Olle Hammar

Google Scholar

Peer-Reviewed Publications

A Commentary on "Gender Bias in Parental Attitude: An Experimental Approach" by Begum, Grossman, and Islam (2018)
Demography 12344620, 2025 (with Carl Bonander, Gunther Bensch, Niklas Jakobsson and Abel Brodeur)

Begum et al. (2018) examined gender bias in parental attitudes using an experimental approach in rural Bangladesh. Households were reported as randomly assigned to treatment conditions in a lab-in-the-field allocation task. We show that the group assignment was inherited from Islam (2019), a previous, nonrandomized experiment conducted in the same region. The lack of randomization contradicts the design descriptions provided by the authors in Begum et al. (2018) and elsewhere and raises concerns about the validity of comparisons across treatment groups. This also points to serious shortcomings in the reporting and transparency of the study design—issues that mirror those that led to the retraction of Islam (2019) from the European Economic Review.

Material: Online Appendix, Replication Package, Authors' Response, Editorial Note
Coverage: Medium
Working Paper: I4R DP 267

"Try to Balance the Baseline": A Comment on "Parent-Teacher Meetings and Student Outcomes: Evidence from a Developing Country" by Islam (2019)
European Economic Review 175: 105021, 2025 (with Carl Bonander, Niklas Jakobsson, Gunther Bensch, Felix Holzmeister and Abel Brodeur)

Islam (2019) reports results from a cluster randomized field experiment in Bangladesh that examines the effects of parent-teacher meetings on student test scores in primary schools. The reported findings suggest strong positive effects across multiple subjects. In this report, we demonstrate that the school-level randomization cannot have been conducted as the author claims. Specifically, we show that the nine included Bangladeshi unions all have a share of either 0% or 100% treated or control schools. Additionally, we uncover irregularities in baseline scores, which for the same students and subjects vary systematically across the author's data files in ways that are unique to either the treatment or control group. We also discovered data on two unreported outcomes and data collected from the year before the study began. Results using these data cast further doubt on the validity of the original study. Moreover, in a survey asking parents to evaluate the parent-teacher meetings, we find that parents in the control schools were more positive about this intervention than those in the treated schools. We also find undisclosed connections to two additional RCTs.

Material: Online Appendix, Replication Package, Author's Response, Retraction Notice
Coverage: The Australian, Medium, Retraction Watch, I4R, The Australian, Medium, Berghs betraktelser
Working Paper: I4R DP 214, IZA DP 17781

Where Would Ukrainian Refugees Go if They Could Go Anywhere?
International Migration Review 57(2): 587-602, 2023 (with Mikael Elinder and Oscar Erixson)

We present estimates of the number of refugees expected to flee Ukraine and to which countries they are expected to migrate based on migration preferences data from the Gallup World Poll. This is important in terms of both immediate refugee assistance efforts and long-term integration policies. Our key finding is that as many as twelve million people may want to leave Ukraine permanently and that refugee policies in potential destination countries are likely to have a substantial impact on the distribution of Ukrainian refugees between different countries. More specifically, international solidarity in response to the migration crisis would significantly reduce the refugee flows to EU countries, incur a limited burden on non-EU countries, and, at the same time, better take the preferences of the Ukrainians into account.

Material: Online Appendix
VoxUkraine: Where Do Ukrainians Want to Go? Migration Aspirations and Destination-Country Preferences in Ukraine
Video: Delmi, EBA, ESO
Coverage: Svenska ESF-rådet, ABF Stockholm, SVT Forum, ESO, VoxUkraine, openDemocracy, Bohusläningen, Kristianstadsbladet, Norra Skåne, Strömstads Tidning, Västerbottens-Kuriren, Free Network, SvD Debatt
Working Paper: IFN WP 1440

Social Networks and Immigrant Integration: Experimental Evidence from Sweden
Journal of Development Economics, Accepted, Pre-Results Review, 2022 (with Mounir Karadja and Akib Khan)

