生育率演变的生态学逻辑及人口增长的长期趋势

人口增长的长期过程一直是充满困惑与引发争论的话题,将人类复归到生态系统的普通成员,按照生态学逻辑构建一个由替代生育率内生引导、人口容量外生制约的人口增长新模型,以代替用具体社会经济因子解释短周期人口变动的传统思路,探讨生育率转变的一般模式及人口发展的长期趋势。工业革命以来,全球人口已经或正在经历着第一次、第二次生育率转变,全球生育率演变可以聚类为欧美、亚非拉、撒哈拉以南非洲和东亚四种区域模式;在计划生育政策的推动下,我国在短短的三十年内完整经历了两次生育率转变。极限替代生育率是生育率演变的长期目标,但当前已有一些国家跌破更替水平,这也许会成为各个国家的普遍经历,预示着人口容量约束的日益显性化;世界及主要国家的人口规模正在日益逼近其容量极限,并会在惯性驱动下突破容量限制,达到峰值后再以负增长方式趋近人口容量,同期的生育率也将向极限替代生育率递增复归。按趋势模拟世界和中国的可持续人口容量分别约为65亿人和12亿人。研究设计出测量人口增长惯性的新指标——人口增长惯性系数,它是生育率与实时替代生育率之比或出生率与死亡率之比,相比常用的人口惯性因子更为简便易行。     

Completing the Demographic Transition

Despite ongoing declines in fertility in many countries, the population of the world is experiencing a period of rapid expansion, and its size is expected to reach 10 billion by the end of the demographic transition. Three causes of this growth are identified and quantified: 1) fertility above the replacement level of two surviving children per woman, 2) continuing declines in mortality, and 3) population momentum resulting from a young age structure. A set of simple analytic expressions is proposed for estimating these factors from standard demographic indicators. Population momentum is shown to be the main cause of future growth in most countries and regions.     

How High is Hispanic/Mexican Fertility in the United States? Immigration and Tempo Considerations

In this article, I demonstrate that the apparently much higher fertility of Hispanic/Mexican women in the United States is almost exclusively the product of period estimates obtained for immigrant women and that period measures of immigrant fertility suffer from three serious sources of bias that together significantly overstate fertility levels: difficulties in estimating the size of immigrant groups; the tendency for migration to occur at a particular stage in life; and, most importantly, the tendency for women to have a birth soon after migration. When these sources of bias are taken into consideration, the fertility of native Hispanic/Mexican women is very close to replacement level. In addition, the completed fertility of immigrant women in the United States is dramatically lower than the level obtained from period calculations. Findings are consistent with classical theories of immigrant assimilation but are a striking departure from the patterns found in previous studies and published statistics. The main implication is that without a significant change in immigration levels, current projections based on the premise of high Hispanic fertility are likely to considerably exaggerate Hispanic population growth, its impact on the ethno-racial profile of the country, and its potential to counteract population aging.     

Family Benefits and Fertility: An Econometric Analysis

A population's growth potential is significantly underestimated by conventional calculations of population momentum which assume an immediate drop to replacement level fertility. Here we assume that the growth rate of births linearly declines to zero over a specified time interval, and find simple and intuitively meaningful expressions for the size of the ultimate birth cohort and the resultant population momentum. In particular, we find that the increase in the number of births over the transition is equal to growth at the initial rate for half the time needed to attain a constant birth level. Thus our formula readily calculates the growth potential of a population under a gradual approach to stationarity without the need for a numerical projection. Calculations for actual and hypothetical populations are presented to show the demographic impact of such gradual approaches to zero growth.     

Modeling momentum in gradual demographic

The analysis of population momentum following a gradual decline in fertility to replacement level provides valuable insights into prospects for future population growth. Here, we extend recent work in the area by applying a new form of the quadratic hyperstable (QH) model, which relates exponentially changing fertility to the resultant exponentiated quadratic birth sequence. Modeling gradual transitions from an initial stable population to an ultimate stationary population indicates that such declines in fertility increase momentum by a product of two factors. The first factor is a previously noted continuation of stable growth for half the period of decline. The second is a not previously appreciated offsetting factor that reflects the interaction between the decline in fertility, the changing age pattern of fertility, and the changing age composition of the population. Numerical examples using both hypothetical and actual populations demonstrate that for declines of any length, the product of the two factors yields momentum values that closely agree with the results of population projections. The QH model can examine monotonic transitions between any two sets of constant vital rates. As a generalization of the fixed-rate stable model, it has great potential value in numerous areas of demographic analysis.     

