Female-biased sex ratios in urban centers create a “fertility trap” in post-war Finland

Jenni E Pettay, Virpi Lummaa, Robert Lynch & John Loehr (2021): Behavioral Ecology
DOI: 10.1093/beheco/arab007

Abstract

Because sex ratios are a key factor regulating mating success and subsequent fitness both across and within species, there is widespread interest in how population-wide sex ratio imbalances affect marriage markets and the formation of families in human societies. Although most modern cities have more women than men and suffer from low fertility rates, the effects of female-biased sex ratios have garnered less attention than male-biased ratios. Here, we analyze how sex ratios are linked to marriages, reproductive histories, dispersal, and urbanization by taking advantage of a natural experiment in which an entire population was forcibly displaced during World War II to other local Finnish populations of varying sizes and sex ratios. Using a discrete time-event generalized linear mixed-effects model, and including factors that change across time, such as annual sex ratio, we show how sex ratios, reproduction, and migration are connected in a female-dominated environment. Young childless women migrated toward urban centers where work was available to women, and away from male-biased rural areas. In such areas where there were more females, women were less likely to start reproduction. Despite this constraint, women showed little flexibility in mate choice, with no evidence for an increase in partner age difference in female-biased areas. We propose that together these behaviors and conditions combine to generate an “urban fertility trap” which may have important consequences for our understanding of the fertility dynamics of today including the current fertility decline across the developed world.

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John Loehr with his workgroup received EUR 225.000 grant from Kone Foundation in 2016 for their project Learning from the past: the effect of forced migration from Karelia on family life.
Karelia-project had their kick-off meeting at the University of Turku 19.4.2017. Intense discussions, good spirit and a lot of inspiration among the team!

Menikö luonnonvalinnalla jotain pieleen: Miksi nainen elää menopaussin jälkeen lähes saman mokoman vaikkei voi saada jälkeläisiä?

Virpi Lummaa

Our multidisciplinary research team is looking for a post-doctoral researcher for a three-year project investigating life history, social integration and the influence of kin in forced migrants in a 20th century Finnish population.

The project is an exciting opportunity to investigate the consequences of forced migration of over 400000 people during World War II from an evolutionary ecology and sociology viewpoint. These migrants encountered much the same traumas and faced similar prejudices and resentment that current migrants face today, making the study of this population particularly appropriate to gain insight into the present and future of current migrants.
 

John Loehr with his workgroup received EUR 225.000 grant from Kone Foundation in 2016 for their project Learning from the past: the effect of forced migration from Karelia on family life.

The plight of migrants has come to the forefront recently as masses of people have migrated to Europe seeking asylum from predicaments faced at home. Many people in Finland seem to have forgotten that over 400,000 Finnish people had to abandon their homes in Karelia as a result of World War II. In this cross-disciplinary project, directed by John Loehr, an ecological scientist, biologists, sociologists, historians and demographic researchers study how enforced migration has affected family relations, having children, and integration into the community.

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Carly, Verane, Simon, Kimmo, Virpi, Jenni, Samuli, Martin, Mirkka

The research group spent three intense days having a brilliant Project Meeting in Tampere, Finland in August 2016. The venue was the most beautiful place by the lake, surrounded by the pristine Finnish nature. A perfect venue for the best conference ever! Special thanks to our hosts Jenni and Esko.
Photos from the Project Meeting in Kesämaa, Finland, August 2016. Photos by Esko Pettay / Wild TechPhotos Oy.

Virpi Lummaa's Group: Project meeting in Finland, August 2016. Photo by Esko Pettay

Virpi Lummaa is an outstanding evolutionary biologist and her work has led to significant advances in our understanding of the ecological causes and evolutionary consequences of variation in reproductive success and longevity. Her research on humans, based on detailed pedigrees, and birth and death records, revealed the selection pressures shaping life history in pre-industrial populations, and in so doing allowed for the first rigorous, scientific examination of human behavioural ecology. These findings have revealed the complex trade-offs shaping recent human evolution.
The Scientific Medal, Britain's zoological Oscar, is awarded to scientists with up to 15 years postdoctoral experience for distinguished work in zoology.

Virpi Lummaa Scientific Medal 2016 Zoological Society of London