Jane Menken

Age, Period, and Cohort Effects in Demography: A Review

This paper considers only the vital events of demographic measurement, the factors influencing the rate at which those events occur and then investigates the consequences of patterns of these events. It reviews the state of the art of age, period and cohort analysis for demographic dependent variables. Major examples of such analyses are given in both mortality and fertility studies. In the area of mortality the conventional approach to such analysis apears to be well suited to a wide range of applications yielding useful results. The reasons for this suitability are: early childhood experience is important in many major disease and death processes, so that cohorts are legitimately viewed as acquiring early on a certain fixed susceptibility; data sometimes stretch back far enough that stationary standards of age patterns can be developed empirically, and applied to later experience; and, logarithmic or logistic transformations linearize comparisons of age schedules or mortality so that standard statistical procedures are suitable. Applications of age, period, and cohort analysis are not always routine; external constraints are required, in the form of theoretically based and mathematically expressed age patterns of mortality, in order to distinguish effectively between period and cohort effects. A set of models of age patterns of mortality that are based on cohort as well as period experience could be constructed with useful applications. With fertility analysis the conventional approach is much less suitable. Once goal directed behavior is introduced, empirical examinations must be based on theories or assumptions about how such goals are formulated and pursued. Conventional analysis might suffice only if one is prepared to accept the assumption that all pertinent goals and strategies are formulated before the initiation of childbearing and remain unaffected by subsequent events. This assumption is untenable for modern developed populations and the forms of analysis appropriate to age period cohort investigations of fertility will have to develop along with theories of reproductive behavior.

Demographic foundations of family change.

As a result of the decline in mortality current cohorts have the theoretical potential to spend more time in the conjugal family statuses of parent child and spouse. To determine the extent to which this potential has been realized in the US a model was developed to simulate demographic conditions in 1800 1900 1960 and 1980 and then calculate how long women remained in the various family statuses in each historical period. Despite declines in fertility and increases in divorce the 1980 simulated cohort spent more years as children as parents as currently married spouses and in conjugal family units than people in the 1800 and 1900 cohorts. The 1980 cohort spent about 27 adult years with spouse and surviving children--4 years more than the 1800 cohort. The proportion of adult life spent in the status as child of at least one parent over 65 years of age has increased from 15% in 1800 to 29% in 1980. On the other hand time spent as a spouse is far lower than its potential. If 1800 marriage patterns obtained concurrently with 1980 mortality years married would increase from 27 to 45 whereas the actual 1980 figure is 35 years. If 1800 mortality were combined with 1980 marriage patterns 15 of the 1980 years married would be lost showing the preponderant effect of mortality. Compared to earlier cohorts contemporary cohorts have chosen to spend a smaller proportion of their adult years as a spouse as a parent of a young child and as a member of a conjugal family unit. Time spent with spouse and children declined from 56% of adult lifetime in 1800 to 43% in 1980. The proportion of lifetime spent as a parent of a child under 18 years has declined by half and that spent as a parent of a child under 5 years has declined by 2/3 as a result of reductions in family size preferences. More attention is urged to the changing time spent in various family statuses as a potential source of social change.

Age and fertility: How late can you wait?

In the Presidential Adress presented at the Annual Meeting of the Population Association of America Boston Massachusetts March 28-30 1985 the 2 main issues discussed were infertility and demographic changes. Recent studies most notably one published in the New England Journal of Medicine indicate that fecundity declines more rapidly with age than previously thought. More and more women in their late 20s and early 30s are childless but when polled many do not plan on remaining childless. Given the recent studies it is possible that more of these women will remain childless than expect to do so. The present furor over infertility may be exaggerated however. Studies of historical data for populations with little evidence of fertility control have reviewed changes in fertility with age and found that biological infertility rises only moderately with age until the late 30s or early 40s. There are 3 other reasons why infertility may be overestimated. In the past 25 years emphasis has been placed on controlling unwanted fertility not on infertility. 2nd there may be a compositional explanation: among the smaller group who want to have children there may be a higher proportion who have infertility problems. 3rd current medical practices tend to lead to exaggerated diagnoses of infertility. To study demographic change computer simulations were used to examine whether or not a woman in a cohort was alive at each age and the status she occupied in her family of origin and adulthood. The number of years children have living parents has increased due mainly to lower mortality rates. There has also been an increase since 1900 in the number of years that women are responsible for children and elderly parents. This figure was highest in the 60s when women also entered the labor force in increasing numbers. This led to a higher demand on the public sector as the financial responsibility of care for the elderly shifted away from the private institution of the family. This generations solution as seen in the lower figures in 1980 for years of responsibility to children and parents seems to be to have fewer children and to have them later in life.

Proportional hazards life table models: an illustrative analysis of socio-demographic influences on marriage dissolution in the United States.

