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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:


  1. Greenwood, P. J. Mating systems, philopatry and dispersal in birds and mammals. Anim. Behav. 28, 1140–1162 (1980).

    Article  Google Scholar 

  2. Dobson, F. S. Competition for mates and predominant juvenile male dispersal in mammals. Anim. Behav. 30, 1183–1192 (1982).

    Article  Google Scholar 

  3. Emlen, S. T. The evolution of helping. I. An ecological constraints model. Am. Nat. 119, 29–39 (1982).

    Article  Google Scholar 

  4. West, S. A. et al. Cooperation and the scale of competition in humans. Curr. Biol. 16, 1103–1106 (2006).

    Article  CAS  Google Scholar 

  5. Ortega, F. & Peri, G. The Causes and Effects of International Migrations: Evidence from OECD Countries 1980–2005 (National Bureau of Economic Research, 2009).

  6. Glazer, N. & Moynihan, D. P. Beyond the melting pot. The Negroes. Puerto Ricans, Jews, Italians, and Irish of New York City. JAMA 186, 1027 (1963).

    Article  Google Scholar 

  7. Hamilton, W. D. The genetical evolution of social behaviour. I. J. Theor. Biol. 7, 1–16 (1964).

    Article  CAS  Google Scholar 

  8. Trivers, R. L. The evolution of reciprocal altruism. Q. Rev. Biol. 46, 35–57 (1971).

    Article  Google Scholar 

  9. Greenwald, B. C. & Stiglitz, J. E. Externalities in economies with imperfect information and incomplete markets. Q. J. Econ. 101, 229–264 (1986).

    Article  Google Scholar 

  10. Lawrence, P. R. & Lorsch, J. W. Differentiation and integration in complex organizations. Adm. Sci. Q. 12, 1–47 (1967).

    Article  Google Scholar 

  11. Putnam, R. D., Leonardi, R. & Nanetti, R. Y. Making Democracy Work: Civic Traditions in Modern Italy (Princeton Univ. Press, Princeton, 1994).

  12. Putnam, R. Bowling Alone: the Collapse and Revival of American Community (Simon and Schuster, New York, 2000).

    Google Scholar 

  13. Allport, G. W. The Nature of Prejudice (Addison, New York, 1954).

  14. Pettigrew, T. F. & Tropp, L. R. A meta-analytic test of intergroup contact theory. J. Pers. Soc. Psychol. 90, 751–783 (2006).

    Article  Google Scholar 

  15. Olson, M. Logic of Collective Action: Public Goods and the Theory of Groups (Harvard Economic Studies) (Harvard Univ. Press, Cambridge, 1965).

  16. Alesina, A. & La Ferrara, E. Participation in heterogeneous communities. Q. J. Econ. 115, 847–904 (2000).

    Article  Google Scholar 

  17. Friedberg, R. M. You can’t take it with you? Immigrant assimilation and the portability of human capital. J. Labor Econ. 18, 221–251 (2000).

    Article  Google Scholar 

  18. Horvat, E. M., Weininger, E. B. & Lareau, A. From social ties to social capital: class differences in the relations between schools and parent networks. Am. Educ. Res. J. 40, 319–351 (2003).

    Article  Google Scholar 

  19. Borjas, G. J. Assimilation, changes in cohort quality, and the earnings of immigrants. J. Labor Econ. 3, 463–489 (1985).

    Article  Google Scholar 

  20. Quillian, L. Prejudice as a response to perceived group threat: population composition and anti-immigrant and racial prejudice in Europe. Am. Sociol. Rev. 60, 586–611 (1995).

    Article  Google Scholar 

  21. Fennelly, K. & Federico, C. Rural residence as a determinant of attitudes toward US immigration policy. Int. Migr. 46, 151–190 (2008).

    Article  Google Scholar 

  22. Marsden, P. V. Core discussion networks of Americans. Am. Sociol. Rev. 52, 122–131 (1987).

    Article  Google Scholar 

  23. Bott, E. & Spillius, E. B. Family and Social Network: Roles, Norms and External Relationships in Ordinary Urban Families (Routledge, Oxon, 2014).

