Claire Leroy – PhD student in economics at CREST, École polytechnique, IP Paris

In September 2025, she will be a post-doctoral fellow at the Institute for Fiscal Studies (IFS) in London for one year. She will then take up, in September 2026, a position as assistant professor at LMU University of Munich.

Her research interests are in public economics and applied microeconomics.

She studies the impact of public policies on the economy and ways to improve their design, using empirical and theoretical approaches. She focuses on the role of imperfect information and other frictions in how citizens perceive and react to policies.

Her thesis, Raising Take-up of Welfare Programs: Evidence from a Large French Reform, was supervised by the economists Pierre Boyer and Antoine Bozio.

1. What have been the key stages of your research journey?

I began my thesis at CREST in September 2020, in the middle of the Covid crisis. As a result, my first year was a bit special: most activities, like the weekly seminars, were held online. There was little contact with other members of CREST because access to the department was still quite limited. From the end of the first year, things started to return to normal. And from that point on, my thesis was anything but solitary. I really began to build ties with other PhD students and researchers. Looking back, I think that this first “Covid” year made me realize how essential exchanges are—whether informal discussions between PhD students or seminar presentations—in research work.

A key moment of my thesis was my five-month research stay at the University of Berkeley. There, I was able to interact with Emmanuel Saez, the professor who had invited me, as well as with other researchers and PhD students. Given the level of this university, the atmosphere really pushes you to be ambitious in your research projects. This stay allowed me to meet many people, in particular other visiting PhD students, and to build a real network. I still come across people I met there at conferences.

But the experience that perhaps marked me the most was the “job market” period—that somewhat mythical moment everyone talks about during the PhD. You often hear that it’s super stressful, complicated… For my part, I ultimately found it much easier to navigate than expected. I think the mutual help between PhD students played a big role, and the atmosphere at CREST as well: it really is an environment where candidates are well supported in this stage.

2. Did you benefit from a working environment favorable to the advancement of your research—seminars, working groups, collective research activities?

CREST offers very good working conditions for PhD students. On the material side, I was able to obtain funding to do a research stay abroad, to attend conferences, and to obtain an additional year of doctoral contract.
But what I found just as important is the general atmosphere of the department. It is a very friendly environment, with little hierarchy compared to other departments. Within the economics department, everyone knows each other. There are not really barriers between PhD students and professors—it is quite normal to have lunch together, for example. There is also a real spirit of mutual help among PhD students, without competition. That makes the PhD, which can be a somewhat difficult period, much more livable. For several years now, PhD students have been organizing “PhD Breakfasts,” where useful information is shared with newcomers (about teaching, the job market, research stays abroad, etc.). And there are plenty of other initiatives in the same spirit, which are quite unique to CREST.

What makes it work, I think, is that people often come on site, mingle, and there is a real willingness among PhD students to get involved in organizing these moments. From cohort to cohort, PhD students pass on tips, advice, and often encouragement. I think this atmosphere and this collective involvement is a very precious intangible asset… but also a bit fragile. Preserving it really requires everyone’s involvement.

3. Did the PhD confirm or change your initial professional plan?

Before starting my thesis, I worked for three years at the Institut des Politiques Publiques (IPP) as a junior economist. I had already had in mind the idea of doing a thesis since my research master’s, but at the time, I had not found supervisors with whom it would have been possible.

When I began the thesis, I told myself that if an opportunity for an academic career arose, I would be delighted to seize it, while being aware that positions are rare and that it is a demanding path. I was therefore also open to the idea of continuing in a more applied direction, similar to what I had known at IPP. I approached the thesis as a kind of test: I wanted to check whether academic research really corresponded to what I was looking for.

At the end of the doctorate, I finally chose to apply for professor positions, because this test proved conclusive. Even though research obviously involves challenges, I felt in my place throughout the thesis, and this experience confirmed my desire to commit to a career as a teacher-researcher.

4. What advice would you give to a PhD student at the beginning of the journey?

I think one can give a lot of advice on very practical aspects of the thesis, but one of the most important undoubtedly concerns PhD life in general. As we know, it is a period that can be demanding, and many PhD students encounter at one time or another difficulties that can weigh on mental well-being. There is no magic recipe, but just as we invest time and energy in our research, my advice is to realize that living one’s thesis well requires personal work that also takes time and effort: learning to identify moments of blockage, knowing how to seek support when necessary, reflecting on one’s own ways of functioning. I think this work, which we don’t talk about much, is fully part of the doctoral journey.

