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On Population Growth and Parental Investment

Voland et al. (CA 38:129–35) have recently presented data that seem to suggest a correlation between population growth rates and an index of differential parental investment. Seeking to augment previous work on the way sex-biased parenting may be influenced by factors such as social and economic status, they want to include difference in population growth rates as another factor that may affect a parent’s “fitness calculations” regarding the sex in which there should be greater parental investment. Data from six 18thand 19th-century rural German populations are used, with each population divided into upper and lower classes. The index of differential parental investment, TW, compares the female/male ratio for infant mortality, q(f)/ q(m), for the upper class with the same ratio for the lower class (i.e., a ratio of ratios) for each of the six regions. Voland et al. note that upper-class q(f)/q(m) values are consistently higher, thus showing a class difference in degree of sex bias toward sons and daughters. When they correlate TW with the growth rate for each of these six rural groups they obtain a value of rs p 0.829, which they accept as statistically significant (n p 6, p p 0.05, one-tailed). The linkage between differential sex bias and growth rates, they argue, stems from higher values of rs suggesting “expansion competition” and lower values suggesting “resource competition.” They conclude (p. 134): “It seems that social-rank effects on sex-biased parental investment can go in either direction, depending on whether the prevailing kind of competition is displacement or expansion.” A closer examination of the data provided by Voland et al. makes it evident that the matter is more complex and the conclusions drawn may be premature. First, according to the data they report (table 1) rs p 0.69 (n p 6, p p 0.13, two-tailed), not 0.829, and so the relationship between TW and growth rate is not statistically significant as they claim. Second, when Krummhörn, the region with the second-to-lowest growth rate, is compared with Geest, the region with the highest growth rate, the