  24. Bevelander, P. & Irastorza, N. Economic Integration of Intermarried Labour Migrants, Refugees and Family Migrants to Sweden: Premium Or Selection? (IZA, 2014).

  25. Boustan, L. P., Fishback, P. V. & Kantor, S. The effect of internal migration on local labor markets: American cities during the Great Depression. J. Labor Econ. 28, 719–746 (2010).

    Article  Google Scholar 

  26. Wilson, S., Bassiou, E., Denli, A., Dolan, L. C. & Watson, M. Traveling groups stick together: how collective directional movement influences social cohesion. Evol. Psychol. 16, 1474704918792134 (2018).

    Article  Google Scholar 

  27. Silk, J. B., Alberts, S. C. & Altmann, J. Social bonds of female baboons enhance infant survival. Science 302, 1231–1234 (2003).

    Article  CAS  Google Scholar 

  28. Choi, J.-K. & Bowles, S. The coevolution of parochial altruism and war. Science 318, 636–640 (2007).

    Article  CAS  Google Scholar 

  29. Kulu, H. et al. Fertility by birth order among the descendants of immigrants in selected European countries. Popul. Dev. Rev. 43, 31–60 (2017).

    Article  Google Scholar 

  30. Pailhé, A. The convergence of second-generation immigrants’ fertility patterns in France: the role of sociocultural distance between parents’ and host country. Demogr. Res. 36, 1361–1398 (2017).

    Article  Google Scholar 

  31. Jejeebhoy, S. J. Women’s Education, Autonomy, and Reproductive Behaviour: Experience from Developing Countries (OUP, Oxford, 1995).

  32. Coleman, D. A. Trends in fertility and intermarriage among immigrant populations in western Europe as measures of integration. J. Biosoc. Sci. 26, 107–136 (1994).

    Article  CAS  Google Scholar 

  33. Goldscheider, C. & Uhlenberg, P. R. Minority group status and fertility. Am. J. Sociol. 74, 361–372 (1969).

    Article  CAS  Google Scholar 

  34. Hacker, J. D. & Roberts, E. The impact of kin availability, parental religiosity, and nativity on fertility differentials in the late 19th-century United States. Demogr. Res. 37, 1049–1080 (2017).

    Article  Google Scholar 

  35. Sosis, R. Why aren’t we all hutterites? Hum. Nat. 14, 91 (2003).

    Article  Google Scholar 

  36. Wiley, N. F. The ethnic mobility trap and stratification theory. Soc. Probl. 15, 147–159 (1967).

    Article  Google Scholar 

  37. Wilensky, H. L. & Edwards, H. The skidder: ideological adjustments of downward mobile workers. Am. Sociol. Rev. 24, 215–231 (1959).

    Article  Google Scholar 

  38. Kramer, K. L. Cooperative breeding and its significance to the demographic success of humans. Annu. Rev. Anthropol. 39, 417–436 (2010).

    Article  Google Scholar 

  39. Sear, R. & Coall, D. How much does family matter? Cooperative breeding and the demographic transition. Popul. Dev. Rev. 37, 81–112 (2011).

    Article  Google Scholar 

  40. Portes, A. & Sensenbrenner, J. Embeddedness and immigration: notes on the social determinants of economic action. Am. J. Sociol. 98, 1320–1350 (1993).

    Article  Google Scholar 

  41. Lavery, J. E. The History of Finland (Greenwood, Westport, 2006).

  42. Haapala, P. & Toivo, R. M. Suomen Historian Kartasto (Karttakeskus, Helsinki, 2007).

  43. Syrjänen, K., Honkola, T., Lehtinen, J., Leino, A. & Vesakoski, O. Applying population genetic approaches within languages: Finnish dialects as linguistic populations. Lang. Dyn. Change 6, 235–283 (2016).

    Article  Google Scholar 

  44. Granovetter, M. The strength of weak ties: a network theory revisited. Sociol. Theory 1, 201–233 (1983).

    Article  Google Scholar 

  45. Elwert, A. et al. Opposites attract—evidence of status exchange in ethnic intermarriages in Sweden. Lund Pap. Econ. His. 2016, 147 (2016).

