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Twenty-six of 480 Mexicans and 58 of 609 Peruvians measured have a close maternal ancestor with the same surname as a paternal one. Such isonymy usually implies inbreeding and when standardized within subseries, 7 of 26 anthropometric variables of the Mexicans are significantly small and may represent “inbreeding depression.” Neither the Peruvians nor the combined series show any such depression, however. Thus, on average, isonymy does not consistently lead to biologically distinct offspring by anthropometric standards.