“What Strange Creatures Siblings Are.” The Importance of Siblings in the Iberian Peninsula and the Azores,1750–1900

This study compares the role of siblings in the survival of newborns from 1750 to 1900. Utilizing nearly one million longitudinal life history trajectories from Spain and Portugal, we examined whether cooperative breeding and resource dilution theories explain the observed patterns in southern Europe. Our analysis employs both descriptive statistics and probit regression models. Our findings confirm that in all cases, a high number of older siblings significantly increased the chances of newborn survival (by 10–30 percent). Additionally, there were notable differences based on the gender of the baby, with girls born in rural Spain being more adversely affected.