In brief Over the past half century, the world has witnessed an unprecedented decline in human fertility rates. This analysis reviews the various socioeconomic, cultural, and biological factors involved in driving this change and considers whether low fertility rates are a temporary or permanent feature of our future demographic profile. Abstract Since the early 1960s, the world has witnessed the spectacular collapse of human fertility. As a result of this phenomenon, several countries are already seeing their population numbers fall and more will follow in the coming decades. The causes of this fertility decline involve a complex interplay of socio-economic, environmental, and biological factors that have converged to constrain fertility in posterity’s wake. Since large numbers of offspring are no longer needed to compensate for high infant mortality in contemporary society, couples have opted to have small families in a quality-over-quantity investment in their progeny’s future. Simultaneously, increases in female education, the enhanced participation of women in the paid workforce, and a resultant delay in childbearing has placed limits on achievable family size. Progressive urbanization, the improved availability of contraceptives, and the socio-economic pressures experienced by young adults in ageing societies are also contributing to fertility’s demise. These factors, together with the individualism that pervades modern society and the increasing social acceptability of voluntary childlessness, have firmly established a low fertility ethos in most post-transition countries. Since none of these forces are about to relent, it looks as if extremely low fertility might be with us for some time to come. This may have long-term consequences. The lack of selection pressure on high fertility genotypes, the ability of ART to retain poor fertility genotypes within the population, and sustained exposure to reproductive toxicants in modern industrialized environments may all contrive to leave a permanent mark on the fecundity of our species.
Abstract This article estimates the effect of crime on migration rates for counties in U.S. metropolitan areas and makes three contributions to the literature. First, I use administrative data on migration flows between counties, which gives me more precise estimates of population changes than data used in previous studies. Second, I am able to decompose net population changes into gross migration flows in order to identify how individuals respond to crime rate changes. Finally, I include county-level trends so that my identification comes from shocks away from the trend. I find effects that are one-fiftieth the size of the most prominent estimate in the literature; and although the long-run effects are somewhat larger, they are still only approximately one-twentieth as large. I also find that responses to crime rates differ by subgroups, and that increases in crime cause white households to leave the county, with effects almost 10 times as large as for black households.
Competition is among the most important factors regulating plant population and community dynamics, but it is not well understood how different vital rates respond to competition and jointly mediate competitive population dynamics and species coexistence. We used integral projection models (IPMs) to model the population growth of 112 pairwise combinations of 14 competing herbaceous plant species across an elevation gradient (n = 324 IPM models in total). We showed that the response of individual growth and seedling establishment contributed most strongly to competition-induced declines in population growth compared to survival, flowering probability and fecundity that frequently showed complementary responses that occurred in 92% of species pairs. Complementary responses significantly promoted population growth under competition by 22% on average and strengthened species coexistence. Our study emphasises the need to investigate demographic processes to better understand competitive population dynamics and species coexistence.
Abstract There is a population sex imbalance in India. Despite a consensus that this imbalance is due to excess female mortality, the specific source of this excess mortality remains poorly understood. I use microdata on child survival in India to analyze the proximate sources of the sex imbalance. I address two questions: when in life does the sex imbalance arise, and what health or nutritional investments are specifically responsible for its appearance? I present a new methodology that uses microdata on child survival. This methodology explicitly takes into account both the possibility of naturally occurring sex differences in survival and possible differences between investments in their importance for survival. Consistent with existing literature, I find significant excess female mortality in childhood, particularly between the ages of 1 and 5, and argue that the sex imbalance that exists by age 5 is large enough to explain virtually the entire imbalance in the population. Within this age group, sex differences in vaccinations explain between 20% and 30% of excess female mortality, malnutrition explains an additional 20%, and differences in treatment for illness play a smaller role. Together, these investments account for approximately 50% of the sex imbalance in mortality in India.
Abstract Demographers are often interested in the environmental impacts of population growth. I examine the impact of growth specifically on air quality in California. In recent decades, California has suffered from notoriously polluted air and has experienced rapid population growth. Despite the population .growth, air quality actually has improved since the early 1980s due to aggressive regulatory efforts. Using data for 56 counties, I analyze the contribution of population growth to trends in atmospheric emissions of five regulated pollutants from 1980 to 1990, controlling for trends in per capita income and regulatory efforts. The analysis is disaggregated by source of emissions and demonstrates that population growth is strongly associated with some sources of emissions but not with others. Thus, the overall impact of population growth depends upon the composition of production and consumption activities in each county. I also explore whether the trend in number of households predicts better than the trend in number of persons, and whether the impact of population growth depends upon the age structure or source of growth (immigration or domestic increase). Generally, these alternative specifications of population do not improve the models of atmospheric emissions.