Immigrant integration is key to realizing the potential of international migration in economic development. However, integration is often thought to be hampered by immigrants' limited social networks beyond their co-ethnics. We study how contact with natives affects immigrants' social and economic outcomes through a field experiment. We have partnered with an NGO in Sweden that annually matches over one thousand pairs of native Swedes and immigrants for informal meeting to promote new social connections and facilitate integration. Using an RCT and a combination of survey and administrative register data, we will study how such matches with native Swedes affect immigrants' social relations and values, and whether they lead to better labor and housing market outcomes via access to information or referrals. We will also examine changes in immigrants' attachment to their country of origin and remittances to family and friends outside Sweden.

Trial Registration: AEA RCT Registry 6714
Video: Lindau Nobel Laureate Meetings
Coverage: Folkuniversitetet, Sylff, ABF Stockholm, Next Gen Economics, Publikt, UR Samtiden

Global Earnings Inequality, 1970-2018
Economic Journal 130(632): 2526-2545, 2020 (with Daniel Waldenström)

We estimate trends in global earnings dispersion across occupational groups by constructing a new database that covers 68 developed and developing countries between 1970 and 2018. Our main finding is that global earnings inequality has fallen, primarily during the 2000s and 2010s, when the global Gini coefficient dropped by 15 points and the earnings share of the world's poorest half doubled. Decomposition analyses show earnings convergence between countries and within occupations, while within-country earnings inequality has increased. Moreover, the falling global inequality trend was driven mainly by real wage growth, rather than changes in hours worked, taxes or occupational employment.

Material: Online Appendix, Replication Package
Data: The Global Earnings Inequality Database
Virtual Issue: The Economics of Income and Wealth Distribution
VoxEU: Global Earnings Inequality: Evidence from a New Database
Podcast: IFN-podden
Coverage: VoxEU, LIS Newsletter, VoxDev, Research Briefs, FAZ, DW, LTN, Bergh, Kvartal, SvD, Wikipedia
Working Paper: CEPR DP 12019, IFN WP 1166, IZA DP 10762, UU NEK WP 2017:7

The Impact of Social Beliefs on Microfinance Performance
Journal of International Development 27(7): 1074-1097, 2015 (with Katarzyna Burzynska)

We apply a panel of 331 microfinance institutions from 37 countries to investigate the relationship between social beliefs and microfinance financial performance over the period of 2003-2011. We find that microfinance institutions in countries with higher levels of trust and more collectivist culture have lower operating and default costs and charge lower interest rates. These results provide the first large cross‐country evidence that social beliefs are important determinants of microfinance performance.

Video: Lund University
Working Paper: Knut Wicksell WP 2014:5

Working Papers

Comparing Human-Only, AI-Assisted, and AI-Led Teams on Assessing Research Reproducibility in Quantitative Social Science
I4R Discussion Paper 195, 2025 (with Abel Brodeur, David Valenta, Alexandru Marcoci, Juan P. Aparicio, Derek Mikola, Bruno Barbarioli, Rohan Alexander, Lachlan Deer, Tom Stafford, Lars Vilhuber, Gunther Bensch, et al.)

This study evaluates the effectiveness of varying levels of human and artificial intelligence (AI) integration in reproducibility assessments of quantitative social science research. We computationally reproduced quantitative results from published articles in the social sciences with 288 researchers, randomly assigned to 103 teams across three groups — human-only teams, AI-assisted teams and teams whose task was to minimally guide an AI to conduct reproducibility checks (the “AI-led” approach). Findings reveal that when working independently, human teams matched the reproducibility success rates of teams using AI assistance, while both groups substantially outperformed AI-led approaches (with human teams achieving 57 percentage points higher success rates than AI-led teams, p < 0.001). Human teams were particularly effective at identifying serious problems in the analysis: they found significantly more major errors compared to both AI-assisted teams (0.7 more errors per team, p = 0.017) and AI-led teams (1.1 more errors per team, p < 0.001). AI-assisted teams demonstrated an advantage over more automated approaches, detecting 0.4 more major errors per team than AI-led teams (p = 0.029), though still significantly fewer than human-only teams. Finally, both human and AI-assisted teams significantly outperformed AI-led approaches in both proposing (25 percentage points difference, p = 0.017) and implementing (33 percentage points difference, p = 0.005) comprehensive robustness checks. These results underscore both the strengths and limitations of AI assistance in research reproduction and suggest that despite impressive advancements in AI capability, key aspects of the research publication process still require human substantial human involvement.

