Natural Fertility Populations Research Articles

The impact of eliminating sterility on population growth.

The issue of this discussion is whether and to what extent treatment of infertility will contribute to a significant increase in fertility, and whether treatment of sexually transmitted diseases and infertility is justified as a humane effort to alleviate suffering, death, and the extent of impact on men, women, fetuses, and infants. The Brunham model produces estimates of the difference in the total fertility rate (TFR) due to sterility of between 9.7 and 18.0. However estimates with more realistic assumptions about life expectancy, menarche, menopause, and union formation yield TFR differences between 2.1 and 2.5. Accounting for secondary sterility and a more realistic simulation yields a change in the population growth rate of 0.8% and 0.9% compared to 2.5% and 1.9% in the Brunham model. The conclusion is that population growth rates would not be very likely to increase if sterility is eliminated. The Brunham model is an important attempt to model the effects of different assumptions about rates of partner exchange on disease prevalence and on sterility levels. The Brunham conclusion is that population growth rates increase by 50% or more when gonorrhea has a prevalence of 20% and a 12% probability of sterility per 6-month duration of illness. Chlamydia-related secondary sterility would not have quite as large an effect. Many other models are possible because of the many other fertility inhibiting factors. The Brunham model does not account for any age variation in fertility and mortality rates, which is not a realistic view of human behavior. The Brunham model also uses a very high mortality pattern, comparable to TFRs of 8.5 to 9.5 for a noncontracepting Hutterite natural fertility population, with short breast feeding, and universal marriage. When fecundability, exposure to risk of intercourse, postpartum and lactational amenorrhea, and coital frequency are taken into account, the impact of sterility is reduced. The proposed model accounts for sterility and postpartum amenorrhea, union formation, and coital frequency with realistic ages of menarche and menopause.

The end of fertility: age, fecundity and fecundability in women

Onset of capacity for childbearing in women is dated biologically by menarche, although actual onset may be delayed. The end of childbearing is less understood but recent demographic and biological research on fertility at older ages in clarifying the end of fertility. The demographic view of declining fertility with age is based on age-specific fertility in natural fertility populations, artificial insemination and pregnancy rates by age and World Fertility Survey data. New data from the Demographic and Health Surveys on exposure to the risk of pregnancy shows that whereas older women biologically need longer exposure to pregnancy, exposure declines on behavioural grounds such as duration of marriage. Actual fecundity is obscured by factors of fecundability. Recent research on medically assisted conception is adding to the understanding of declining fecundity with age, especially the relative contributions of endometrial and ovarian ageing. This paper reviews the available information on declining fertility with age and discusses the implications of the extension of fertility through new medical technologies.

Changes in fertility relative to starting, stopping, and spacing behaviors in a migrating Mennonite community, 1775-1889.

Fertility change over time in a migrating Mennonite church congregation is reconstructed through genealogies developed from church registries of vital events during 1725-1924. The congregation was located in Prussia from 1725-1821, in Russia from 1822-1874, and in Kansas, U.S.A., from 1875-1924. Age-specific marital fertility rates were relatively low and usually peaked for women aged 25-29. Total fertility rates ranged from 1.19 to 5.29. These relatively low figures for a natural fertility population may partly reflect underreporting of births and deaths of infants, but it also reflects the heterogeneity in fertility evident for this population. Some women had many children while others were having either few or none. This pattern changed twice. Fertility was lowest during residence late in Prussia and early in Russia, peaked during residence late in Russia, and has decreased slightly for women born in the United States.

A model of the age patterns of births by parity in natural fertility populations

This research develops a convolution model to express the age patterns of fertility at each birth order in natural fertility populations in terms of six parameters, directly representing the proximate determinants of fertility, and a series of parity level indicators. The parity level indicators at each birth order are simply the proportions of women in a cohort who will eventually have births at each birth order it the age‐related fecundity decline is controlled. The Coale‐McNeil nuptiality model is adopted to represent the age pattern of first marriage rates and the natural fertility schedule employed in the Coale‐Trussell fertility model is incorporated to adjust age effects. The fast Fourier transform is used in solving the model numerically. It proves that the model is able to provide excellent fits to fertility for rural Chinese women in the 1950s.

Hutterite fecundability by age and parity: Strategies for frailty modeling of event histories

Effective fecundability declines with age and parity. Furthermore, women differ in their effective fecundability: some women have persistently low or high monthly chances of live-birth conception. Estimates are presented concerning the magnitude of these effects in a natural-fertility population: 406 Hutterite women in North America who had 3,206 births, largely in the 1940s and 1950s. The estimates are based on models that incorporate the effects of persistent heterogeneity and that use the full information provided by multiple-spell duration data. In addition, hazards rather than probabilities are modeled, piecewise linear hazard functions are used, and age and parity effects are decomposed systematically. These methods permit the development of more elaborate models of changing fecundability and of heterogeneity in postpartum amenorrhea.

