Why childhood survival may depend on sibling dynamics

A new study reports that the mix of older brothers and sisters in a family can change a child’s odds of surviving early childhood. The work looks backward, using records from two Alpine parishes to ask a very human question with modern statistical tools.

Mark Spa, an expert in the Department of Biology at the University of Turku, is the lead author of the study.

The team analyzed lives from the mid-1700s through the 1800s, when disease and scarcity were constant pressures. They found that older brothers born close in time to a girl were linked to lower survival for that girl, while older sisters close in age were linked to higher survival for both girls and boys.

Sibling survival question

The core question here is simple, who helps a child survive and who competes with them for limited family resources. That question sits inside the resource dilution hypothesis, which says parental time and money are finite and are divided among children.

Past work often pooled brothers and sisters together, which can blur effects that differ by sex. This study separated siblings by sex and by whether they were close in age, under 5 years older, or farther apart by 5 years or more.

The researchers also accounted for other family factors. They considered parental ages, whether a parent died while the child was young, socioeconomic status, and the presence of grandparents.

How the research was conducted

The dataset covered 2,941 children born between 1750 and 1870 in Elm and Linthal, two Swiss parishes with high fertility and high childhood mortality. Those communities kept careful registers of births and deaths, which let the scientists reconstruct family trees and survival to age 5.

The team used a generalized linear mixed model (GLMM) to estimate links between sibling composition and survival while controlling for family and cohort differences. That approach helps isolate associations within families rather than just across the whole population.

To sharpen the picture, the researchers decomposed the single count of older siblings into categories. They looked at brothers versus sisters, alive at the younger child’s birth versus already deceased, and close in age versus farther apart in age.

Sibling dynamics and survival

When brothers and sisters were all lumped together, the total number of older siblings did not predict survival to age 5. Once the categories were split, clear patterns emerged that depended on sex and spacing.

Girls with older brothers close in age had lower survival than girls without such brothers. At the same time, having older sisters close in age was associated with higher survival for both girls and boys.

The experts found that older brothers close in age reduced the survival of girls. The same analysis showed no such penalty for boys with closely spaced older brothers.

Why sibling sex matters

Across species, males and females can differ biologically in size, energy needs, and immunity. Evidence in humans suggests males are often more vulnerable to infection in early life, which can amplify competition when resources are tight.

Biology is only part of the story. Cultural rules can tilt resources toward sons or toward daughters, which changes how sibling competition or help shows up in survival patterns.

In households where the economic or social benefits of boys are prioritized, parents may shield sons from the costs of competition. That can worsen the squeeze for girls who are close in age to older brothers.

The role of culture and resources

The positive association with older sisters close in age suggests help, not just competition, is in play. Help can look like child care, household work that frees parents’ time, or small but steady contributions to family labor.

Other populations show that the balance of help and competition can flip with context. In pre-industrial Finland, elder siblings sometimes boosted survival to adolescence, and the presence of elder siblings improved the chances of younger siblings surviving to sexual maturity, according to a Finnish research paper.

Spacing matters because it tracks dependency. Siblings under age 5 at a younger child’s birth are themselves dependent on parents, increasing competition, while those 5 or more years older are more capable of helping.

What this means today

Modern health systems are far different from those in the 1700s and 1800s, but family dynamics still matter. When children are born in quick succession, parental attention, money, and energy get split across more needs at once.

The Swiss records remind us that siblings are not interchangeable units. Brothers and sisters can influence a child’s path in different ways, and those effects can depend on their ages and the expectations placed on them.

For public health and social policy, the lesson is to watch for invisible pressure points inside families. Support that reduces the crunch during closely spaced births could be especially valuable for girls in settings where they face greater competition or lower priority.

Limits of sibling survival

This work is observational and historical, so it cannot prove cause and effect. The experts note that some associations were modest in size, and that younger siblings, who arrive later, were not modeled directly.

Even so, the careful breakdown by sex and age spacing offers a clearer map of where to look next.

Future studies can test mechanisms directly in living populations, linking sibling help and competition to measurable behaviors in the home.

The study is published in the journal Proceedings of the Royal Society B.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–