Population change in Meso-America: The Tip of the demographic Iceberg

This paper examines the demographic situation in Mexico and Central America (Meso-America) and looks at the momentum for growth implied by recent population shifts. Despite successes in reducing fertility levels in many Meso-American countries, the dramatic declines in mortality among infants and children have given rise to a "death dearth" that is contributing to a new "baby boom" in these countries; thus our subtitle, "The Tip of the Iceberg."The impacts of such population growth on education, the economy and migration are considered in some detail. The anticipated inability of country economies to provide jobs for the thousands of young adults entering the labor force in future years could result in significant increases in the number seeking to migrate in a northerly direction. Thus, the United States is also vitally interested in the demographic shifts taking place south of its border.The need for a more unified regional approach to some of the social and economic problems facing these nations is pointed out as is the need for a more rational immigration policy on the part of the United States in light of the potential increase in immigration in future years.     

中国妇女生育水平变动考证和未来人口发展的策略选择

1990年以来各类人口调查和相关学者的研究结果显示,目前的低生育水平的实现有外在政策强力的作用,更有中国社会经济发展内生化的作用;真实的生育水平既不是调查包括人口普查所显示的那样低(低于1.5),也不是有关部门和政府所想象的那样高。调查显示农村居民平均生育意愿不会高于2个孩子,这是生育率持续稳定的根本。众所周知,为了保证人口稳定持续的发展,应使妇女生育率保持更替水平或至少接近更替水平。从人口发展规律和人口发展与社会经济发展相适应的规律出发,结合当前我国居民生育意愿和生育水平走低的实际,我们应该抓住生育政策调整的最佳时期,调整应该在最佳时期即未来5-10年内完成。     

World population in transition

The world's population growth rate peaked at slightly over 2%/year in the late 1960s and in 1986 is down to 1.7% and falling. Annual numbers added continue to rise because these rates apply to a very large base, 4.9 billion in 1986. According to UN medium variant projections, world population growth will peak at 89 million/year in the late 1990s and then taper off until world population stabilizes in the late decade of the 21st century at about 10.2 billion. Close to 95% of this growth is occurring in less developed countries (LDCs) of Africa, Asia (minus Japan), and Latin America. LDC fertility rates are declining, except in sub-Saharan Africa and parts of Latin America and South Asia, but most have far to go to reach the replacement level of 2.1 births/woman. Fertility is below replacement in virtually all more developed countries. For LDCs, large numbers will be added before stabilization even after attainment of replacement level fertility because of the demographic momentum built into their large and young population bases. This complicates efforts to bridge gaps between living standards in LDCs and industrialized countries. From a new debate about whether rapid population growth deters or stimulates economic growth, a more integrated view has emerged. This view recognizes the complementary relationship between efforts to slow population growth and other development efforts; e.g., to improve health and education, upgrade women's status, increase productivity. Most effective in the increased contraceptive prevalence and fertility declines seen in many LDCs has been the combination of organized programs to increase access to family planning information and supplies with socioeconomic development that enhances the desire for smaller families.     

Momentum and the growth-free segment of a population

In the scenario of a sudden drop in fertility to replacement level. Preston (1986) argued that the population segment under age T, the length of generation, remained growth-free. Here we first present a new relationship for the momentum of any observed population as the ratio of 1) the proportion of the observed population under the mean age at childbearing to 2) the proportion of its life table population under that mean age. We then use that relationship to demonstrate Preston's approximation. Growth factors for other population segments are also presented. When the initial population is stable, momentum can be approximated as a function of the net reproduction rate alone.     

The Momentum of Real Population Under Linear Fertility Transition

Population momentum is the ratio of a population's ultimate size after a demographic transition to its initial size before the transition. For stable population and instantaneous drop to replacement fertility, Keyfitz found a simple formula for the momentum. Although Keyfitz's formula has been extended to cover gradual demographic transitions, the stable initial population, however, is still an obstacle to approaching reality. Using the solution of the Lotka equation with time-varying vital rates and extending it to negative values of time, exact and accurate formulae for the momentum of any initial population that undergoes a linear fertility transition are expressed. Examples using the data from the United Nations indicate that the accurate formula works well for the age structures that are far from stable such as the more and less developed regions, as well as that close to stable such as the least developed region.     

