Does birth order matter? Over the past hundred years, this simple question has inspired a rancorous debate. For the past ten years, however, the growing consensus has been that yes, it does matter. It is becoming increasingly clear that, relative to first borns, later born siblings within the same family have lower educational attainment (Black, Devereux and Salvanes, 2005), lower cognitive ability (Bjerkedal et al., 2007; Barclay, 2015a), and worse health in adulthood (Barclay and Myrskylä, 2014).
Studies have shown that these patterns are likely to be explained by social upbringing within the family of origin rather than any pre-natal in utero experiences (Kristensen and Bjerkedal, 2007; Barclay, 2015b). The main social explanations that have been offered to account for the connection between birth order and long-term outcomes concern the idea that first borns benefit from the exclusive attention of their parents when they are the only child in the household (Blake, 1981; Zajonc, 1976). As more siblings are born, those parental resources are divided further and further. If it is parental investment in the first few years of life that matters the most for development, this should lead to a cumulative advantage for first and earlier born siblings relative to later born siblings.
In a recent study, my co-author, Martin Kolk, and I examined the relationship between birth order and mortality using Swedish population register data on 1.7 million Swedes (Barclay and Kolk, 2015). As figure 1 shows, we found that later born siblings in Sweden have higher rates of mortality than their older brothers and sisters. Furthermore, this pattern is particularly pronounced for mortality attributable to cancers of the respiratory system and external causes, meaning accidents, suicides, and events of undetermined intent.
One of the many strengths of the Swedish register data is that it provides information on family linkages. This means that it was possible for us to connect individuals to their biological mothers and fathers, and then to link them to their siblings. Having identified each individual’s siblings, we were able to run our statistical analyses using a within-family comparison approach. The idea of this approach is that siblings within the same family share genetics as well as the social environment during childhood. By comparing brothers and sisters to one another, the assumption is that we don’t have to worry about the factors that that these siblings share, such as genes, parents, neighbourhood, or the general home environment, and we can focus on factors that vary amongst siblings, such as their birth order. This is important, as concerns about statistical confounding have plagued birth order research in the past as most of that research compared individuals across different families. That is to say, there was a strong concern that the finding that later borns did worse than first borns was not explained by birth order itself, but by some factor that was related to both birth order and long-term outcomes such as educational attainment or cognitive development (Rodgers, 2001). However, when it’s possible to compare siblings within the same family to one another we have much stronger evidence that it is birth order itself that is driving the pattern rather than some other factor.
Using Swedish administrative register data that covers the whole Swedish population to study individuals born between 1938 and 1960, we found that when comparing siblings within the same family, later borns have higher age-specific mortality than first borns. Interestingly, this seems to be particularly true for women. Although we cannot say why this is the case with any certainty, previous studies have shown that women tend to find a stronger social role within the family than men, and because of that, women might be more affected by internal family dynamics.
Another interesting result was that mortality attributable to cancers of the respiratory system and external causes is much higher for later born siblings, and this was especially true for women. This finding is intriguing. One potential explanation for this is that previous studies have shown that later born siblings are more likely to begin smoking and drinking at younger ages than first borns (Bard and Rodgers, 2003; Blane and Barry, 1973). Older siblings who smoke or drink act as role models for their younger siblings, and could also facilitate access to alcohol and cigarettes. We know from other research that those who start smoking at a younger age are more likely to continue smoking into adulthood, so this could drive the relationship between birth order and lung cancer (Khuder, Dayal and Mutgi, 1999). This idea seems plausible, but since the Swedish data is an administrative register we didn’t have any information on health behaviours, so we weren’t able to specifically test this potential mechanism.
Overall, our study provides support for the idea that early life experiences are important for the long run. The hypothesis here is that early disadvantage accumulates over time, and that long chains of risk connect experiences over the life course. This study goes to show that it is not only differences between families that can explain social disparities, but that relative advantange and disadvantage within each family can also contribute to explaining why siblings turn out differently.
Barclay, K. J. (2015a). A within-family analysis of birth order and intelligence using population conscription data on Swedish men. Intelligence, 49, 134-143.
Barclay, K. J. (2015b). Birth order and educational attainment: evidence from fully adopted sibling groups. Intelligence, 48, 109-122.
Barclay, K., & Kolk, M. (2015). Birth order and mortality: a population-based cohort study. Demography, 52(2), 613-639.
Barclay, K., & Myrskylä, M. (2014). Birth order and physical fitness in early adulthood: Evidence from Swedish military conscription data. Social Science & Medicine, 123, 141-148.
Bard, D. E., & Rodgers, J. L. (2003). Sibling Influence on Smoking Behavior: A Within‐Family Look at Explanations for a Birth‐Order Effect. Journal of Applied Social Psychology, 33(9), 1773-1795.
Bjerkedal, T., Kristensen, P., Skjeret, G. A., & Brevik, J. I. (2007). Intelligence test scores and birth order among young Norwegian men (conscripts) analyzed within and between families. Intelligence, 35(5), 503-514.
Black, S. E., Devereux, P. J., & Salvanes, K. G. (2005). The more the merrier? The effect of family size and birth order on children’s education. The Quarterly Journal of Economics, 669-700.
Blake, J. (1981). Family size and the quality of children. Demography, 18(4), 421-442.
Blane, H. T., & Barry, H. (1973). Birth order and alcoholism: a review. Quarterly Journal of Studies on Alcohol, 34(3), 837-852.
Khuder, S. A., Dayal, H. H., & Mutgi, A. B. (1999). Age at smoking onset and its effect on smoking cessation. Addictive Behaviors, 24(5), 673-677.
Kristensen, P., & Bjerkedal, T. (2007). Explaining the relation between birth order and intelligence. Science, 316(5832), 1717-1717.
Rodgers, J. L. (2001). What causes birth order–intelligence patterns? The admixture hypothesis, revived. American Psychologist, 56(6-7), 505.
Zajonc, R. B. (1976). Family configuration and intelligence: Variations in scholastic aptitude scores parallel trends in family size and the spacing of children. Science, 192(4236), 227-236