Demography is a science short on theory, rich in quantification. Nevertheless, demography has produced one of the best documented generalizations in the social sciences: the demographic transition. What is the demographic transition? Stripped to its essentials it is the theory that societies progress from a pre-modern regime of high fertility and high mortality to a post-modern regime of low fertility and low mortality. The cause of the transition has been sought in the reduction of the death rate by controlling epidemic and contagious diseases. Then, with modernization, children become more costly. Cultural changes weaken the importance of children. The increasing empowerment of women to make their own reproductive decisions leads to smaller families. Thus there is a change in values, emphasizing the quality of children rather than their quantity. In short, the fertility transition is becoming universal phenomenon, in which every country may be placed on a continuum of progress in the transition.

The complexity of host–parasite interactions makes it difficult to predict how host–parasite systems will respond to climate change. In particular, host and parasite traits such as survival and virulence may have distinct temperature dependencies that must be integrated into models of disease dynamics. Using experimental data from Daphnia magna and a microsporidian parasite, we fitted a mechanistic model of the within-host parasite population dynamics. Model parameters comprising host aging and mortality, as well as parasite growth, virulence, and equilibrium abundance, were specified by relationships arising from the metabolic theory of ecology. The model effectively predicts host survival, parasite growth, and the cost of infection across temperature while using less than half the parameters compared to modeling temperatures discretely. Our results serve as a proof of concept that linking simple metabolic models with a mechanistic host–parasite framework can be used to predict temperature responses of parasite population dynamics at the within-host level.

This study presents an assessment of fertility trends in 23 countries of sub-Saharan Africa. It examines trends and differentials in proximate determinants and fertility preferences. Findings from the Demographic and Health Surveys for these countries over a period of 15 years show that desired family size has decreased significantly. Two-thirds of the countries examined show evidence of fertility decline, a particularly rapid decline in the cases of Kenya and Zimbabwe. Areas with higher education for women and lower child mortality experienced larger reductions in fertility and desired family size. Contraceptive use far exceeds other proximate determinants in explaining these changes. The striking regularity in fertility reduction across all ages indicates that contraception is practiced mostly for birth spacing and that contraceptive methods have gained wide acceptance among younger cohorts. Good prospects are seen for further intensification of fertility declines in East Africa and urban West Africa. However, low levels of education and high child mortality make rapid changes unlikely in rural West Africa.

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