The proportional hazards life table is a recently developed approach to the analysis of survival data when mortality risks vary among individuals. It assumes that at a given age (or duration since the start of a life) the force of mortality is a constant (specific to that age) multiplied by a proportionality factor which is determined by the characteristics of the individual and does not change unless these covariates do. In this paper, the method is reviewed for the case where the covariates are fixed at the start of the lifetime and illustrated by an application to marital dissolution in the United States.

Early childbearing and subsequent fertility.

Data from the National Survey of Family Growth which was conducted by the U.S. National Center for Health Statistics in 1973-1974 were searched for the relationship between early childbearing and subsequent fertility. The data show that early age at 1st birth especially if the mother is a teenager correlates with more rapid and higher levels of subsequent fertility. Women who start their childbearing in their teen years tend to have more unwanted and out-of-wedlock births. This is true within racial educational and religious subgroups. Marital status at 1st birth had little effect on subsequent fertility. Previously observed fertility differences between blacks and whites are largely attributable to differences in age at 1st birth. Blacks in fact seem to have slowed the pace of their current childbearing perhaps due to the growth of federally subsidized family planning programs in recent years. There are still differences in subsequent fertility between Catholics and non-Catholics when age at 1st birth is controlled. Subsequent childbearing seems to have no relation to whether the 1st birth was considered wanted or unwanted. Education proves to be an important predictor of contraceptive use and success. The reasons for this are complex.

Ageing and adult health status in eight lower-income countries : the INDEPTH WHO-SAGE collaboration

Background: Globally, ageing impacts all countries, with a majority of older persons residing in lower- and middle-income countries now and into the future. An understanding of the health and well-being of these ageing populations is important for policy and planning; however, research on ageing and adult health that informs policy predominantly comes from higher-income countries. A collaboration between the WHO Study on global AGEing and adult health (SAGE) and International Network for the Demographic Evaluation of Populations and Their Health in developing countries (INDEPTH), with support from the US National Institute on Aging (NIA) and the Swedish Council for Working Life and Social Research (FAS), has resulted in valuable health, disability and well-being information through a first wave of data collection in 2006–2007 from field sites in South Africa, Tanzania, Kenya, Ghana, Viet Nam, Bangladesh, Indonesia and India. Objective: To provide an overview of the demographic and health characteristics of participating countries, describe the research collaboration and introduce the first dataset and outputs. Methods: Data from two SAGE survey modules implemented in eight Health and Demographic Surveillance Systems (HDSS) were merged with core HDSS data to produce a summary dataset for the site-specific and cross-site analyses described in this supplement. Each participating HDSS site used standardised training materials and survey instruments. Face-to-face interviews were conducted. Ethical clearance was obtained from WHO and the local ethical authority for each participating HDSS site. Results: People aged 50 years and over in the eight participating countries represent over 15% of the current global older population, and is projected to reach 23% by 2030. The Asian HDSS sites have a larger proportion of burden of disease from non-communicable diseases and injuries relative to their African counterparts. A pooled sample of over 46,000 persons aged 50 and over from these eight HDSS sites was produced. The SAGE modules resulted in self-reported health, health status, functioning (from the WHO Disability Assessment Scale (WHODAS-II)) and well-being (from the WHO Quality of Life instrument (WHOQoL) variables). The HDSS databases contributed age, sex, marital status, education, socio-economic status and household size variables. Conclusion: The INDEPTH WHO–SAGE collaboration demonstrates the value and future possibilities for this type of research in informing policy and planning for a number of countries. This INDEPTH WHO–SAGE dataset will be placed in the public domain together with this open-access supplement and will be available through the GHA website (www.globalhealthaction.net) and other repositories. An improved dataset is being developed containing supplementary HDSS variables and vignette-adjusted health variables. This living collaboration is now preparing for a next wave of data collection. Access the supplementary material to this article: INDEPTH WHO-SAGE questionnaire (including variants of vignettes), a data dictionary and a password-protected dataset (see Supplementary files under Reading Tools online). To obtain a password for the dataset, please send a request with ‘SAGE data’ as its subject, detailing how you propose to use the data, to global.health@epiph.umu.se

