An Evolutionary Ecological Perspective on Demographic Transitions: Modeling Multiple Currencies BOBBI S. LOW, 1 CARL P. SIMON, 2 AND KERMYT G. ANDERSON 3 1 School of Natural Resources and Environment Population Studies Center, The University of Michigan, Ann Arbor, Michigan 2 Department of Mathematics, School for Public Policy, and Center for the Study of Complex Systems, The University of Michigan, Ann Arbor, Michigan 3 Population Studies Center, Institute for Social Research, The University of Michigan, Ann Arbor, Michigan ABSTRACT Life history theory postulates tradeoffs of current versus future reproduction; today women face evolutionarily novel versions of these tradeoffs. Optimal age at first birth is the result of tradeoffs in fertility and mortality; ceteris paribus, early reproduction is advantageous. Yet modern women in developed nations experience relatively late first births; they appear to be trading off socioeconomic status and the paths to raised SES, education and work, against early fertility. Here, [1] using delineating parameter values drawn from data in the literature, we model these tradeoffs to determine how much socioeconomic advantage will compensate for delayed first births and lower lifetime fertility; and [2] we examine the effects of work and education on women’s lifetime and age- specific fertility using data from seven cohorts in the Panel Study of Income Dynamics (PSID). Am. J. Hum. Biol. 14:149–167, 2002. Ó 2002 Wiley-Liss, Inc. More is known about human populations than about populations of any other species. In addition, with six billion humans on the planet, and diverse and rapid changes in demographic patterns, there is deep concern about the ability to make and test predic- tions about population patterns, causes, and effects. Biologists and demographers both study patterns of population growth, births, and deaths––however, because they typi- cally ask different questions, their meas- urements tend to differ. Demography, as a social science (Caldwell, 1996), has focused on proximate determinates of aspects of fertility and mortality; the importance of such knowledge to a variety of concerns has led to internal tensions and cross-fertiliza- tion in various endeavors whose practition- ers call themselves demographers (e.g., Kirk, 1996). In some ways, the analyses of biologists concerned with life history theory and patterns (Roff, 1992; Stearns; 1992; Charnov, 1993), even those who examine human life histories (Hill, 1993; Low, 1998) are simpler. Although they may explore mechanisms, they focus more on trait–en- vironment correlations (see also Bock, 1999): Under what circumstances do par- ticular patterns occur? The concerns involve explaining variation in behavior as a func- tion of ecological (including social) context; adaptive payoffs are crucial. In general, biologists focus on ÔÔnetÕÕ results as the outcomes of differing strate- gies––aggregating influences, but disag- gregating results, whereas demographers disaggregate variables (birth rates and death rates, rather than number reaching independence, for example), and aggregate results. It is thus sometimes hard to com- pare, or to translate, from one mode to the other. Both perspectives have great utility, depending on the questions asked; they differ in the level at which explanations are sought––and thus their explanations are complementary, rather than alternatives. Combining the two levels of endeavor could be profitable (e.g., Bock, 1999). Cer- tainly demography is crucial to life-history analysis. Stearns (1992, p. 20) called it the ÔÔkey to life history theory, allowing us to calculate the strength of selection on life history traits for many conditionsÕÕ Life history theory, with its generation of spe- cific and testable hypotheses derived from theoretical expectations, may be useful if we can sufficiently delineate predictions for complicated human situations. Correspondence to: Bobbi S. Low Received 11 September 2000; Revision received 23 September 2001; Accepted 5 October 2001 AMERICAN JOURNAL OF HUMAN BIOLOGY 14:149–167 (2002); DOI 10.1002/ajhb.10043 ª 2002 Wiley-Liss, Inc.