Podcast: Allegedly does not replicate
Working Paper: IZA DP 17645

How Has the War in Ukraine Affected Russian Sentiments?
IZA Discussion Paper 17457, 2024 (with Mikael Elinder and Oscar Erixson)

We analyze the effects of the war in Ukraine on various sentiments in the Russian population, using the exogenous timing of Gallup and Levada surveys around key war events. The invasion spurred strong rally-'round-the-flag effects, while the mobilization was disliked. The Wagner rebellion had no significant impact. High war support is sustained—despite high casualties—through strategic recruitment and economic compensations. Russians abroad have turned against Putin, aligning with global views. Analyzing different surveys, less sensitive questions, monthly data, and potential respondent selection, we find no evidence that our conclusions would be driven by selection or false reporting.

Working Paper: IFN WP 1510, LNU WP in Economics and Statistics 14, arXiv 2410:00663

Mass Reproducibility and Replicability: A New Hope
I4R Discussion Paper 107, 2024 (with Abel Brodeur, Derek Mikola, Nikolai Cook, et al.)

This study pushes our understanding of research reliability by reproducing and replicating claims from 110 papers in leading economic and political science journals. The analysis involves computational reproducibility checks and robustness assessments. It reveals several patterns. First, we uncover a high rate of fully computationally reproducible results (over 85%). Second, excluding minor issues like missing packages or broken pathways, we uncover coding errors for about 25% of studies, with some studies containing multiple errors. Third, we test the robustness of the results to 5,511 re-analyses. We find a robustness reproducibility of about 70%. Robustness reproducibility rates are relatively higher for re-analyses that introduce new data and lower for re-analyses that change the sample or the definition of the dependent variable. Fourth, 52% of re-analysis effect size estimates are smaller than the original published estimates and the average statistical significance of a re-analysis is 77% of the original. Lastly, we rely on six teams of researchers working independently to answer eight additional research questions on the determinants of robustness reproducibility. Most teams find a negative relationship between replicators’ experience and reproducibility, while finding no relationship between reproducibility and the provision of intermediate or even raw data combined with the necessary cleaning codes.

Video: Toronto Workshop on Reproducibility
Coverage: OSIRIS, Open Philanthropy, Ekonomistas, Marginal Revolution, IZA Newsroom
Working Paper: IZA DP 16912

Job Market Paper

The Cultural Assimilation of Individualism and Preferences for Redistribution
Job Market Paper, November 2022

I analyze the relationship between individualism and preferences for redistribution, using variation in immigrants’ countries of origin to capture the impact of cultural values and beliefs on personal attitudes towards income redistribution and equality. Using global individual-level survey data for almost one million individuals (including 65,000 migrants) in a large number of countries around the world, I find strong support for the hypothesis that more individualistic cultures are associated with lower preferences for redistribution. At the same time, cultural assimilation in this dimension seems to take place relatively fast, where the impact of the destination culture starts to dominate the origin culture when an individual has lived as long in the country of destination as she did in her country of origin. Moreover, I find no statistically significant effect of the origin culture on an individual’s preferences for redistribution if migration took place before the age of 10. The results are confirmed using a variety of robustness checks, including the grammatical rule of a pronoun drop as an instrumental variable.

