kinship and fertility in the Icelandic population. We point out that the data further suggest that fertility initially increases with kinship and then decays. This is supported by another large study on the Danish population suggesting a superposition of effects of inbreeding and outbreeding depression on human fertility. Helgason et al. (1) reported a positive association between kinship and human fertility based on comprehensive registers containing long historical series of the Icelandic population. Their results, interpreted as a manifestation of outbreeding depression, are in line with other similar findings in nonhuman populations (2–4). In parallel, we presented a positive association between fertility and marital radius (the distance between mates ’ birthplaces), based on a large Danish cohort (all women born in Denmark in 1954) (5). As a consequence of the classic Malécot theory on spatial genetic structure of populations (5–9), which states that the kinship is a decreasing function of marital radius, the results in (5) suggest a negative association between consanguinity and human fertility. Although the results of (1) and(5) appear to contradict each other, we argue that a closer look at both data sets reveals that this is not the case. Both studies corroborate the hypothesis that the superposed contrary forces of inbreeding and outbreeding depression have an effect on human fertility. The coincidence of the conclusions of these two independent large-scale studies suggests that this is a general phenomenon in human populations. Although we do not cast doubt on the existence of a positive association between kinship and fertility as presented in (1), we remark that the relation between the mean number of children and kinship is not the same in the different periods reported. Examination of figure S2 in (1) reveals that the curve representing the number of children as a function of the kinship coefficient is not monotonic in the periods 1900 to 1924, 192
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