Explaining the human demographic transition

In her recent TREE review1xBorgerhoff Mulder, M. Trends Ecol. Evol. 1998; 13: 266–270Abstract | Full Text | Full Text PDF | PubMed | Scopus (140)See all References1, Borgerhoff Mulder discusses several hypotheses for the human demographic transition from an evolutionary point of view, and her conclusion suggests that human reproductive decisions might be driven by a psychology designed to maximize material wealth. There are weaknesses in this idea.To propose a ‘theory of fertility rate of a human population’ as is a simplistic approach that does not take into account the variation of fertility rates of subpopulations. A portion of the population does not reproduce at all, another portion is having more children than average. Having no children can be (1) a voluntary decision, (2) the result of impotency, or (3) the result of sterility caused by environmental pollution2xHumanokologie. Nentwig, W. CrossrefSee all References2. Approximately 20–25% of German couples have no children for one of these reasons. Reason (3) alone concerns an estimated 10% of all German couples, and this trend is increasing. For comparison, about 25% of US couples are childless, and among African women up to 30–40% cannot have children2xHumanokologie. Nentwig, W. CrossrefSee all References, 3xDie Weltbevolkerung. Birg, H. See all References.The proposed theory focuses on decisions made by single individuals or couples. Analyses in developing countries, however, show that there is a discrepancy between the number of offspring desired and the actual fertility rate, that is, many women have (to have) more children than they would like. Common reasons are unavailability of contraceptives, insufficient education and the females’ weak social position2xHumanokologie. Nentwig, W. CrossrefSee all References2. To maintain their social status, men force their wives to have more children than they would like to have. Additionally, undesired teenage pregnancies become more and more prevalent, and contribute significantly to the world population increase. This means that, for most women, becoming pregnant is not a free decision. Thus, a theory on fertility decline should consider gender-specific aspects of education and social hierarchies.The theory of demographic transition, as described in the second half of the 19th century by several demographers as a scientific reply to Malthus4xAn Essay on the Principle of Population. Malthus, Th.R. See all References4, contains a decline in both mortality and fertility, that is, a transition from high to low levels of mortality and reproduction. In the first third of this transition, population growth is medium. It is getting stronger in the second third; subsequently it declines at the end of the demographic transition when both mortality and fertility reach low levels. In very technologically advanced societies the fertility rate may even be lower than the mortality rate, and consequently the population is shrinking (European average for the year 1997: −0.1%, Ref. 5xSee all ReferencesRef. 5). Since fertility decline follows mortality decline, the first is usually interpreted as a response to the latter. People realize that medical supply becomes more easily available and is of higher quality. Infant mortality decreases, and at the same time life expectancy and general prosperity increase. Children are no longer needed as a service to the elderly; they become more and more of a burden because of increasing costs of education. From this perspective, the theory of material wealth maximization is an oversimplification because it overlooks the causal link to the previous decline in mortality.Last but not least, according to population growth projections2xHumanokologie. Nentwig, W. CrossrefSee all References, 3xDie Weltbevolkerung. Birg, H. See all References, 6xWorld Population Projections for the 21st Century—Theoretical Interpretations and Quantitative Simulations. Birg, H. See all References the world population is now at approximately half its projected final size of about 12 billion, which will be reached in approximately 150–200 years. Today, the world is already overpopulated and we suffer the consequences of this extreme high population density (numerous stress-dependent diseases, malnutrition, hunger7xSee all References7, environmental pollution, greenhouse effect2xHumanokologie. Nentwig, W. CrossrefSee all References2). It is far from clear if the Earth can support 12 billion people; there are many indications that the actual six billion are already too many2xHumanokologie. Nentwig, W. CrossrefSee all References2. Under comparable living conditions, animal populations suffer from density-dependent stress syndromes (examples in 8xEcology. Begon, M., Harper, J.L., and Townsend, C.R. See all References, 9xOkologie. Remmert, H. See all References) and reduce their reproduction. This is exactly the effect which we observe now as human fertility declines, and it can be interpreted as an attempt to approach the carrying capacity of our world.I agree with Borgerhoff Mulder’s conclusion that ‘the human demographic transition is a highly complex phenomenon with no single form and no common set of causes’. Any hypothesis that wants to explain this transition should therefore take into consideration cohort life tables, mortality decline and a widespread lack of free decision.