Famines in Historical Perspective

This paper presents a challenge to the Malthusian view that historically famines have been a consequence of overpopulation relative to available resources. The Malthusian paradigm has been used to explain periods of population stability in the past and to account for modern population growth once subsistence crises were no longer a threat. Using premodern Asian populations as a model the sequence of demographic changes that could be expected to have accompanied and followed famines in the past are simulated in this paper. Critical to an assessment of the demographic importance of famine is an understanding of the time it takes for a population to recover: the shorter the time to achieve prefamine population size the more frequent famines would have to be to serve as a check to population growth. The information used in this simulation of the consequences of famine includes the age composition of the population at the time the crisis began and the magnitude duration and age pattern of changes in fertility and mortality rates when the crisis was over. The simulation results suggest that the only way famines and other mortality crises could have been a major deterrent to longterm population growth is if they occurred with frequency and severity far exceeding that recorded in history. The simulations indicate that the low rate of natural increase set by normal levels of fertility and mortality constitutes a more plausible explanation for the longterm slow growth of large populations in the past. Moreover the control of normal mortality is most likely responsible for the onset of modern population growth. In general famines were sufficiently localized and brief that their demographic impact was modest and even relatively low rates of natural increase quickly erased their impact. Unless famine intensities increase greatly there is little likelihood that famines will be a major determinant of population growth in the future. Overall the social and economic effects of famines have probably been more farreaching than their demographic impact.

Prevalence of HIV among those 15 and older in rural South Africa

A greater knowledge of the burden of HIV in rural areas of Southern Africa is needed, especially among older adults. We conducted a cross-sectional biomarker survey in the rural South African Agincourt Health and Socio-demographic Surveillance site in 2010–2011 and estimated HIV prevalence and risk factors. Using an age–sex stratified random sample of ages 15+, a total of 5037 (65.7%) of a possible 7662 individuals were located and 4362 (86.6%) consented to HIV testing. HIV prevalence was high (19.4%) and characterized by a large gender gap (10.6% for men and 23.9% for women). Rates peaked at 45.3% among men and 46.1% among women – both at ages 35–39. Compared with a similar study in the rural KwaZulu-Natal Province, South Africa, peak prevalence occurred at later ages, and HIV prevalence was higher among older adults – with rates above 15% for men and 10% for women through to age 70. High prevalence continues to characterize Southern Africa, and recent evidence confirms that older adults cannot be excluded from policy considerations. The high prevalence among older adults suggests likely HIV infection at older ages. Prevention activities need to expand to older adults to reduce new infections. Treatment will be complicated by increased risk of noncommunicable diseases and by increasing numbers of older people living with HIV.

The nutrition fertility link: an evaluation of the evidence.

The determinants of age-specific marital fertility rates in non-contracepting populations are discussed and the physiological link between nutrition and reproduction in women who are chronically malnourished rather than famished are examined. Initial attention is directed to menarche and menopause for these set the outer bounds to the time of childbearing. This is followed by a discussion of the determinants of birth interval. Birth interval and its components are of primary concern because if populations are to fit different numbers of births into similar childbearing spans the intervals between these births must vary. There is reason to believe that the age of menarche might be affected by the level of nutrition. In some countries where nutrition has remained inadequate the age of menarche is comparatively high. Like the age at menarche the age at menopause is likely to have had little influence on overall fertility in Europe. The meager evidence available suggests that the mean age at last birth appears to vary little among populations which do not practice birth control. The length of postpartum anovulation the period after a birth when there is no ovulation is a primary determinant of variation in the length of the birth interval. Research in which the direct measurement of the duration of breastfeeding was possible found that breastfeeding was the primary determinant of the length of postpartum amenorrhea. Researchers in Bangladesh and in Guatemala have found a slight negative relation between nutritional status and the duration of postpartum amenorrhea.

The health and social consequences of teenage childbearing.

The proportion of teenage girls becoming pregnant is increasing the proportion of total births which are being born to women aged 15-19 is increasing and the proportion of out-of-wedlock births to teenagers is increasing. Nearly 60% of all births to 15-19 year olds are conceived out of wedlock and 27% are born out of wedlock. For both black and white teenagers stillbirths deaths in the 1st month and deaths during the 1st year are greater than for women in their 20s. For white teens the rates are twice those of women in their 20s; for blacks 1 1/2 times. This reflects the higher infant death rates for black women at all age groups. Maternal mortality low birth-weight babies complications of pregnancy such as toxemia prolonged labor and iron-deficiency anemia and defective infants are all more common among teenagers. Lack of prenatal care and poor diets play as great a part as physical immaturity for the late teens. More distressing is the social aspect of teenage childbearing. The teenage mother often discontinues h er education has another child quickly and is locked into a poverty syndrome. Teenage marriages break up faster than other marriages and teenage marriages in which the bride was pregnant at time of marriage have the highest divorce rate of all. The only bright note comes from a survey in Japan which shows that women who have their 1st baby before age 18 have 1/3 less chance of breast cancer than women who have their 1st baby after age 35. In fact not until after age 40 is the life and health of both mother and fetus in greater danger than it is in the teen-age pregnancy. If all births to women under 18 were eliminated the population would be 5% smaller in 50 years than if these rates remain constant. If all teenage births were eliminated the net reproduction rate would be reduced from 1.16 to just over the stationary value of 1.00.