For me, the PhD is a test period that allows you to see whether you can do research without the ups and downs of the profession—uncertainty, self-doubt about one’s abilities, stress—taking up too much space. And if it becomes too difficult, one must also know that ending one’s thesis is a completely legitimate decision, which we should normalize more. That said, I think it is important to deconstruct the very widespread idea that a “normal” PhD student must necessarily be suffering. That is not true. A thesis can also go very well, with of course a few harder moments, but overall, it can be an extremely rich experience.

5. How would you define an “in-work benefit” and what is its main objective within social policies? In what way is this type of measure important in combating precarity and improving the well-being of beneficiaries?

Two of my thesis articles deal with in-work benefits, a particular type of social assistance that appeared in the 1970s, first in the United States with the establishment of the Earned Income Tax Credit (EITC), then adopted in many countries from the 1990s onwards. In France, these measures have taken several forms: the Prime pour l’emploi (PPE) in 2001, the RSA “activité” in 2009, then the prime d’activité, which exists today and replaces these two other measures. This type of aid, which sometimes takes the form of tax credits, has a dual objective: reducing poverty while encouraging beneficiaries to remain in or return to employment.

Unlike traditional social assistance, such as the RSA in France, which targets people without any resources—and therefore without professional activity—in-work benefits are aimed at the working poor, that is, people in employment but with low incomes. By their design, these measures seek to make work financially more attractive, ensuring that “work pays.” They thus aim to limit what are called “poverty traps,” often associated with traditional benefits—situations, in theory, in which individuals are discouraged from returning to work because benefits reduce the net gain from employment.

That said, the incentive effects of these benefits depend on the economic context. They are relevant when unemployment or inactivity is explained by a lack of labor supply on the part of individuals. Conversely, if difficulties in accessing employment come mainly from a lack of demand on the part of firms, these considerations become secondary. More generally, we observe that the two types of aid—traditional aid and in-work benefits—often coexist within the same socio-fiscal system. Moreover, several works in theoretical economics show that optimal social policy generally rests on a mix between these two approaches.

6. In France, what were the effects of the 2019 reform of the prime d’activité? What conclusions can be drawn from your analysis regarding the concrete impact of this reform?

The prime d’activité, which is part of the family of in-work benefits, is a social benefit paid under means testing to households in which at least one member is in employment. It typically targets people paid at the level of the minimum wage or those with paths marked by alternations between employment and inactivity, with unstable incomes.

At the end of 2018, in the context of the Yellow Vests movement, the government announced a major reform of the prime d’activité. The reform consisted of increasing both the amount of the benefit and the number of eligible households. In the second chapter of my thesis, I focus on a very striking phenomenon: the explosion in the number of applications from January 2019, with nearly one million additional applicants. My objective is to understand what explains such an influx. The results show that this increase cannot be explained solely by the mechanical effects of the reform, that is, the broadening of eligibility criteria.

Alongside this mechanical effect, a take-up phenomenon is observed: many families who were already eligible for the prime d’activité before the reform, but who did not claim it, finally applied. This is what we call a reduction in non-take-up. According to a 2017 estimate by the DREES, about 27% of eligible households did not receive the benefit. My study shows that this rate was halved following the reform. What is interesting is that this very significant effect was unexpected, since it was not the stated aim of the reform. Faced with the increase in public expenditure generated by these new beneficiaries, the State had to revise upwards the credits allocated to the prime d’activité.

7. In your view, what are the main obstacles to the effective take-up of social benefits, despite their proven effectiveness? Are there structural or behavioral factors that explain this phenomenon of non-take-up?

Research, in economics as in other disciplines such as sociology, has highlighted a large number of factors that can explain situations of non-take-up of social benefits. Three main categories of obstacles are generally distinguished: transaction costs (linked to administrative procedures, the complexity of forms, which are costly in terms of time but also psychologically for beneficiaries), stigma (the feeling of shame towards oneself or others associated with receiving aid), and information problems (lack of awareness of the scheme’s existence, poor understanding of eligibility criteria). These factors are often interpreted as belonging to the individual sphere: it would then be the action—or inaction—of potential beneficiaries that explains non-take-up. Some situations of non-take-up can also be attributed to decisions by the administration in charge of the scheme: for example, a wrongful refusal to grant aid, whether by error or deliberately.