Video: Faculti, PhD-EVS, Economics of Migration
Coverage: Migration Facts, Nonicoclolasos

Work in Progress

The Swedish Income Distribution, 1968-2016
(with Paula Roth and Daniel Waldenström)

We provide new evidence on income inequality levels and trends in Sweden from 1968 to 2016. By combining data from tax and population registers, we construct a new dataset that includes the distribution of pre-tax total and post-tax disposable income from labor, business, capital, transfers and benefits, for the full Swedish population since 1968. As such, we can also compare income inequality for both individuals and various household concepts. Our estimates extend Statistics Sweden's official inequality series based on full-population registers (available since 2011) by more than four decades. Our results indicate that the 1980s was the decade with the lowest level of overall income inequality in Sweden (as measured by the Gini coefficient), while income inequality as measured by top income shares for the very top (top 0.001%) has increased steadily over the studied period.

Wealth Inequality in Sweden, 1999-2020
(with Paula Roth, Felicia Stokke and Daniel Waldenström)

This paper presents new evidence on individual and household wealth for the full Swedish population and estimates of wealth inequality between 2000 and 2020. The data come from a large number of administrative and private corporate registers, many of which have never been used before in this context. The aggregate household balance sheet is made consistent with the national and financial accounts of Sweden in all years. Results are shown for the Gini coefficient, top and bottom wealth shares, and decompositions across asset classes and wealth holder groups.

Coverage: Omni, SVT Nyheter, Ekonomibyrån, Ownershift

Community Sponsorship for Refugee Integration: A Randomized Evaluation
(with Mounir Karadja and Akib Khan)

We use an RCT to evaluate the UN refugee-agency UNHCR's Community Sponsorship (CS) model in Sweden. The CS model matches newly arrived refugees with local contacts from the host municipality in order to improve language skills, cultural exchange, and new social networks. We have partnered with several municipalities that work closely with the local CSOs to create these matches. Our study will be the first randomized evaluation of this popular program, which has helped resettle hundreds of thousands of refugees worldwide. As such, this project will contribute to the broad research and societal discussion on integration by studying an innovative and scalable intervention. We evaluate the intervention based on its effects on social and economic integration, as well as values, health, and well-being.

Trial Registration: AEA RCT Registry 13394

The Global Distribution of Human and Nonhuman Wealth
(with Daniel Waldenström)

In this paper, we introduce novel estimates of wealth inequality, integrating the standard household wealth concept with newly assessed individual human capital. Using microdata and national accounts from numerous countries since 2000, we explore the distribution across age, gender, education, and occupation. Our analysis reveals two key findings: human capital is more evenly distributed than financial capital, and total wealth, the sum of human and financial capital, is significantly more equal than financial wealth alone. This study offers a groundbreaking perspective on global wealth dynamics, emphasizing the critical, yet often overlooked, role of human capital in wealth distribution.

Video: Institute for Futures Studies

Longitudinal Changes in Non-European Migrant Values and Social Norms in Sweden
(with Bi Puranen and Christian Welzel)

What happens with an individual's values when he or she migrates to a new country with other social norms than in the country of origin? In this project, we study cultural change among migrants by analyzing individual-level survey data from a large sample of non-European migrants in Sweden collected through the Migrant World Values Survey. The case of Sweden is interesting both for being a country that has recently experienced a large increase in its foreign-born population share, and also for having cultural values quite distinct from the average among its recent immigrant waves. In the study, we first analyze how selected the Swedish immigrants are by comparing the values of newly arrived migrants to the average in their origin country or cultural zone. Second, by comparing similar immigrants that have lived differently long in Sweden, we study the speed of cultural convergence in different cultural dimensions including values related to trust, choice, family, religion, and politics. Third, we also analyze the potential determinants of this cultural integration speed, including gender, age, income, education, employment, language, demographics, origin and cohort effects. Our study contributes to the literature on migration and cultural change, and will have important policy implications regarding the inclusion and integration of migrants.

