The unprecedented gains in life expectancy over the past 300 years, the reductions in disease prevalence, and the increasing age at onset of disability have all contributed to raise the number of years free of disease and disability that a person born today can expect to live. In addition, the development of cures for many conditions and the pro-vision of effective symptom management for those conditions that cannot be cured have eliminated or reduced significantly the age-specific rates of functional impairment that used to be associated with many diseases. The immediate effect of longer lives is that now more people will be able to use their accumulated experience longer, and that they are more likely to share more of their life span with their children and grandchildren. As a result of improvements in human physiology and major advances in medicine, the number of disability and symptom-free years of life that remain at any given age is now much larger than it has ever been. This creates strong incentives for individuals to undertake measures aimed at preserving physical functioning and cognitive ability, also referred to as investments in human capital. Individuals respond by under-taking more of these investments, which include purchases of preventive and rehabilitative medical services as well as the acquisition of new skills and knowledge. For instance, in 1910, only 13% of adults in the United States were high school graduates and only 3% were college graduates. By 1998, the comparable percentages were 83 and 24, respectively (Caplow et al., 2000). It is no coincidence that, at the beginning of the twenty-first century, healthcare and educational services constitute two of the fastest growing sectors of the US economy, as they do in most other OECD nations. Not only do these activities maintain or improve the quality of life but they also enhance labor productivity.
Productivity-induced demographic and economic change in the USA
The relationships between technological development, nutrition, body size, and economic change have become most apparent over the course of the past century. They are perhaps best illustrated by examining the consequences of the dramatic improvements in labor productivity experienced by the agricultural sector in the United States since the end of World War II. From 1948 to 1994, agricultural output more than doubled, expanding at an average annual rate of 1.9% (Ahearn et al., 1998). During the same period, total hours worked in agriculture, adjusted for quality, fell by more than two-thirds, or 2.7% annually.
These figures imply that between 1948 and 1994 US agricultural output per hour rose at an average rate of 4.6% per annum, a more than nine fold increase over the span of fifty years. This surge in agricultural labor productivity is attributable to steadily improving yields and an increase in the acreage cultivated per hour. For instance, the introduction of pesticides, herbicides, and fertilizer, combined with higher-yielding crop varieties raised the amount of potatoes per harvested acre by a factor of almost 2.5 between 1948 and 1994 (US Department of Agriculture, 2000). Similarly, the number of acres cultivated per hour has been raised dramatically by the mechanization of agriculture, at an average annual rate of about 3%. As agricultural labor became more productive, the numbers of annual hours per worker as well as the number of workers were cut without curtailing agricultural output. Although annual hours per agricultural worker declined by 1% per year, the number of agricultural workers fell even more rapidly, by 1.7% per year (Ahearn et al., 1998). Those workers who were released from the agricultural sector found employment in other sectors of the economy, where they helped to raise output of other goods that consumers wanted, or they stopped working altogether. The fraction of the labor force employed in agriculture fell from 13% in 1948 to 3.2% in 1998 (US Bureau of the Census, 1976; Braddock, 1999; Bureau of Labor Statistics, 2001). Despite the sharply declining number of hours worked, the growth of US agricultural output has been outpacing the growth of the population during the past 50 years. Whereas from 1948 to 1994 agricultural output grew by 1.9% annually, the population of the United States grew on average by 1.2% per annum (US Department of Commerce, 2000). As a result, agricultural output per capita increased at an annual rate of approximately 0.7%. Compounded over the second half of the twentieth century, therefore, agricultural output per capita, which can be used to assess a country’s capacity to supply its inhabitants with calories, increased by about 40%.
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