Some general relationships in population dynamics

This paper extends the Lotka system of stable population equations to any population. The authors present this new general system and describe its duality with the recent Preston-Coale system. They derive these results by considering the calculus of change on the surface of population density defined over age and time. They show that analysis of this Lexis surface leads to all the known fundamental relationships of the dynamics of single-region human populations, several interesting new relationships, and a duality between period and cohort life tables.     

Age-specific mortality trends in France and Italy since 1900: Period and cohort effects

Although France and Italy currently exhibit very similar expectations of life, their mortality patterns by age and sex are not the same. The differences were much greater at the beginning of the century than they are now. Graphical presentation of the data since 1900 by age, period and birth cohort is used to explore in detail the differences in trends and to bring out, in particular, differences between the two countries in period and cohort effects. In addition to providing more details on well-known period effects — the secular decline in mortality in both countries (with Italy largely ‘catching up’ with France), and the immediate effects of the world wars — similarities and differences in cohort effects are also apparent. The two countries exhibit clear differences not only in terms of immediate casualties of the wars, but also in terms of the long-term impact of unfavourable wartime living conditions on the cohorts most affected.     

Lively Questions for Demographers about Death at Older Ages

This is an expanded version of comments on the future of the demography of aging at an invited session of the 2008 annual meeting of the Population Association of America. In an introduction, John Haaga offers reasons for a revival of interest in population aging, including greater realization of plasticity in aging trajectories at both individual and societal levels. Linda Martin proposes that population scientists working in aging emulate those studying fertility and family planning in previous decades, learning from interventions (in this case, aimed at increasing retirement savings and reducing disability at older ages). Changes in family structure will increasingly affect new cohorts of the elderly, and Linda Waite speculates on the ways in which changes in the economy, medicine, and the legal environment could affect the social context for aging. Research on mortality at older ages is “alive and well” asserts James Vaupel, who sets out six large questions on mortality trends and differentials over time and across species. Lastly, Wolfgang Lutz expands the scope of projections, showing the considerable uncertainty about the timing and pace of population aging in the developing world and the effects on future elderly of the increases in educational attainment in much of the world during the second half of the twentieth century.     

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.     

Association of late childbearing with healthy longevity among the oldest-old in China

Statistical analysis of a large and unique longitudinal data-set demonstrates that childbearing after age 35 or 40 is associated with survival and healthy survival among very old Chinese women and men. The association is stronger for women than for men. The estimates are adjusted for a variety of confounding factors: demographic characteristics, family support, social connections, health practices, and health conditions. Further analysis based on an extension of the Fixed-Attributes Dynamics method shows that late childbearing is positively associated with long-term survival and healthy survival from ages 80-85 to 90-95 and 100-105. This association exists among oldest-old women and men, but, again, the effects are substantially stronger for women than for men. We discuss four possible factors that may explain why late childbearing affects healthy longevity at advanced ages: (1) social factors; (2) biological changes caused by late pregnancy and delivery; (3) genetic and other biological characteristics; and (4) selection.     

The impact of heterogeneity in individual frailty on the dynamics of mortality

Life table methods are developed for populations whose members differ in their endowment for longevity. Unlike standard methods, which ignore such heterogeneity, these methods use different calculations to construct cohort, period, and individual life tables. The results imply that standard methods overestimate current life expectancy and potential gains in life expectancy from health and safety interventions, while underestimating rates of individual aging, past progress in reducing mortality, and mortality differentials between pairs of populations. Calculations based on Swedish mortality data suggest that these errors may be important, especially in old age.     

Extremes are not normal: a reminder to demographers

Journal of Population Research - Demographers have always held great interest in extremal phenomena. Extreme value distributions are tailor made to model extremes but demographers do not often take...     

Heterogeneity's Ruses: Some Surprising Effects of Selection on Population Dynamics

As a cohort of people, animals, or machines ages, the individuals at highest risk tend to die or exit first. This differential selection can produce patterns of mortality for the population as a whole that are surprisingly different from the patterns for subpopulations or individuals. Naive acceptance of observed population patterns may lead to erroneous policy recommendations if an intervention depends on the response of individuals. Furthermore, because patterns at the individual level may be simpler than composite population patterns, both theoretical and empirical research may be unnecessarily complicated by failure to recognize the effects of heterogeneity.     

Targeting lifesaving: Demographic linkages between population structure and life expectancy

Life expectancy in a heterogeneous population can be increased by lowering mortality rates or by averting deaths at different ages, from different causes, or for different groups, as well as by changing the proportions of individuals in various risk groups, perhaps by altering the transition rates between groups. Understanding how such changes in population structure affect life expectancy is useful in evaluating alternative lifesaving policies.