Distributional National Accounts in the Welfare State: Sweden, 1930-2020
(with Paula Roth, Daniel Waldenström and Gabriel Zucman)

This paper presents new evidence on the evolution of income and wealth inequality in Sweden since 1930. The basis for the analysis is the distributional national accounts (DINA) methodology and to match individual register data with macroeconomic totals from the national accounts and thereby compute new estimates of pre- and post-tax/transfer distributions of income and wealth in Sweden. The paper makes several contributions to the literature on income and wealth distribution. It is the first paper to apply the recent DINA methodology using full-population administrative registers that cover multi-decadal panels. This allows for an extensive scrutiny of a range of the assumptions and method approaches used in past DINA-studies of, e.g., inequality in the US and France. Furthermore, the paper will be the first one that studies the DINA series for a Scandinavian welfare state, Sweden, over its entire evolution since 1930.

Replication Reports

A Comment on "Your Place in the World: Relative Income and Global Inequality" by Fehr, Mollerstrom and Perez-Truglia (2022)
I4R Discussion Paper 228, 2025 (with Erwan Dujeancourt and Francesca Foliano)

Fehr, Mollerstrom and Perez-Truglia (2022) studied individual preferences for policies addressing global inequality by conducting a two-year, face-to-face survey experiment on a representative sample of Germans from the Socio-Economic Panel (SOEP). They found that Germans systematically underestimated their true place in the global income distribution; that these misperceptions were persistent; and that correcting those misperceptions did not affect their support for polices related to global inequality (while information provision about national relative income affected demand for national redistribution, but only for left-of-center respondents). In this replication report, we present the results from a computational reproduction and robustness replication of Fehr et al. (2022). While direct access to the SOEP microdata is restricted, we were able to obtain access for these reproduction and replication purposes. We confirm that the original study is computationally reproducible and that the main results are generally robust to the following alterations: controlling for political left instead of party; using an extended set of control variables; and dropping observations with missing values.

Material: Replication Package

A Comment on "Income and Inequality in the Aztec Empire on the Eve of the Spanish Conquest"
I4R Discussion Paper 221, 2025

Alfani and Carballo (2023) estimate the levels of income inequality in the Aztec Empire around 1492, that is, before the Spanish conquest. Their main estimate finds that the Gini index was 50.4. They conclude that income inequality in the Aztec Empire was high even before the Spanish conquest, questioning to what extent today's high levels of economic inequality in Mexico can be explained by the Spanish conquest and extractive institutions imposed by the colonizers. First, I confirm that the main outcomes are computationally reproducible from the analysis data provided in the replication package. Second, I detect two inconsistencies with respect to the Kolmogorov-Smirnov test and the IV analysis, but after correcting for those I obtain qualitatively similar results. Third, I test the robustness reproducibility of the relationship between population density and per capita income through eleven different robustness tests, confirming a positive and statistically significant relationship but with large variation in the point estimates. Finally, I use these estimates as inputs for robustness measures of the main outcomes. On average, these robustness reproductions yield very similar results to those in the original paper. However, they also indicate large uncertainty about the exact estimates, for example, with Gini index estimates ranging between 38.8 and 65.6. As such, the conclusion that the level of income inequality was higher in the Aztec Empire than in modern Mexico does not appear to be a robust finding. The finding that it was more unequal than in contemporary United States, however, seems to be robust.

Material: Replication Package

Replication Report: A Comment on Gethin, Martínez-Toledano & Piketty (2022)
I4R Discussion Paper 19, 2023 (with Da Gong)

Gethin, Martínez-Toledano and Piketty (2022) analyze the long-run evolution of political cleavages using a new database on socioeconomic determinants of voting from approximately 300 elections in 21 Western democracies between 1948 and 2020. They find that, in the 1950s and 1960s, voting for the "left" was associated with lower-educated and low-income voters. After that, voting for the "left" has gradually become associated with higher-educated voters, while high-income voters have continued to vote for the "right". In the 2010s, there is a disconnection between the effects of income and education on voting. In this replication, we first conduct a computational reproduction, using the replication package provided by the authors. Second, we do a robustness replication testing to what extent the original results are robust to i) restricting the sample to "core" left and right parties, ii) analyzing the top 80% versus bottom 20%, iii) weighting by population, iv) dropping control variables, and v) using country fixed effects. The main results of the paper are found to be largely replicable and robust.