signed to reach Mars: consumer durables may be designed to last until the end of the warranty period. Such designs, however, generally result in substantial spans of life after the target age. The Pioneer space probe was still functioning when it escaped the Solar System; some washing machines and refrigerators continue to function years after their warranties have expired. A body design that gives an organism a good chance of surviving long enough to reproduce may be a sufficiently robust design that some of the organisms can survive long thereafter (Hayflick, 1994).

Automobiles are popular pieces of complicated equipment. They are sufficiently standardized that it is meaningfully possible to count their numbers on an age-specific (model-year) basis. Using automobile registration data gathered by the various states of the United States, R.L. Polk and Company did so for July 1, 1941, and then for July 1 of every year from 1947 to the present. These data, which are closely analogous to population count data collected by national statistical offices and census bureaus, have been assembled and analyzed by Vaupel and C.R. Owens (unpublished work). Using standard demographic methods, they estimated, among other summary statistics, the age-trajectory of mortality for automobiles in various cohorts and in various periods, as shown in Figures 2-6 and 2-7. Depending on the period or cohort, it was possible to compute central death rates—the number of deaths in an interval divided by the average number of individuals alive in the interval—from age 1 to an age up to age 17. Several aspects of the age-trajectories of automobile mortality are intriguing and are addressed by Vaupel and C.R. Owens.

What is important here is the deceleration and leveling off of mortality at older ages. If the titles and legends of Figures 2-6 and 2-7 were erased, an unsuspecting observer might think the trajectories pertained to Drosophila or nematode worms or some other living species. The trajectories are also somewhat similar to the human pattern at older ages.

The deceleration and leveling off of automobile mortality is partially due to heterogeneity—the composition of the automobile fleet changes with age because some kinds of cars are built better and last longer than others. As shown in Figure 2-8, however, even if a single make of car were chosen and a single model year were followed, there is still evidence of deceleration and leveling off. Among cars of the same make and model year—roughly analogous to genotype, although there are different models of the same make—there are certainly differences in "phenotype" (i.e., in observable characteristics of individuals), arising from various errors made in constructing particular cars, from the environment in which the car is driven, from the temperament of the car's owner, etc. Lemons driven on dirt roads by careless, impetuous drivers will tend to be selected out.

Beyond the effects of such compositional change, there are almost certainly "physiological and behavioral" changes that contribute to mortality deceleration. Older cars are repaired. Older cars may be driven less frequently and more



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