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CHAPTER 7 ANSWERS
A7.1 As used in biodemography, “facultative” means that environmental influences cause mortality rates, or the trajectories of mortality rates, to have significant plasticity, rather than being
fixed like those predicted by Gompertz analysis. Although the overall population mortality rate
may take on a particular Gompertz pattern, there are subpopulations that will have significantly
different mortality rates. Many biodemographers believe that such subpopulations had a significant influence on the origins of human longevity
A7.2 Human mortality rates remain variable, even after the confounding factors of age-related
disease and the environment are eliminated. Experiments in Mediterranean fruit flies demonstrated that long-lived flies had different mortality rates than shorter-lived flies, suggesting that
different genotypes for longevity exist within a population—that is, demonstrating the plasticity
of genetic factors.
A7.3 The intelligence of early H. sapiens led to simple technologies, such as tools for hunting
and creating protection from the elements, that resulted in better nutrition and increased survival
of offspring. Parents could have fewer children and invest more resources into those children,
resulting in better-quality offspring—which would have increased longevity.
A7.4 Studies evaluating the correlation between average life span of parents and average life
span of offspring found very weak correlations. Results from studies of monozygotic and dizygotic twins suggested that heredity accounts for no more that 25% of the variance in life span.
Moreover, average life span began to increase when humans began to improve environmental
conditions (such as water supply and sanitation) that had been detrimental to health.
A7.5 Human intelligence, as based on brain size, took a substantial leap forward when our species parted from H. erectus. The life-span difference between A. afarensis and H. erectus is approximately 15 years, with a brain-weight increase of 450 grams that took place over 3 million years.
H. sapiens developed a brain weight almost 400 grams greater than that of H. erectus in under
700,000 years; this growth in brain weight translated into a gain in life span of 60–70 years.
A7.6 The microscope allowed scientists to visualize bacteria. Throughout the eighteenth and
nineteenth centuries, biologists would discover that bacteria were the cause of infections.
Governments began to enact public health policies aimed at reducing exposure to harmful bacteria, which reduced the prevalence of infection; this increased the human life span.
A7.7 (1) Louis Pasteur provided the methods for producing large quantities of vaccines in the
1880s, which, by 1900–1910, led to more efficient control of infectious diseases. Also, procedures
were developed for ensuring safe drinking water and for treatment of sewage, leading to reduced
exposure to bacteria. (2) In the 1920s, women began having babies in hospitals, which reduced
the rate of maternal deaths during childbirth.
A7.8 Infant mortality rate began to flatten in 1970, but life expectancy at birth continued to
increase at the same rate observed in 1910. Thus, decreasing infant mortality rate was no longer
accounting for the increase in life span. Rather, increases in life expectancy now resulted from
medical treatments for the diseases of older age. An example is the improved prevention, early
detection, and successful medical and surgical treatment of coronary heart disease.
A7.9 (1) The ratio of workers (paying into the Social Security system) to retirees (receiving Social
Security benefits) has decreased. (2) The proportion of the population over retirement age has
increased. (3) Age-specific life expectancy for those 65 and older has increased.
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A7.10 The gender gap is decreasing. The most likely reason is that the number of men participating in behaviors that shorten life span, such as smoking, has been declining. Also, men are at a
greater risk for heart disease at younger ages than are women. Prior to the twenty first century,
men would die from heart disease earlier than women causing a gender gap in life expectancy.
With the introduction of modern treatments for heart disease, men who would once have died are
now able to be treated early and live significantly longer. In turn, the gender gap is narrowing.