Integration involves a trade-off between fertility and status for World War II evacuees


Understanding how refugees integrate into host societies has broad implications for researchers interested in intergroup conflict and for governments concerned with promoting social cohesion. Using detailed records tracking the movements and life histories of Finnish evacuees during World War II, we find that evacuees who intermarry are more likely to be educated, work in professional occupations, marry someone higher in social status and remain in the host community. Evacuees who intermarry before the war have fewer children, whereas those who marry into their host community after the war have more children. These results indicate that life-history and assimilation outcomes depend on key differences between pre-war environments—when migrants are living in their own communities—and post-war environments—when migrants are living in the host community. Overall, this suggests that integration involves a trade-off between reproduction and status such that evacuees who integrate gain social status, whereas those who maintain stronger bonds with their natal communities have higher fertility. We discuss these results within the framework of social capital, intergroup conflict and life-history theory and suggest how they can inform our understanding of evolutionary adaptations that affect tribalism.

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Fig. 1: Evacuee locations before (1938), during (1943) and after (1945) the war.
Fig. 2: Posterior distribution credibility intervals for factors affecting the probability of intermarriage and the number of children for all evacuees.
Fig. 3: Intermarriage is positively associated with marrying up both before and after the war.
Fig. 4: Reproductive costs of intermarriage are only evident before the war.

Code availability

The code used to produce these models, generate all results and produce all of the figures and tables in this paper and the Supplementary Information is available on GitHub,, and is also included in Supplementary Software.

Data availability

The data that were used to generate these results and that support the findings of this study are available on GitHub:


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This research was supported by the Kone foundation. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. We thank J. Pettay for helping to translate much of the source data from Finnish to English, and J. Kallioniemi and T. Salmi for helping to extract the data from the original texts.

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R.L. wrote the first draft of the manuscript, conducted the statistical analysis and created the figures and tables. K.P. and K.M. helped design the statistical models. J.L. and V.L. planned the study and oversaw data collection. All authors helped to write and edit the manuscript.

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Correspondence to Robert Lynch.

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Lynch, R., Lummaa, V., Panchanathan, K. et al. Integration involves a trade-off between fertility and status for World War II evacuees. Nat Hum Behav 3, 337–345 (2019).

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