    Google Scholar 

  46. Mäenpää, E. Homogamy in educational level and parental social class in Finland: a log-linear analysis. Eur. Sociol. Rev. 31, 253–267 (2014).

    Article  Google Scholar 

  47. Chagnon, N. A., Lynch, R. F., Shenk, M. K., Hames, R. & Flinn, M. V. Cross-cousin marriage among the Yanomamö shows evidence of parent–offspring conflict and mate competition between brothers. Proc. Natl Acad. Sci. USA 114, E2590–E2607 (2017).

    Article  CAS  Google Scholar 

  48. Chetty, R., Hendren, N., Kline, P. & Saez, E. Where is the land of opportunity? The geography of intergenerational mobility in the United States. Q. J. Econ. 129, 1553–1623 (2014).

    Article  Google Scholar 

  49. Sarvimäki, M., Uusitalo, R. & Jantti, M. Long-Term Effects of Forced Migration (IZA, 2009).

  50. Pagnini, D. L. & Morgan, S. P. Intermarriage and social distance among U.S. immigrants at the turn of the century. Am. J. Sociol. 96, 405–432 (1990).

    Article  Google Scholar 

  51. Qian, Z. & Lichter, D. T. Social boundaries and marital assimilation: interpreting trends in racial and ethnic intermarriage. Am. Sociol. Rev. 72, 68–94 (2007).

    Article  Google Scholar 

  52. Lichter, D. T., Qian, Z. & Tumin, D. Whom do immigrants marry? Emerging patterns of intermarriage and integration in the United States. Ann. Am. Acad. Pol. Soc. Sci. 662, 57–78 (2015).

    Article  Google Scholar 

  53. Li, W., Airriess, C. A., Chen, A. C.-C., Leong, K. J. & Keith, V. Katrina and migration: evacuation and return by African Americans and Vietnamese Americans in an eastern New Orleans suburb. Prof. Geogr. 62, 103–118 (2010).

    Article  Google Scholar 

  54. McCarthy, K. F., Peterson, D. J., Sastry, N. & Pollard, M. The Repopulation of New Orleans after Hurricane Katrina (Rand, 2006).

  55. Shephard, B. The Long Road Home: the Aftermath of the Second World War (Vintage, London, 2010).

  56. Communication from the Commission to the Council, the European Parliament, the European Economic and Social Committee and the Committee of the Regions: A Common Agenda for Integration—Framework for the Integration of Third-Country Nationals in the European Union COM/2005/0389 (European Commission, 2005).

  57. How Germany is integrating its refugees. The Economist (16 September 2017);

  58. Lynch, R., Lynch, E., Loehr, J., Pettay, J. & Lummaa, V. Open Science Framework. How social class and occupations predict migration patterns and assimilation of forced migrants in wartime Finland (2017). Available at:

  59. Anonymous. Siirtokarjalaisten Tie (Nyky-Karjala, Turku, 1970).

  60. Loehr, J., Lynch, R., Mappes, J., Salmi, T., Pettay, J., & Lummaa, V. (2017). Newly Digitized Database Reveals the Lives and Families of Forced Migrants from Finnish Karelia. Finnish Yearbook of Population Research, 52, 59-70.

    Article  Google Scholar 

  61. Hytönen, T. Sodanjälkeisen asutustoiminnan vaikutus suomen väestökehitykseen (2007). Master’s thesis, University of Jyväskylä.

  62. Waris, H., Jyrkilä, V., Raitasuo, K. & Siipi, J. Siirtoväen Sopeutuminen Tutkimus Suomen Karjalaisen Siirtoväen Sosiaalisesta Sopeutumisesta (Helsingissä Otava, Helsinki, 1952).

  63. McElreath, R. Statistical Rethinking: a Bayesian Course with Examples in R and Stan (Chapman and Hall/CRC, Boca Raton, 2015).

  64. Gelman, A., Hill, J. & Yajima, M. Why we (usually) don’t have to worry about multiple comparisons. J. Res. Educ. Eff. 5, 189–211 (2012).

    Google Scholar 

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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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