Population Density is a Key Factor in Declining Human Fertility

A study carried out by W. Lutz of the International Institute for Applied Systems Analysis in Austria, M. R. Testa of the Vienna Institute of Demography at the Austrian Academy of Sciences and D. J. Penn of the Konrad Lorenz Institute for Ethology also at the Austrian Academy of Sciences, shows that ‘Population Density is a Key Factor in Declining Human Fertility‘.

Using fixed effects models on the time series of 145 countries and controlling for key social and economic variables, the 3 Austrian social scientists found a consistent and significant negative relationship between human fertility and population density. Moreover, they found that individual fertility preferences also decline with population density. These findings suggest that population density should be included as a variable in future studies of fertility determinants. These significant findings were published in 2007.

This study was also quoted by NSP in its alternative Population Plan for Singapore (‘NSP proposes alternative Population Plan for Singapore‘). NSP said:

“Looking at the data both internationally and domestically, it can be seen that when population density increases, fertility decreases. This is supported by independent research in Austria. Increasing Singapore population to 6.9 million by 2030 is therefore, likely to further depress Singapore’s fertility rate, creating a vicious cycle. There is a need to focus on improving Singapore’s fertility rate if we want to continue growing our economy with minimal social problems.”

The paper published by the study, said, “In many parts of Europe and some Asian countries, fertility rates have recently fallen to such low levels that the resultant rapid population ageing and shrinking already causes widespread concern. Various social, economic, political and bio-medical factors are associated with declines in birth rates but few social scientists have considered population density even though density has been shown to be a key determinant in many animal populations.”

In other words, even in the animal kingdom, population density does affect animal populations too.

It said, “If human fertility also depends on population density, this will have important implications for population projections…”

The paper said that population growth in the developing world is mainly due to high fertility rates combined with a very young age structure that results in increasing cohorts of women entering reproductive age. In Europe, by contrast, below-replacement fertility has resulted in an age structure with fewer children and therefore fewer women entering reproductive age in the future.

The paper also noted that a wide range of species — whether microbes, mussels, fruit flies or elephants — have been found to have density-dependent effects on survival and reproduction. Domestic animals have long been known to show reduced reproduction at high population densities and an increasing number of studies have found density-dependent reproduction in the wild. For example, reproduction has been observed to decline with increasing density in birds and mammals and experiments also show that reproduction can be density-dependent in the wild.

Food resource limitation is suspected to be an important mechanism behind density-dependent reproduction, although stress-induced endocrine changes from crowding are also known to curtail reproduction in primates and other mammals. It is not difficult to understand why such responses have evolved. When survival is density-dependent, density-dependent reproduction will provide a selective advantage.

Indeed, experimental studies on birds found that individuals adaptively adjust their number of offspring according to population density. There have been few studies that explicitly addressed the relationship between density and reproduction in humans, although they have generally found a significant negative relationship, even in very different settings. A systematic review of this relationship for historical rural societies also found significant negative elasticities. And yet, over the past two decades mainstream demographic analyses have generally ignored density as a possible determinant of human fertility.

In their search for an explanation for modern fertility declines, evolutionary demographers have focused on the fitness costs of reproduction, and whether low-fertility parents are trading offspring quantity for quality and ignoring density dependence.

To comprehensively assess the relationship between population density (i.e, population per total land area) and human fertility (i.e, number of births per woman), the Austrian scientists assembled a broad range of data, covering 159 countries since 1960.

Using mathematical analysis, the findings of the Austrian scientists imply that population density should be added to the list of the usual factors that are assumed to affect human fertility (i.e., education, women’s status, economic development, etc.). Density-dependence and the importance of these other factors are not mutually exclusive, and density may affect or interact with these other factors (e.g., women may gain more status at higher population densities).

The scientists also found that density is correlated with reduced fertility preferences, as well as actual outcomes, suggesting that it affects individual reproductive decisions. The scientists suggested that the personal perception of density (living space, availability of interpersonal communication, etc.) plays a role in this context. This idea is consistent with the ‘‘frontier effect,’’ which postulates that fertility preferences increase under low density.

Fertility preferences are expected to decline with increased costs of rearing children, such as child-care, housing, education, and trade-offs with economically productive work and these factors can increase with population density.

The paper concluded that a robust, negative association between fertility and population density has far reaching consequences, ranging from the projections of future regional fertility differentials to the way we model human population dynamics.

Singapore

So, what about the situation in Singapore? Looking at the population density and the Total Fertility Rate (TFR) of Singapore from 1975 to 2011, it also appears that there is a significant negative relationship between fertility and population density, as put forward by the paper.

Population density (people per sq. km) in Singapore [Link]:

TFR chart from NPTD’s Population White Paper:

In this regard, NPTD of the Prime Minister’s Office should take heed of such prestigious international studies linking population density to fertility rate. With the increase in population density in Singapore by bringing in another million more foreigners into the country in the next 17 years, our TFR would naturally fall further, according to this Austrian’s study.