This article examines the evolution of the role played by the number and gender of siblings in the survival and biological well-being of individuals in rural Spain during the twentieth century. Our aim is to test how two fundamental theories – the cooperative breeding hypotheis and the resource dilution hypothesis – about how the number of siblings affect the individual come together in this area of study during a period of economic, health, and social transformation. We used a sample of 19,331 individuals born between 1900 and 1979 from 14 rural villages, for whom data on sibling count and various family and environmental variables are available. Using these data, we ran several statistical models to discover the effects of siblings on survival. In addition, we studied the long-term effect of siblings on height using height data from 2,783 male conscripts. Our results show that the number of siblings positively influenced survival, either through the cooperation of older siblings in the care of their younger brothers and sisters or through parents exhibiting higher offspring survival abilities. However, increased reproductive success may come with a disadvantage. The biological well-being, as measured by height, of male conscripts was significantly lower among individuals with more siblings in the early decades of the study. Conversely, in the later decades, the negative relationship between sibship size and height was not statistically significant when the number of living siblings was fewer than five.

This study examines the changes over time of the twinning rate and infant and child mortality across 17 rural villages in the province of Zaragoza (Spain) over a span of 200 years. The aim is to understand how the twinning rate evolved in conjunction with the processes of economic and social modernization, as well as the demographic transition. During the period analyzed, the twinning rate increased by 10%, rising from 1.26 per 1000 births in the second half of the 18th century to 1.38 per 1000 births in the first half of the 20th century. This rate varied due to hereditary issues, biological factors such as the mother’s age and parity, and socioeconomic features like family occupation and the mother’s education level. In terms of child mortality, twins historically faced a significantly higher mortality rate. However, they benefited greatly from the mortality transition, at least in absolute numbers. While nearly 60% of twins did not survive beyond 5 years of age in earlier periods, the mortality rate for twins decreased to 40% by the first half of the 20th century. The excess mortality was particularly severe for girls, driven by a widespread preference for sons, which led to higher mortality rates for girls in the first 5 years of life, especially in the earliest months. These findings help us understand the improvements in uterine and childhood survival rates for contemporary twins, which can be attributed to the socioeconomic and medical advancements of the 20th century.