Alexander Weissa,b,1, James E. Kingc, Miho Inoue-Murayamad, Tetsuro Matsuzawae, and Andrew J. Oswaldf,g
a Scottish Primate Research Group, bDepartment of Psychology, School of Philosophy, Psychology and Language Sciences, University of Edinburgh, Edinburgh EH8 9JZ, United Kingdom; cDepartment ofPsychology, University of Arizona, Tucson, AZ 85721; dWildlife Research Center of Kyoto University, Sakyo, Kyoto 606-8203, Japan; eSection of Language and Intelligence, Primate Research Institute, Kyoto University, Inuyama, Aichi 484-8506, Japan; fDepartment of Economics and Centre for Competitive Advantage in the Global Economy, University of Warwick, Coventry CV4 7AL, United Kingdom; and gInstitute forthe Study of Labor, 53072 Bonn, Germany
Edited by George A. Akerlof, University of California, Berkeley, and approved October 11, 2012 (received for review July 22, 2012)
Recently, economists and behavioral scientists have studied the pattern of human well-being over the lifespan. In dozens of countries, and for a large range of well-being measures, including happiness and mental health,well-being is high in youth, falls to a nadir in midlife, and rises again in old age. The reasons for this U-shape are still unclear. Present theories emphasize sociological and economic forces. In this study we show that a similar U-shape exists in 508 great apes (two samples of chimpanzees and one sample of orangutans) whose well-being was assessed by raters familiar with the individual apes. ThisU-shaped pattern or “midlife crisis” emerges with or without use of parametric methods. Our results imply that human wellbeing’s curved shape is not uniquely human and that, although it may be partly explained by aspects of human life and society, its origins may lie partly in the biology we share with great apes. These ﬁndings have implications across scientiﬁc and social-scientiﬁc disciplines,and may help to identify ways of enhancing human and ape well-being.
We explore an alternative explanation. From a very different research tradition, work on great ape (mostly chimpanzee) development has identiﬁed similarities to humans in the development of psychological domains other than well-being (22). Thus, it is worth considering a heretofore untested theory, namely that the Ushapefound in human studies of age and well-being evolved in the common ancestors of humans and nonhuman primates, particularly the great apes. If one could establish that the U-shape in wellbeing exists in nonhuman primates, the implications would be wide-ranging. This ﬁnding would also recommend new hypotheses for well-being researchers. Results In a sample of 155 chimpanzees from Japanese zoos,research centers, and a sanctuary (sample A), a sample of 181 chimpanzees housed in United States and Australian zoos (sample B), and a sample of 172 orangutans housed in United States, Canadian, Australian, and Singaporean zoos (sample C), multiple regression analyses indicated that linear and quadratic age effects were negative and positive, respectively (Table 1). In other words, all three samplesexhibited a U-shape (Fig. 1). The age-related effects were individually signiﬁcant in sample A, but not samples B or C. The curves’ minima were reached at, respectively, ages 28.3, 27.2, and 35.4, and were thus comparable to human well-being minima, which range from ∼45–50 y. In the fourth regression, for the total sample, the linear and quadratic age effects indicated a U-shape and weresigniﬁcant. Linear and quadratic age effects did not signiﬁcantly differ across the samples (Table 2). Finally, the linear and quadratic effects again described a U-shaped function (Fig. 1) and were signiﬁcant after the interaction terms were dropped (Table 2). The curve’s minimum was at age 31.9. Use of 10 banded age variables revealed the same results (SI Text, and Table S2). Discussion Although great...