This article investigates the fertility of Danish twins born during the periods 1870-1910 and 1953-64 in order to pursue two central questions for understanding human reproduction: Do genetic dispositions influence fertility and fertility-related behavior? Does the relevance of the "nature versus nurture" debate shift over time or with demographic regimes? The authors find that genetic influences on fertility exist, but that their relative magnitude and pattern ale contingent on gender and on the socioeconomic environment experienced by cohorts. Among females born in 1880-90 and after 1955, about 30-50 percent of the variance in fertility is due to genetic influences; these influences are substantially smaller for earlier and for interim birth cohorts. Male fertility is generally subject to smaller genetic and larger shared-environment effects than female fertility. Because genetic effects are most prevalent in situations with deliberately controlled fertility and relatively egalitarian socioeconomic opportunities, the authors propose that the genetic dispositions affect primarily fertility behavior and motivations for having children. Analyses of fertility motivations, measured by age of first attempt to have a child, support this interpretation.

Service industries now dominate the economies of large cities. But recent advances in information technology have made electronic delivery of some services possible, reducing the importance for these industries of face-to-face interaction. The future location of service industries, and therefore the future of cities, depends on whether services are attracted to cities even if the cost of delivering service output declines. This paper finds that the need to be near suppliers and customers accounts for much of the relationship between business service employment and county density. Industry-level information technology usage is associated with a reduction in the tendency of services to locate near suppliers and customers, but an increase in the tendency to locate near types of workers that the industry demands. On balance, information technology usage is associated with faster service employment growth in cities, especially medium-size and large cities with population up to 2.5 million.

This article uses linked county-level and individual-level economic, demographic, and social indicators drawn from three historical U.S, censuses to examine a construct that has been labeled labor market or opportunity structure in historical and demographic multilevel research. Findings are that turn-of-the-twentieth-century localities were diverse opportunity structures that systematically and differentially shaped older men's unemployment rates and their likelihood of work and retirement. This article also highlights issues that researchers might face as they work with linked census data-specifically multidimensionality of contextual influences with resultant collinearity and variable selection problems.

This article uses linked county-level and individual-level economic, demographic, and social indicators drawn from three historical U.S, censuses to examine a construct that has been labeled labor market or opportunity structure in historical and demographic multilevel research. Findings are that turn-of-the-twentieth-century localities were diverse opportunity structures that systematically and differentially shaped older men's unemployment rates and their likelihood of work and retirement. This article also highlights issues that researchers might face as they work with linked census data-specifically multidimensionality of contextual influences with resultant collinearity and variable selection problems.

I investigate the factors that fostered the rise in separate living quarters for older nonmarried women since mid-century by examining the impact of Old Age Assistance on living arrangements in 1940 and 1950. I find that Old Age Assistance substantially increased demand for separate living quarters, but that demand depended upon the rules of the program, in particular whether children were held legally responsible for the care of their aged parents. I argue that more than half of the decline in the fraction of older nonmarried women living with kin from 1950 to 1990 can be attributed to rising Social Security benefits and expanded eligibility and to the fact that Social Security benefits were given with no strings attached.

I show that the trend towards single households among older nonmarried women, the majority of whom were widows, has been ongoing only since 1940 and investigate the factors that fostered the rise in separate living quarters since mid-century by examining the impact of Old Age Assistance on living arrangements in 1940 and 1950. I find that Old Age Assistance substantially increased demand for separate living quarters, but that demand depended upon the rules of the program, in particular whether children were held legally responsible for the care of their aged parents. I argue that almost half of the decline in the fraction of older nonmarried women living with relatives from 1950 to 1990 can be attributed to rising Social Security benefits and expanded eligibility and to the fact that Social Security benefits were given with no strings attached.

This article examines the relationship between different measures of technological change and the change in the wage (employment) share of skilled workers. The capital/output ratio and the share of computer stock in total capital stock are used to examine the effect of both capital deepening and capital quality on work force structure. The results suggest that both capital deepening and investment in computers are positively correlated to the change in the wage share of skilled workers. Comparing the magnitude of different capital investment variables suggests that computer investment has a stronger effect on the change in the wage share of skilled workers than does noncomputer capital investment. In addition, this article examines the effect of other advanced manufacturing technologies, such as flexible manufacturing systems, robots, networks, and numerically controlled machines, on the share of skilled workers. The results indicate a positive effect of these technologies on the work force skill mix and that the effect is stronger over longer periods.

I demonstrate that although socioeconomic differences in birth weight have always been fairly small in the United States, they have narrowed since the beginning of the century. I argue that maternal height, and therefore the mother's nutritional status during her growing years, accounted for most of the socioeconomic differences in birth weight in the past, but not today, implying that in the past health inequality was transmitted across generations. I also show that children born at the beginning of this century compared favorably to modern populations in terms of birth weights, but suffered higher fetal and neonatal death rates because obstetrical and medical knowledge was poorer. In addition, often children in the past were at a disadvantage relative to children today because best practice resulted in insufficient feeding. The poor average health of past populations therefore originated in part in the first days of life.