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Am J Hum Biol. 2012 Nov-Dec;24(6):786-99. doi: 10.1002/ajhb.22316. Epub 2012 Sep 17.

Why do women have more children than they want? Understanding differences in women's ideal and actual family size in a natural fertility population.

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Department of Anthropology, Integrative Anthropological Sciences Program, University of California-Santa Barbara, Santa Barbara, California 93106, USA.



We develop and test a conceptual model of factors influencing women's ideal family size (IFS) in a natural fertility population, the Tsimane of Bolivia. The model posits affects of socioecology, reproductive history, maternal condition, and men's IFS. We test three hypotheses for why women may exceed their IFS despite experiencing socioeconomic development: (H(1) ) limited autonomy; (H(2) ) improved maternal condition; and (H(3) ) low returns on investments in embodied capital.


Women's reproductive histories and prospective fertility data were collected from 2002 to 2008 (n = 305 women). Semistructured interviews were conducted with Tsimane women to study the perceived value of parental investment (n = 76). Multiple regression, t-tests, and analysis of variance (ANOVA) are used to test model predictions.


Women's IFS is predicted by their socioecology, reproductive history, maternal condition, and husband's IFS. Hypotheses 2 and 3 are supported. Couples residing near town have smaller IFS (women = 3.75 ± 1.64; men = 3.87 ± 2.64) and less variance in IFS. However, the degree fertility exceeds IFS is inversely correlated with distance to town (Partial r = -0.189, df = 156, P = 0.018). Women living near town have greater maternal condition but 64% value traditional skills over formal schooling and 88% believe living in town is unfeasible.


While reduced IFS is evident with socioeconomic development, fertility decline may not immediately follow. When perceived benefits of investment in novel forms of embodied capital are low, and somatic wealth and large kin networks persist as important components of fitness, fertility may remain high and increase if maternal condition improves.

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