New perspective in population assistance

The United States Agency for International Development (AID), in a proposal to Congress, has suggested that before assistance is extended to developing countries, its impact on the country's population growth should be considered and development projects accepted that contribute directly to fertility declines. Foreign development projects will be examined for their direct and indirect impact on population growth. Some projects related to education, employment and economic development may be found, by their very nature, to encourage fertility decline and emphasized as a result of the impact examinations. Other projects may be changed to stress the features that are conducive to slowing population growth. The population impact examination proposal has been included in this year's legislation to authorize the continuation of United States development assistance programs for the fiscal year 1978.     

Population momentum expresses population aging

Population momentum and population aging occur when an initially growing population experiences a reduction in fertility to replacement level. Conceptually and empirically, momentum and aging express the same change, albeit on different scales. Fundamentally, they are two manifestations of the underlying process of demographic transformation. We consider three measures of aging over the transition to stationarity: the increase in mean population age, the decrease in the proportion under age 30, and the increase in the proportion over age 65. The three measures of aging are highly correlated, though the relationship to momentum is weakest for the increase in the proportion over age 65. We find that momentum is linearly related to aging. In both model and actual populations, a one-year increase in mean age translates into about 4.5% more population growth. The population below age 30 does not grow over the transition to stationarity, and the ratio of initial to ultimate proportions under age 30 is virtually identical to momentum.     

Future population growth: researchers in three countries quantify causes and propose policies

This article discusses Population Council analyses conducted by social scientists from India, Kenya, and the Philippines. These scientists agreed that population momentum would continue to increase population size, and that governments must strengthen and create a range of economic, health, and social programs and policies to slow population growth. Multiple approaches will be needed. John Bongaarts is credited with being the first to identify the key role of population momentum and to decompose growth into unwanted fertility, high desired fertility, and population momentum. Unwanted fertility is responsible for about 19% of projected population growth in India, 26% in Kenya, and 16% in the Philippines. High wanted fertility accounts for 20% of future growth in India, 6% in Kenya, and 19% in the Philippines. Population momentum can account for under 50% or over 90% of growth. Unwanted fertility can be addressed by fulfilling unmet need and increasing knowledge of methods, reducing the fear of side effects and disapproval, and eliminating poor service. Family planning programs need to be strengthened and integrated with maternal and child health services. Preferred and actual family sizes can be reduced by lowering infant mortality by means of increasing infant and child health services and girls' educational attainment. Population momentum can be addressed by delaying age at marriage and childbearing through improving social conditions. Investments in human development through education, training, and income generation can create the conditions for slowing population growth. Countries should decompose population growth into its components of unwanted and high wanted fertility and population momentum as a means of distributing resources most effectively.     

Shifting shares of the population and U.S. fertility

American fertility, as measured by the total fertility rate, apparently has been climbing since 1988 and could approach replacement level in 1990. Three possible explanations are explored: incorrect denominators, actual fertility increase, and changing ethnic proportions of the population. Using California data as a surrogate for the nation, it is found that at least part of the gain in fertility is attributable to what is called "shifting shares." Given that minorities have higher fertility than the majority, as these groups increase their share of the population, the nation's fertility can be expected to continue climbing. Any attempt to attain zero population growth must therefore be postponed indefinitely.     

Is Low Fertility a Temporary Phenomenon in the European Union?

This article addresses two questions: (i) will the mere end of further postponement of fertility in the EU-countries lead to an appreciable rise in European fertility and bring total fertility rates closer to replacement level, as witnessed in the United States? and (ii) what are the chances that such a stop to postponement is imminent? The answer to the first question is positive, but only if there is enough recuperation of fertility at older ages. Translated in the Bongaarts–Feeney framework, this condition means that the birth-order-specific TFRs would indeed remain constant. In the absence of full recuperation at older ages, the induced rise in the national TFRs would be trivial and by no means restore period and cohort TFRs to replacement levels. Hence, caution is needed when using the Bongaarts–Feeney adjusted TFRs for pro-jective purposes. With respect to the second question, female education and employment trends in tandem with ideational and family disruption data are used to speculate about the prospects for such an end to further fertility postponement and for fertility increases at older ages. Strikingly, EU-countries that have the greatest potential for still later fertility are also the ones with very low TFRs (below 1.5) at present. The overall conclusion is that low to very low fertility in the EU is unlikely to be a temporary phenomenon     

Reproductive ideals and educational attainment among white Americans, 1943-1960

Abstract Would the persistent inverse relation between educational attainment and family size in the United States be removed if actual fertility were equal to ideal? Data on ideal family size from 10 national surveys among white Americans of both sexes (from 1943 to 1960) show that gradeschool level respondents have higher ideals than the more educated even when age, religious affiliation, and farm residence are used as controls. Comparison of these ideals with the actual family size or ever-fertile women in the United States indicates that, on the average, the actual family size of all major educational groups falls below the ideal, but the college-educated are furthest from their ideal. If this group lessened the gap between actual and ideal family size, the educational differential in fertility would decrease, but at the price of increasing the rate of population growth.     

