Academic journal article Demographic Research

Modeling Fertility in Modern Populations

Academic journal article Demographic Research

Modeling Fertility in Modern Populations

Article excerpt

Abstract

The age-specific fertility pattern has a typical shape common in all human populations through years. In order to describe this shape a number of parametric models have been proposed. Recently, the fertility pattern in developed countries exhibits a deviation from the classical one. Recent data sets of the United Kingdom, Ireland and the US show distortions in terms of a bulge in fertility rates of younger women. Furthermore in countries with distorted fertility, the pattern of first births also exhibits an intense hump in younger ages, stronger than that of the total fertility pattern. This heterogeneity indicated by the recent fertility distributions of European countries and the US might be related to marital status, religion, educational level and differences in social and economic conditions. Additionally in the US this heterogeneity in fertility patterns might be related to ethnic differences in the timing and the number of births. As expected, the existing models are unable to describe the new shape of the fertility pattern and therefore the use of more appropriate representations is required.

In this paper, a new flexible model for describing both the old and the new patterns of fertility is proposed. In order to evaluate the adequacy of the model, we fit it to a variety of empirical fertility schedules.

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1. Introduction

Modeling fertility curves has attracted the interest of demographers for many years. A variety of mathematical models have been proposed in order to describe the age-specific fertility pattern. Several of these models have been shown to provide excellent fits to one-year age-specific fertility rate distributions of human populations (Hoem et al, 1981).

In recent years a considerable variation in the pattern of fertility is observed in data sets for populations of developed countries. This variation is related to the form of the fertility curve. While the standard fertility pattern is a bell shaped one, roughly symmetrical though sharper in its left part around its peak placed in an age around 25, in recent years, in data of modern developed populations, a second peak placed in a much younger age than the first one, becomes obvious (Chandola et al., 1999; 2002). In fact, recent fertility data of some English-speaking countries, e.g., the United Kingdom, Ireland and the US display a marked hump in early ages. The heterogeneity in the fertility patterns of the UK, Ireland and the US might be associated to some extent to marital status as well as to educational level and social status of the mothers. Additionally in the US this heterogeneity in fertility patterns may be explained by ethnic differences in the timing and the number of births.

Existing models cannot capture the modern fertility pattern. Chandola et al. (1999; 2002) proposed a mixture of the Hadwiger function in order to describe this new form in the fertility pattern.

In this work, a flexible model in different versions describing both the standard and the new distorted age-specific fertility pattern is presented and evaluated.

In the next section we review hypotheses presented in the literature for explaining heterogeneity in fertility. In Section 3 we shortly present existing models for fitting fertility data. In Section 4 the proposed model is described. Section 5 provides an evaluation of the model by fitting it to a wide set of empirical fertility schedules. Finally in Section 6, a short discussion and an outlook for further research are provided.

2. The fertility pattern in modern populations

It is widely known that the distribution of age-specific fertility rates has a typical shape common in all human populations through time. It begins with a minimum placed at the beginning of reproduction age interval and then it rises until it attains a maximum somewhere in the 30's. Then it declines again to level off near age 50. …

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