On Fertility and Inequality: Evidence from IVF treatments
To what extent do differences in fertility (child-bearing) generate inequality? Studies have shown that fertility is associated with women’s labour market trajectories, wealth, children’s school performance, and is strongly correlated within their social networks. We know less about whether such relationships reflect a causal effect of fertility. This project aims to estimate the causal effect of fertility on the outcomes above by using data on outcomes from IVF(in-vitro-fertilization) treatments. We aim to compare women (and their partners) who end up with different number of children because of differential IVF treatment success and follow their labour market outcomes, wealth, their children’s school performance, and how fertility is affected in their social networks. Since we have established in a previous study, using the same data material, that there is a large degree of randomness in the outcome of a given IVF treatment, this method allows us to study the causal effect of fertility. Our data consists of 32,000 couples from the Danish IVF register that we have linked to labour market and school registers. We are able to observe couples that are both successful and unsuccessful in their IVF treatments and follow them (and their children) up to 20 years. Our results can lead to new knowledge about the causal relationship between fertility and inequality and should therefore be of great interest to the international scientific community and to policy-makers.
Final report
THE PURPOSE AND DEVELOPMENT OF THE PROJECT
This project aimed to examine the causal effects of fertility on various types of inequality in society. The focus was on three central dimensions: (1) inequalities in labor market outcomes and wealth distribution within couples, (2) differences in children’s school performance depending on family size, and (3) the influence of social networks on individuals’ fertility decisions. To achieve this, the project utilized the random variation created by outcomes of IVF treatments as a natural experiment. This method enabled causal analyses of how fertility impacts economic, educational, and social aspects of inequality. The project sought to address significant gaps in previous literature by applying innovative methods and using unique data sources. Specifically, the study aimed to shed light on how fertility interacts with social and economic structures in a context characterized by generous welfare and family policy systems, such as Denmark.
IMPLEMENTATION
The research team, consisting of Lundborg, Plug, and Würtz-Rasmussen, worked intensively and continuously on the project’s execution. The work involved collecting extensive registry data, data processing, and analysis. During the project, a PhD student from the University of Amsterdam, Julius Ilciukas, became involved and co-authored one of the central papers.
The participants met regularly, both in person and online, to ensure effective collaboration and to discuss ongoing analyses. The project’s findings were presented at numerous seminars, workshops, and international economics conferences. These presentations enabled critical feedback and contributed to disseminating the preliminary results to broader research networks. Furthermore, the project resulted in several working papers, some of which are currently under review in top-ranked economics journals.
KEY FINDINGS OF THE PROJECT
1. Parenthood and Labor Market Incomes
The first paper focuses on how parenthood affects long-term labor market incomes, with particular attention to so-called "child penalties"—the income loss often associated with becoming a parent. The study uses data from IVF treatments in Denmark and compares incomes between women and men up to 25 years after their first treatment. The findings show that women initially experience a significant income loss shortly after the birth of their first child. However, this loss gradually diminishes over time and disappears entirely within ten years. After 15 years, women who became mothers even have slightly higher incomes than those who remain childless. For men, no significant effects of parenthood on incomes are observed.
These findings challenge the prevailing notion that children are the primary cause of persistent gender income gaps. Instead, the study highlights how generous family policies and well-developed childcare systems, such as those in Denmark, can mitigate the negative effects of parenthood on women’s labor market outcomes.
2. Siblings’ Effects on Children’s Development
The second paper examines how the presence of siblings affects the cognitive and non-cognitive outcomes of firstborn children. Using data from IVF treatments, the study exploits the randomness of treatment success to isolate the causal effects of having siblings. The findings reveal that the presence of siblings does not significantly impact firstborn children’s school performance in subjects such as math and reading. Nor are their personality traits, such as conscientiousness and emotional stability, or their school satisfaction affected.
These results challenge traditional theories that claim only children are either advantaged or disadvantaged due to their singleton status. The study shows that siblings, at least in the Danish context, neither create significant advantages nor disadvantages for firstborn children. The findings also underscore the importance of supportive family policies in equalizing potential differences related to family structure. Additionally, the study provides important insights into the potential consequences of declining birth rates and changing family structures in modern societies.