Stages of the Demographic Transition from a Child's Perspective: Family Size,Cohort Size,and Children's Resources

This article provides a new characterization of stages of the demographic transition from the perspective of children competing for resources within families and cohorts. In Stage 1 falling mortality increases the size of both families and birth cohorts. In Stage 2 falling fertility overtakes falling mortality to reduce family size, but population momentum causes continued growth in cohort size. In Stage 3 falling fertility overtakes population momentum to produce declining cohort size. We apply our framework to census microdata for eight countries and to United Nations population projections for a larger set of countries. The results suggest that most countries spend two to three decades in Stage 2, with declining family size offset by increasing cohort size. From the perspective of children aged 9–11, many countries enter Stage 3 between 2000 and 2010. Other countries, especially in Africa, will continue to experience increasing cohort size for several more decades.     

The Census Bureau on Prospects for US Population Growth in the Twenty‐First Century

The U.S. Census Bureau periodically releases projections of the US resident population, detailed by age, sex, race, and Hispanic origin. The most recent of these, issued 13 January 2000, for the first time extend to the year 2100 and also include information on the foreign‐bom population. (Earlier projections were carried up to 2080.) The extensive tabulations presenting the new set, and detailed explanation of the methodology and the assumptions underlying the projections, are accessible at the Census Bureau's web site: http://www.census.gov . A brief summary of some of the main results of these projections is reproduced below from United States Department of Commerce News, Washington, DC 20230. (The Census Bureau is an agency of the Department of Commerce.) Uncertainties as to future trends in fertility, mortality, and net migration over a period of some 100 years are very great, as is illustrated by the massive difference in the projected size of the population for 2100 in the three variants produced. The “middle” projected population figure of 571 million (which represents a growth of some 109 percent over its current level) is bracketed by “lowest” and “highest” alternative projections of 283 million and 1.18 billion, respectively. With somewhat lesser force, the point also applies to the 50‐year time span considered in the well‐known country‐by‐country projections of the United Nations. These projections are also detailed in three variants: low, middle, and high. The UN projections (last revised in 1998) envisage less rapid growth in the United States during the first part of the twenty‐first century than do the Census Bureau's. The projected population figures for 2050 in the three variants (low, middle, and high) are as follows (in millions):

Population momentum for gradual demographic transitions

Population momentum is the ratio of a population's ultimate size after a demographic transition to its initial size before the transition. For an instantaneous drop to replacement fertility, Nathan Keyfitz found a simple expression M K for the momentum. However, as Keyfitz pointed out, `no one thinks that any country will drop immediately to stationary reproduction patterns'. We present results concerning the momentum of a population whose demographic transition is completed within a finite time. First, we provide an exact analytical formula for such a population's momentum. Second, for rapid fertility transitions, we obtain a simple exact expression for momentum that reduces to Keyfitz's M K if the transition is instantaneous. We show, by example, that our simpler formulae are accurate approximations to population momentum for transitions that take as long as 100 years. Finally, we show that the speed of fertility decline makes a substantial difference to population momentum.     

The momentum of mortality change

Mortality change is not usually assigned much importance as a source of population growth when future population trends are discussed. Yet it can make a significant contribution to population momentum. In populations that have experienced mortality change, cohort survivorship will continue varying for some time even if period mortality rates become constant. This continuing change in cohort survivorship can create a significant degree of mortality-induced population change, a process we call the 'momentum of mortality change'. The momentum of mortality change can be estimated by taking the ratio of e0 (the period life expectancy at birth) to CAL (the cross-sectional average length of life) for a given year. In industrialized nations, the momentum of mortality change can attenuate the negative effect on population growth of declining fertility or sustained below-replacement fertility. In India, where population momentum has a value of 1.436, the momentum of mortality change is the greatest contributor to its value.