Biodemography (“the mating of biology and demography”) is applied to the increased longevity of human life (fueled by aging baby-boomers, lower mortality rates since the ’50s, and increased chance of surviving).  The authors are trying to explain the “perplexing” decline in old-age mortality.

Prominent theory: we have genetically determined maximum life spans…”post-reproductive span of life should be short because there is no selection against mutations that are not expressed until reproductivity has ceased” (5)

The authors are testing new theories and developing new concepts due to the inadequacy of present ones

Mortality Deceleration
Hypothesis: mortality accelerates with age as reproduction declines.  They looked at fruitflies and discovered that mortality does not increase after “maturity” in all species: a “vexing” finding

• Further studies revealed that for all species for whom large cohorts have been followed to extinction, mortality decelerates (or even declines) at older ages
• In theory, older generations contribute to the propagation of their gene by contributing to the reproductive success of the younger generations; the “effective end of reproduction may be much later than indicated by fertility schedules” (7)

Biodemographic Explanations
How do we reconcile mortality deceleration and declining human mortality at older ages with a theory about aging (i.e. senescence)? Three concepts/hypotheses from biodemography:

1) Mortality correlation hypothesis: like cars, living organisms may face mechanical constraints that impose mortality correlations.  Perhaps both deceleration and mortality correlations are general properties of complex systems (7)

2) Heterogeneity in frailty hypothesis: a select subset of survivors results from frail individuals dying off, creating a problem for aging analyses.  Frailty models can and have been applied; verification of this hypothesis hinges on estimation of variation in frailty w/in a population

3) Induced demographic schedules: Lotka equation determines the growth rate of a population given age schedules of fertility and survival (8)…but the problem is the presumption of fixed fertility and survival schedules.  Alternative schedules can be induced by the environment

Recent Findings/Future Directions/Survivor Attributes

• “Stress-related genes and mechanisms may affect longevity across species” (8)
• “Individuals do not face fixed fertility and survival schedules, but dynamically adopt alternative schedules as the environment and the individual’s capabilities change” (8)
• “Fundamental insight”: research into interactions between fertility and longevity as well as genetic research vis-à-vis mortality trajectories are the future

“The concepts [the three described above] can be tied together by a general question: How important are an individual’s survival attributes (i.e. persistent characteristics, innate or acquired, that affect survival chances) as opposed to current conditions in determining the chance of death?” (9)