On the number of sons born and shorter lifespan in historical Sami mothers.

A recent paper by Cesarini et al. (2009) published in lifespan (F1,575 1⁄4 9.79, p 1⁄4 0.002) and gave birth to 0.6 Proceedings B aimed at replicating our previous finding showing that in historical Sami mothers from Northern Scandinavia (mainly from Finland), each son born was associated with a reduction of 0.65 years of a mother’s post-menopausal lifespan (Helle et al. 2002). This finding supports the hypothesis that energetic resources invested in reproduction by the mothers may compromise those resources available for longevity (Kirkwood & Rose 1991). Using similar but a larger dataset of Sami mothers from Sweden, a neighbouring country of Finland, Cesarini et al. (2009) found no evidence for the association between sons born and the mothers’ post-menopausal lifespan. The authors made two main conclusions: (i) our results were likely to be false-positive because of smaller sample size and (ii) that current evidence for the association between offspring sex and maternal longevity is rare and restricted to these Finnish Sami only. We disagree with both conclusions. First, our original analysis was based on 375 postmenopausal Finnish Sami mothers (Helle et al. 2002), whereas the analysis of Cesarini et al. (2009) included, at maximum, 930 Swedish Sami mothers (ranging from 304 to 930, depending on how confident they were on the ethnicity of the mothers). Based on their larger sample size and non-significant results, Cesarini et al. (2009) seem to suggest that our findings were false-positive. Our original publication was based on all Sami women resident in three Finnish Sami populations between 1640 and 1870, but we were unable to include all these women in our analysis because of missing records on their age at death. Since the study of Helle et al. (2002), we have supplemented our database on these Sami using additional historical records available to fill in missing information. Now, this enables us to repeat our original analysis with 481 post-menopausal mothers. Using this larger sample and the same statistical model as previously (i.e. controlling for the number of daughters born and spouse’s lifespan), we find a similar association between the number of sons born and maternal longevity as reported previously (b(95% CIs) 1⁄4 20.65(21.17, 20.12), p 1⁄4 0.016). In this model, the regression coefficient of sons statistically differs from that of daughters (20.65 versus 0.40, F1,477 1⁄4 6.73, p 1⁄4 0.0098). To further increase our sample size, we applied multiple imputation to account for the missing values (96 cases, 16.6%) of spouse’s lifespan (a predictor of the wife’s post-reproductive survival; see Helle et al. 2005) that originally led to the exclusion of these women from the sample. These women had a 3.7 years lower mean