The age pattern of human mortality can be characterized in various ways. Figure 4 shows the American mortality levels in 1900 and 1995 from three perspectives. The first panel shows death rates by age. These death rates are used to construct a life table, which describes the experience of a hypothetical cohort subject throughout its life to the death rates of a given year. Thus, the middle and last panels show the distribution of deaths and the proportion of survivors at each age among members of such a hypothetical cohort.
Together, these three panels illustrate some major features of the mortality decline that has taken place over this time interval. First, death rates have fallen across the age range, but they have fallen most sharply (in relative terms, since the graph has a semilogarithmic scale) at younger ages. The distribution of ages at death has shifted to the right and become much more compressed. At the same time, the survival curve has shifted to the right and become more "rectangular" in shape. This last change is often referred to as the "rectangularization" of the human survival curve.
It was once asserted that this process of rectangularization reflected the existence of biological limits affecting human longevity (21). This notion of limits to the human life span enjoys little empirical support, as discussed below. Nevertheless, the historical process of rectangularization was both real and extremely significant. It is perhaps best thought of as a "compression of mortality," as documented in the middle panel of Figure 4. As the average level of longevity has increased, so has our certainty about the timing of death.
One measure of this variability is the interquartile range of deaths in the life table or the age span of the middle 50% of deaths over the life course. In the 1750s, in Sweden, the life table interquartile range was about 65 years, so that deaths were spread out widely across the age range. The distribution of age at death became more and more compressed over the next two centuries until the life table interquartile range was around 15 years in industrialized countries by the 1950s. Since 1960, there has been little further reduction in the variability of age at death in the developed world even though the average age at death (as reflected in life expectancy at birth) has continued to increase (22).
Like the historic rise of life expectancy, this compression of mortality was due largely to the reduction of juvenile mortality. Once most juveniles had been saved from premature death, a pattern emerged in which deaths are concentrated in the older age ranges. As mortality falls today among the elderly, the
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When over eighty years of age, the poet Bryant said that he had added more than ten years to his life by taking a simple exercise while dressing in the morning. Those who knew Bryant and the facts of his life never doubted the truth of this statement.