These obstacles to take-up vary greatly depending on contexts and schemes. In the case of the prime d’activité in France, sociological work shows that some traditionally important barriers seem to play a more limited role. Stigma, for example, appears relatively weak. This benefit is aimed at workers and is generally seen as legitimate compensation in the face of wages deemed insufficient. Moreover, the administrative costs associated with applying for the prime d’activité are relatively low. The procedure is entirely dematerialized and does not require supporting documents. On the other hand, other, more indirect forms of costs may play a role. Beneficiaries must, for example, log back in online every quarter to declare their resources, a step that some forget or consider constraining. We also observe a high frequency of overpayments (amounts unduly received that beneficiaries must repay) and arrears (sums due but paid late). These situations, often sources of stress or misunderstanding, can discourage some potential beneficiaries from applying for the benefit, for fear of finding themselves in complicated administrative procedures.

8. Your study shows that the increase in the amount of the benefit had a limited effect on take-up, unlike an “information shock.” How do you explain this gap? By what means did you measure the impact of media coverage on the behavior of potential beneficiaries?

In the case of the prime d’activité, my work seeks to understand why some families began to apply for the benefit after the 2019 reform—and thus, by extension, what initially explained their non-take-up.
A first hypothesis is that of transaction costs. If non-take-up stemmed from a rational cost-benefit calculation—in other words, if families judged that the administrative procedures were not worth it given the amount of the benefit—then the increase in the amount paid should have encouraged some of them to take the step. But my empirical analysis allows me to reject this hypothesis.

What I show, on the contrary, is that the increase in take-up is explained mainly by an informational shock. This analysis is based on comparing the evolution of applications between the prime d’activité on the one hand and the RSA (a benefit for which there was no reform or informational shock) on the other. Following the announcement of the reform, the prime d’activité—which remained a relatively recent and still little-known scheme (in 2018, one in four people did not know of its existence)—was the subject of very wide media coverage. It was massively relayed in the press, on radio and television, with explanations of eligibility criteria and target groups. Survey data confirm that a large number of people discovered the existence of the scheme at that time, which led to a sharp increase in applications at the beginning of 2019.

These results therefore suggest that a significant share of non-take-up before the reform was explained by a simple information problem, in particular a lack of knowledge of the very existence of the benefit. That said, it is also necessary to recall the limits of this “natural experiment” approach: while one can identify the effects of the reform on a particular population—those who reacted to this media shock—one cannot necessarily generalize the results to all non-take-up cases. It is likely that even today, many eligible people do not apply for the prime d’activité, but my study does not make it possible to determine which obstacles play a role for those people.

9. What lessons does your research bring for the design of public policies aimed at improving access to social benefits? In what way can your results contribute to a broader reflection on the effectiveness of social policies and on how to better reach the targeted populations?

The fact that the media coverage of the 2019 prime d’activité reform had such a marked impact on take-up makes it possible to consider avenues for reform in other contexts, whether for other social policies or in other countries.
Traditionally, economists and policymakers consider that, faced with information problems, one of the most effective responses is to implement targeted information policies (for example, personalized letters sent to individuals identified as potentially eligible and informing them of their rights in a personalized way). These targeted approaches, however, raise a number of difficulties: how to correctly identify people in situations of non-take-up? How to ensure that the message is received, understood, and perceived as legitimate? The results of studies on the subject show that this kind of intervention does not seem to work among populations that are not, or are barely, in contact with the administration.

By contrast, by extrapolation, my results suggest that a less targeted intervention—such as a national information campaign—can, paradoxically, prove more effective in combating non-take-up linked to a lack of information. We observe that the people who began to apply for the benefit following the reform were disproportionately individuals who had never been in contact with the Caisse des Allocations Familiales (CAF), the organization in charge of managing this benefit. This tends to show that such broad information campaigns can reach audiences that targeted approaches do not always manage to reach. Moreover, when a message is widely disseminated, it can also circulate through network effects, between eligible and non-eligible people, thereby strengthening its impact.

Of course, these campaigns have a cost, often higher than that of targeted mailings. It is therefore essential to balance these different elements—cost, reach, effectiveness—when thinking about the best tools to mobilize when we want to improve the take-up of social benefits.