3. Social Networks and Fertility Decisions
The third subproject investigates how individuals’ fertility decisions are influenced by other members of their social networks, such as siblings, coworkers, and close friends. Previous research suggests that social networks can amplify or dampen the effects of fertility decisions through norms, information exchange, and shared experiences. Using Danish registry data, the study analyzes these effects by leveraging IVF treatments to create random variation in fertility. The aim of this study was to isolate the causal effects of an individual’s fertility on the fertility decisions of other network members.
This work is still ongoing, and the results are not yet finalized. The findings are expected to provide valuable insights into how social networks shape fertility patterns and how these, in turn, may influence broader demographic trends.
NEW RESEARCH QUESTIONS
The project also generated several new research questions based on the same data and methodological approaches:
• Parenthood and Crime: A new project investigates how parenthood affects men’s propensity to commit crimes. Previous research suggests that parenthood is often correlated with reduced criminal behavior in men, but it is unclear whether this relationship is causal.
• Grandparents’ Labor Market Behavior: Another project examines how grandparents adjust their labor supply and retirement decisions when they become grandparents. This aims to provide a broader understanding of how different generations within families influence each other economically and socially.
These new research questions pave the way for further work and deeper analyses of how fertility and family dynamics impact various aspects of society. At the same time, the project provides important insights that can contribute to the design of future family policy reforms.
DISSEMINATION OF RESEARCH AND COLLABORATION
All project members have actively disseminated the findings from subprojects 1 and 2 through presentations at numerous seminars, workshops, and conferences. The results have also been shared, or are planned to be shared, through publications in established working paper series, such as IZA Discussion Papers. Subproject 1 has received significant attention, resulting in interviews with project members in prominent media outlets like The New York Times and The Economist, where they also participated in the podcast “Money Talks.” The findings have further been highlighted by publications such as Forbes and Frankfurter Allgemeine, underscoring the project’s relevance and impact. The project has also led to new collaborations, including with a PhD student at the University of Amsterdam, strengthening academic ties.
LIST OF PUBLICATIONS
• Lundborg, Petter & Plug, Erik & Rasmussen, Astrid Würtz, 2024. "Is There Really a Child Penalty in the Long Run? New Evidence from IVF Treatments," IZA Discussion Papers 16959, Institute of Labor Economics (IZA).
• Lundborg, Petter, Ilciukas, Julius, Plug, Erik & Rasmussen, Astrid Würtz, 2024. "The Only Child," manuscript.
LINKS TO WEBSITES
• Petter Lundborg: https://sites.google.com/site/atpetterlundborg/
• Jan Bietenbeck: https://www.janbietenbeck.com/
• Dan-Olof Rooth: https://sites.google.com/site/danolofrooth/
This project aimed to examine the causal effects of fertility on various types of inequality in society. The focus was on three central dimensions: (1) inequalities in labor market outcomes and wealth distribution within couples, (2) differences in children’s school performance depending on family size, and (3) the influence of social networks on individuals’ fertility decisions. To achieve this, the project utilized the random variation created by outcomes of IVF treatments as a natural experiment. This method enabled causal analyses of how fertility impacts economic, educational, and social aspects of inequality. The project sought to address significant gaps in previous literature by applying innovative methods and using unique data sources. Specifically, the study aimed to shed light on how fertility interacts with social and economic structures in a context characterized by generous welfare and family policy systems, such as Denmark.
IMPLEMENTATION
The research team, consisting of Lundborg, Plug, and Würtz-Rasmussen, worked intensively and continuously on the project’s execution. The work involved collecting extensive registry data, data processing, and analysis. During the project, a PhD student from the University of Amsterdam, Julius Ilciukas, became involved and co-authored one of the central papers.
The participants met regularly, both in person and online, to ensure effective collaboration and to discuss ongoing analyses. The project’s findings were presented at numerous seminars, workshops, and international economics conferences. These presentations enabled critical feedback and contributed to disseminating the preliminary results to broader research networks. Furthermore, the project resulted in several working papers, some of which are currently under review in top-ranked economics journals.
