Fertility and Well-Being

At a Glance Projects Publications Team


Consequences of Sibling Group Structure

Conducted by Mikko Myrskylä; Kieron Barclay; in Collaboration with Anna Baranowska-Rataj (Umeå University, Sweden), Martin Kolk, Martin Hällsten (both: Stockholm University, Sweden), Dalton Conley (Princeton University, USA)

Detailed Description

The focus of this project is to examine how fertility decisions made by parents, which affect sibling group structure and the early-life conditions experienced within the family, affect a range of health, educational, and socioeconomic outcomes for children at various stages of the life course.

We look at parental age at the time of birth and focus on the consequences of birth spacing, family size, and birth order across low-, medium-, and high-income countries. Previous research has suggested that short birth-spacing increases the risk of poor perinatal and poor long-term socioeconomic outcomes. Existing research has shown that both only children (i.e., children without any siblings) and later-born siblings in multi-child sibling groups have worse long-term socioeconomic and health outcomes. As a result, it is important to understand how these various factors influence offspring outcomes.

To address how birth spacing, family size, and birth order affect long-term socioeconomic outcomes for the offspring, we use Swedish population register data and Demographic and Health Surveys from 77 low- and middle-income countries. To minimize residual confounding and to identify the net effect of birth spacing and birth order on long-term outcomes, we use sibling fixed effects. To study the long-term health outcomes of only children, we have also used cousin fixed effects to minimize confounding from factors shared across the wider kin group.

In contrast to much previous work, we have found that in a high-income country such as contemporary Sweden, short nor long birth intervals neither seem to have any negative effect on birth outcomes such as preterm birth, low birth weight, nor on the probability of hospitalization during childhood. Further work of ours has shown that birth spacing has little or no consequences for long-term health and mortality in high-income countries either. However, the story is very different in low- and middle-income countries, where short birth spacing is very strongly associated with the probability of infant mortality even after adjusting for unobserved maternal characteristics. Another of our studies has revealed that the association between short birth spacing and the risk of infant mortality grows weaker with increasing economic development, suggesting that differences in the public health environment and in social and economic conditions explain the disparities in the effects of short intervals across low-, middle-, and high-income countries.

In our research on the importance of family size during childhood for health and mortality, we have found that only children have a higher mortality in adulthood than children raised in multi-child sibling groups, even when comparing cousins. Finally, we have found that birth order has important effects on long-term educational outcomes. There are large and significant effects of birth order on college major choice in Sweden. First-borns are more likely to study engineering or pursue a medical degree, and they are generally more likely to select college majors that lead to higher expected earnings. These results indicate that within-family conditions related to birth order are associated with preference formation during childhood.

Research Keywords:

Fertility Development, Health Care, Public Health, Medicine, and Epidemiology


Keenan, K.; Barclay, K. J.; Goisis, A.:
Population Studies. forthcoming. (2022)    
Barclay, K. J.; Lyngstad, T. H.; Conley, D.:
European Sociological Review 37:4, 607–625. (2021)    
Barclay, K. J.; Baranowska-Rataj, A.; Kolk, M.; Ivarsson, A.:
Population Studies 74:3, 363–378. (2020)    
Keenan, K.; Barclay, K. J.; Goisis, A.:
MPIDR Working Paper WP-2020-004. (2020)    
Spilerman, S.; Barclay, K. J.:
MPIDR Working Paper WP-2020-017. (2020)    
Molitoris, J.; Barclay, K. J.; Kolk, M.:
Demography 56:4, 1349–1370. (2019)    
Barclay, K. J.:
Research in Social Stratification and Mobility 54, 56–65. (2018)    
Barclay, K. J.; Kolk, M.:
Demography 55:3, 929–955. (2018)    
Baranowska-Rataj, A.; Barclay, K. J.; Kolk, M.:
Population Studies 71:1, 43–63. (2017)    
Barclay, K. J.; Hällsten, M.; Myrskylä, M.:
Social Forces 96:2, 629–660. (2017)    
Barclay, K. J.; Myrskylä, M.; Tynelius, P.; Berglind, D.; Rasmussen, F.:
Drug and Alcohol Dependence 167, 15–22. (2016)    
Barclay, K. J.; Myrskylä, M.:
Social Science and Medicine 123, 141–148. (2014)    
The Max Planck Institute for Demographic Research (MPIDR) in Rostock is one of the leading demographic research centers in the world. It's part of the Max Planck Society, the internationally renowned German research society.