Effect of producing sons on maternal longevity in premodern populations.

Helle et al. (1) showed that producing and raising sons reduced the longevity of preindustrial Sami women, whereas daughters tended to have the opposite effect. They suggested that the life-shortening effect of sons could be attributed to higher endocrinological or physiological costs of producing sons compared with those of producing daughters, and that the life-elongating effect of raising daughters might be the outcome of daughters helping in the household of their mothers. To examine the validity of those ideas, we explored the costs of producing sons rather than daughters in the two premodern populations of Krummhörn (Ostfriesland, Germany, 1720 to 1874) and St. Lawrence valley (Quebéc, Canada, 1608 to 1760), applying the same data selection criteria and linear regression model as Helle et al. (2). For the Krummhörn population, we found the same negative relationship between the number of sons born and the age of the mother at death, although this result was not statistically significant. There was also a very slight and likewise not significant positive correlation concerning the number of daughters born (sons: – 0.180 0.200, t – 0.900, P 0.368; daughters: 0.049 0.209, t 0.234, P 0.815) (Fig. 1, A and B). In the Quebéc population, we found a positive relationship between the number of sons or daughters born and maternal age, but only the latter relationship is significant (sons: 0.081 0.092, t 0.881, P 0.378; daughters: 0.197 0.095, t 2.062, P 0.039) (Fig. 1, C and D). The Krummhörn case looks quite similar to what was found for the Sami, but the results are difficult to interpret. Broken down by economic status, we find a strong— although not significant—negative relationship between the number of sons and maternal age for the wealthy farmers ( –0.889 0.348, t 1.326, P 0.186, n 66) but an opposite effect for the poor workers ( 0.462 0.728, t –1.217, P 0.228, n 303) who were analyzed. With the Quebéc population, the effect of the number of sons was reversed relative to what was seen in the Sami population. Even less evidence for a relationship between number of sons or daughters and maternal longevity could be found when the analyses were restricted to the children raised to adulthood (2). It thus seems that the higher physiological costs of producing sons are unlikely to have a strong impact on maternal longevity in general. Instead, these effects—if they exist at all—are modulated decisively by yet unknown factors, probably of a sociocultural nature. Life-history studies of historical populations, although fruitful in many aspects (3), sometimes suffer from a lack of information concerning influential covariates. In addition, specific sociocultural or population genetic conditions may make it difficult to generalize effects observed in such a population.