KEY FINDINGS OF THE PROJECT
1. Parenthood and Labor Market Incomes
The first paper focuses on how parenthood affects long-term labor market incomes, with particular attention to so-called "child penalties"—the income loss often associated with becoming a parent. The study uses data from IVF treatments in Denmark and compares incomes between women and men up to 25 years after their first treatment. The findings show that women initially experience a significant income loss shortly after the birth of their first child. However, this loss gradually diminishes over time and disappears entirely within ten years. After 15 years, women who became mothers even have slightly higher incomes than those who remain childless. For men, no significant effects of parenthood on incomes are observed.
These findings challenge the prevailing notion that children are the primary cause of persistent gender income gaps. Instead, the study highlights how generous family policies and well-developed childcare systems, such as those in Denmark, can mitigate the negative effects of parenthood on women’s labor market outcomes.
2. Siblings’ Effects on Children’s Development
The second paper examines how the presence of siblings affects the cognitive and non-cognitive outcomes of firstborn children. Using data from IVF treatments, the study exploits the randomness of treatment success to isolate the causal effects of having siblings. The findings reveal that the presence of siblings does not significantly impact firstborn children’s school performance in subjects such as math and reading. Nor are their personality traits, such as conscientiousness and emotional stability, or their school satisfaction affected.
These results challenge traditional theories that claim only children are either advantaged or disadvantaged due to their singleton status. The study shows that siblings, at least in the Danish context, neither create significant advantages nor disadvantages for firstborn children. The findings also underscore the importance of supportive family policies in equalizing potential differences related to family structure. Additionally, the study provides important insights into the potential consequences of declining birth rates and changing family structures in modern societies.
3. Social Networks and Fertility Decisions
The third subproject investigates how individuals’ fertility decisions are influenced by other members of their social networks, such as siblings, coworkers, and close friends. Previous research suggests that social networks can amplify or dampen the effects of fertility decisions through norms, information exchange, and shared experiences. Using Danish registry data, the study analyzes these effects by leveraging IVF treatments to create random variation in fertility. The aim of this study was to isolate the causal effects of an individual’s fertility on the fertility decisions of other network members.
This work is still ongoing, and the results are not yet finalized. The findings are expected to provide valuable insights into how social networks shape fertility patterns and how these, in turn, may influence broader demographic trends.
NEW RESEARCH QUESTIONS
The project also generated several new research questions based on the same data and methodological approaches:
• Parenthood and Crime: A new project investigates how parenthood affects men’s propensity to commit crimes. Previous research suggests that parenthood is often correlated with reduced criminal behavior in men, but it is unclear whether this relationship is causal.
• Grandparents’ Labor Market Behavior: Another project examines how grandparents adjust their labor supply and retirement decisions when they become grandparents. This aims to provide a broader understanding of how different generations within families influence each other economically and socially.
These new research questions pave the way for further work and deeper analyses of how fertility and family dynamics impact various aspects of society. At the same time, the project provides important insights that can contribute to the design of future family policy reforms.
DISSEMINATION OF RESEARCH AND COLLABORATION
All project members have actively disseminated the findings from subprojects 1 and 2 through presentations at numerous seminars, workshops, and conferences. The results have also been shared, or are planned to be shared, through publications in established working paper series, such as IZA Discussion Papers. Subproject 1 has received significant attention, resulting in interviews with project members in prominent media outlets like The New York Times and The Economist, where they also participated in the podcast “Money Talks.” The findings have further been highlighted by publications such as Forbes and Frankfurter Allgemeine, underscoring the project’s relevance and impact. The project has also led to new collaborations, including with a PhD student at the University of Amsterdam, strengthening academic ties.
LIST OF PUBLICATIONS
• Lundborg, Petter & Plug, Erik & Rasmussen, Astrid Würtz, 2024. "Is There Really a Child Penalty in the Long Run? New Evidence from IVF Treatments," IZA Discussion Papers 16959, Institute of Labor Economics (IZA).
• Lundborg, Petter, Ilciukas, Julius, Plug, Erik & Rasmussen, Astrid Würtz, 2024. "The Only Child," manuscript.
LINKS TO WEBSITES
• Petter Lundborg: https://sites.google.com/site/atpetterlundborg/
• Jan Bietenbeck: https://www.janbietenbeck.com/
• Dan-Olof Rooth: https://sites.google.com/site/danolofrooth/