support their ageing parents, and will command dowries from their wives' families. In some Hindu communities, sons are also regarded as spiritually ‘superior’; only a man's prayers can send souls to heaven. Daughters, on the other hand, are often less welcome, as dowries will have to be found for them unless they are to remain with their parents for their adult lives. This dislike of daughters is at its most extreme in societies that practise female infanticide.
In many developing countries, both food and medical supplies are scarce, which will limit the mother's fertility. Of fertilizations that do occur, it is estimated that more than one-third fail because they are biologically abnormal, or because the mother develops an infection that terminates the pregnancy. Some of these infections can prevent the woman from conceiving again. New-born babies are very vulnerable to injury and disease, and this is another natural means of population control. If the mother is undernourished, it may be some time before she can conceive again, even if she is taking no measures to prevent conception. Breastfeeding can in some cases prevent ovulation – this is known as lactational amenorrhoea – so if a mother is still feeding her previous baby she is less likely to start another pregnancy.
Thus, to some extent, family size, and hence population, is limited by the environment.
The ‘natural’ ways of controlling family size sound quite draconian to most of us, based as they are on death and famine. In the developed world, and increasingly in developing countries, we have recourse to other methods to limit our families, and it is to these that we turn next.