Too much for the Dutch? How the Dutch baby boom affected kinship size in 2018

Dutch kinship network data, Vera de Bel argues, suggest that the Dutch baby boom has trickled down from one generation to the next, leading to larger numbers of aunts, uncles, and cousins for people born thirty years after the boom.

Demographic changes are reshaping our kinship networks. Falling fertility not only affects the number of children people have, but also the number of siblings, cousins, and grandchildren available to future generations. Furthermore, population aging influences the number of years we share with parents and grandparents, as well as the caring responsibilities that come with them. Lastly, divorce, re-partnering, and subsequent multi-partner fertility introduce new types of kin into our networks, such as half-siblings and step-grandparents.

To gain a better understanding of these changes, it is essential to map kinship networks empirically (see e.g. Jiang 2025). However, data that allow us to do so are scarce. Kolk et al (2023) recently published a study in which they counted how many relatives Swedes have. We replicated their approach work using kinship network data from the Dutch population registers (Bel et al. 2025). In our study, we focus on three birth cohorts to anchor the comparison with Sweden: 1935 to represent the pre-WW2 cohort; 1955 to represent the baby-boom generation born in the years following WW2; and 1985 to represent the generation born after the onset of the Second Demographic Transition (SDT).

Figure 1 shows the number of kin Dutch people have by age. At first glance, this picture is quite similar to the one in Sweden. However, two remarkable differences can be observed: one related to the Dutch baby boom, the other to higher multi-partner fertility in Sweden.

Dutch baby boom

Table 1 shows the number of children, nieces and nephews, siblings, cousins, and aunts and uncles that people have. It presents these counts for individuals born in 1935, 1955, and 1985. The numbers for Sweden are taken from the Swedish study (Kolk et al., 2023), while the figures for the Netherlands are newly calculated. 

Dutch women born in 1935, representing the pre-WW2 cohort, had 2.3 children on average, and 16% had four or more children, compared to 9% in Sweden (not shown in Table 1). Swedish women in the same cohort had 2.0 children on average, or 15% fewer. 

People born in 1955, the baby-boom generation, have 2.7 living siblings and 5.5 living nieces and nephews on average, i.e. 42% more siblings and 34% more nieces and nephews than their Swedish counterparts. Looking at the next generation, born in 1985, allows us to observe what we interpret as a trickle-down effect of the Dutch baby boom: people born in 1985 have 38% more aunts and uncles (5.1) and cousins (11.2) than their Swedish peers.

Multi-partner fertility in Sweden

We also observe a reversal in the average number of siblings between the two countries. Dutch people from the 1955 cohort have 2.7 siblings on average, compared to 1.9 in Sweden, which amounts to a difference of 42%. However, Dutch people born 30 years later, in 1985, have an average of 1.8 siblings, compared to 2.2 in Sweden, which is 18% fewer.

As a result of higher divorce rates in Sweden than in the Netherlands (2.8 vs. 2.0 per 100 persons in 2013; Eurostat, 2024), multi-partner fertility and the presence of half-siblings became increasingly common from the mid-20th century onward (Turunen and Kolk 2017). Consequently, the 1985 Dutch cohort has just 0.1 paternal half-sibling and on average fewer than 0.1 maternal half-siblings, compared to 0.4 paternal and 0.3 maternal half-siblings in Sweden (not shown in Table 1).

Conclusion

Our study shows similarities between Dutch and Swedish kinship networks, but it also highlights two interesting differences. First, our results suggest that the Dutch baby boom has trickled down from one generation to the next, leading to larger numbers of aunts, uncles, and cousins for people born 30 years after the boom. Second, Dutch kinship networks show a lower number of half-siblings compared to Sweden, which can be explained by lower divorce rates in The Netherlands.

References

de Bel, V., Bokányi, E., Hank, K., & Leopold, T. (2025). A parallel kinship universe? A replication of Kolk et al. (2023) with Dutch register data on kinship networks. Demographic Research, 52, 915–938. https://doi.org/10.4054/DemRes.2025.52.28

Eurostat. (2024). Divorce indicators. https://doi.org/https://doi.org/10.2908/DEMO_NDIVIND

Jiang, S. (2025) The kinship gap. When just a few years of difference make a difference, N-IUSSP, 17 November. https://www.niussp.org/fertility-and-reproduction/the-kinship-gap-when-just-a-few-years-of-difference-make-a-difference/

Kolk, M., Andersson, L., Pettersson, E., & Drefahl, S. (2023). The Swedish Kinship Universe: A Demographic Account of the Number of Children, Parents, Siblings, Grandchildren, Grandparents, Aunts/Uncles, Nieces/Nephews, and Cousins Using National Population Registers. Demography, 60(5), 1359–1385. https://doi.org/10.1215/00703370-10955240

Turunen, J., & Kolk, M. (2017). The prevalence of half-siblings over the demographic transition in Northern Sweden 1750–2007. IUSSP International Population Conference.