Toward an Ecological Demography

An innovative way of looking at demographic patterns is based on paradigms from the fields of behavioral and evolutionary ecology. Just as the analytic techniques of demography have been important to biological investigations the theoretical advances of the last 25 years in biology may offer important perspectives on central problems in demography and provide paths to fruitful ecologically informed solutions. The authors summarize the pertinent theory in behavioral ecology highlight two important differences in the way biologists and demographers partition problems and illustrate how behavioral ecology might contribute to a new understanding of several important issues in classical demography namely natural versus controlled fertility population regulation and allocation of resources to offspring. (SUMMARY IN FRE AND SPA) (EXCERPT)

Scope of Perspective in Fertility Models

author replies to comments by both Roth and Cashdan on his article The First Demographic Transition. It is maintained that both critiques made use of underconceptualized fertility models. Thus Roths findings do not undermine the authors argument about the role of settlement in bringing about fertility increases; it is not settlement per se that brings about fertility increases but rather changes in diet and social relations. question is how long such changes would take to work themselves out 10000 years ago. Similarly contrary to Cashdan there is no empirical evidence that decisions to promote or restrict fertility account for an appreciable amount of the variance in fertility between either foragers or settled cultivators. It is conceded to Cashdan however that birth control practiced for the purpose of spacing cannot be inferred from the existence of an extended age pattern of childbearing (natural fertility). To the extent that pretransition societies are concerned with birth control for spacing purposes rather than family limitation their age pattern of childbearing is likely to resemble that in natural fertility populations but will be truncated under conditions where birth spacing increases progressively with age widows remain celibate or gonorrhea is endemic. Needed are formal models that specify which variables have effects of what magnitude in which circumstances and why and how these sets of determinants change. In addition it is necessary to distinguish between micro and macro-level effects and to specify interlevel linkages.

Sterility in a Population with Natural Fertility

In this paper we examine the age pattern of sterility in a natural fertility population of 16 English parishes. We examine estimators of sterility proposed by historical demographers. We demonstrate through Monte Carolo simulation of reproductive histories that the estimators proposed earlier work well only if the ages to which the estimates pertain are substantially modified from the original formulation. The new estimates show a much larger positive effect of childbearing on sterility than would the earlier ones. We also present estimates of the age pattern of sterility due solely to the process of ageing by eliminating secondary sterility induced by childbearing. This curve rises slowly until age 40, after which the proportion sterile increases rapidly with age. We find no evidence of a sharp rise in the risk of sterility in the 30s. We find strong evidence of a decline in fecundity by examining age-specific fertility rates only for those women who are known to be fecund because they later bear children. The evidence suggests only a moderate decline until ages 35–39 and a much steeper decline thereafter.Finally, we illustrate the danger of the use of a clinical test of infertility commonly employed. We show that if women are judged to be infertile because they have not become pregnant within one year of unprotected exposure, then a large fraction of those so judged will be falsely diagnosed. We conclude that the one-year period is too short.

Birth spacing and fertility limitation: a behavioral analysis of a nineteenth century frontier population

Our analysis of changing birth interval distributions over the course of a fertility transition from natural to controlled fertility has examined three closely related propositions. First, within both natural fertility populations (identified at the aggregate level) and cohorts following the onset of fertility limitation, we hypothesized that substantial groups of women with long birth intervals across the individually specified childbearing careers could be identified. That is, even during periods when fertility behavior at the aggregate level is consistent with a natural fertility regime, birth intervals at all parities are inversely related to completed family size. Our tabular analysis enables us to conclude that birth spacing patterns are parity dependent; there is stability in CEB-parity specific mean and birth interval variance over the entire transition. Our evidence does not suggest that the early group of women limiting and spacing births was marked by infecundity. Secondly, the transition appears to be associated with an increasingly larger proportion of women shifting to the same spacing schedules associated with smaller families in earlier cohorts. Thirdly, variations in birth spacing by age of marriage indicate that changes in birth intervals over time are at least indirectly associated with age of marriage, indicating an additional compositional effect. The evidence we have presented on spacing behavior does not negate the argument that parity-dependent stopping behavior was a powerful factor in the fertility transition. Our data also provide evidence of attempts to truncate childbearing. Specifically, the smaller the completed family size, the longer the ultimate birth interval; and ultimate birth intervals increase across cohorts controlling CEB and parity. But spacing appears to represent an additional strategy of fertility limitation. Thus, it may be necessary to distinguish spacing and stopping behavior if one wishes to clarify behavioral patterns within a population (Edlefsen, 1981; Friedlander et al., 1980; Rodriguez and Hobcraft, 1980). Because fertility transition theories imply increased attempts to limit family sizes, it is important to examine differential behavior within subgroups achieving different family sizes. It is this level of analysis which we have attempted to achieve in utilizing parity-specific birth intervals controlled by children ever born.(ABSTRACT TRUNCATED AT 400 WORDS)

A new method for estimating the level of natural fertility in populations practicing birth control

Abstract The natural fertility schedule of a population is the schedule of age-specific marital fertility we would observe if no birth control were being practiced. In natural fertility (no birth control) populations we can observe the natural fertility schedule directly, but in populations practicing birth control the natural fertility schedule is disguised by the marital fertility rates in those age intervals in which control is exercised, the marital fertility rates being below the natural rates. This paper elaborates a method for estimating the natural fertility schedules of populations practicing birth control. Two alternative models are presented, one nonlinear and one linear. The use of these models is then illustrated with reference to the Old Order Amish population of Lancaster County, Pennsylvania. It is shown that for Amish women born between 1860 and 1900 both the nonlinear and linear models yield an estimate of .843 as the ratio between Amish and Hutterite natural fertility schedules.