Economics serving society

Working papers

Working papers serie, “Macroeconomic Risk” Chair, SCOR-PSE

WP n°2021-05 - Sovereign default and imperfect tax enforcement

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By Francesco Pappadà (Paris School of Economics and Banque de France) and Yanos Zylberberg (University of Bristol, CESifo and the Alan Turing Institute)

Executive Summary
Sovereign default risk typically decreases in response of fiscal consolidations. However, the response of sovereign default risk to fiscal policy is dampened when tax enforcement is weak. A fiscal consolidation leads to an expansion of the informal sector, thereby limiting fiscal surpluses, but also hampering future tax collection and failing to reduce default risk. For instance, during the European sovereign debt crisis of 2009 - 2014, several economies with relatively low tax enforcement implemented fiscal consolidations that led to significant welfare costs but limited effects on default risk.

In this paper, we study the dynamics of fiscal policy and default risk when tax enforcement is imperfect. The contribution of the paper is threefold. We first document stylized facts about tax compliance and its dynamics in economies with imperfect tax enforcement, most notably the relationship with fiscal policy and default risk. We then provide a model of sovereign debt with limited commitment in order to understand how the dynamics of tax compliance---disciplined by the empirical moments---affects optimal fiscal policy and default risk. Finally, we quantify the ignored, yet important, welfare cost associated with imperfect tax enforcement: a responsive tax compliance significantly constrains optimal fiscal policies, which, ultimately, has an impact on consumption smoothing.

We uncover novel empirical facts about the dynamics of tax compliance and its impact on default risk. First, we show that tax compliance is volatile and there is large heterogeneity in volatility across countries. Tax compliance is volatile because it strongly responds to fiscal policy and business cycle fluctuations. The heterogeneous volatilities across economies reflect large heterogeneity in such responses. In some economies with imperfect tax enforcement, a larger share of taxpayers hide their activity in downturns and in periods of austerity. In contrast with the standard behavioral response, the magnitude of fluctuations in tax compliance implies sharply decreasing returns to taxes, and some economies display an extreme form of fiscal fatigue. Second, the response of tax compliance to fiscal policy alters the relationship between fiscal policy and default risk. We find that fiscal consolidations are associated with a marked decrease in default risk, but only in countries where tax compliance is inelastic. Instead, when tax compliance strongly responds to taxes, this adjustment directly affects default risk and significantly limits the returns to fiscal consolidations.

We explore the implications of fluctuations in tax compliance on the dynamics of optimal fiscal policy in a model of sovereign debt where a benevolent government uses fiscal policy as a consumption-smoothing instrument. In our quantitative analysis, we evaluate how the dynamic properties of tax compliance affect optimal fiscal policy and welfare by comparing two economies differing along the tax compliance response to fiscal policy and business cycle fluctuations around the (same) steady-state level. The baseline economy differs from the low-response economy in two important dimensions. First, the baseline economy is ten times more likely to experience a default (with a yearly probability of 0.2%, and a yearly probability to be excluded from financial markets of 1.8%). Default is more likely, even though the baseline economy accumulates far less debt on average (10% of output versus 21%). Second, fiscal policy in the baseline economy is less able to smooth fluctuations in consumption: household consumption is much more volatile around the same average levels. We use the model to quantify the costs of such fluctuations and find that they are equivalent to a 2.2% decrease in certainty equivalent consumption. These findings illustrate that fluctuations in tax compliance constrain the set of feasible fiscal policies and significantly lower welfare.

WP n°2020-04 - Waiting for the Prince Charming: Fixed-Term Contracts as Stopgaps

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By Normann Rion (Paris School of Economics, ENS-PSL)

Executive Summary
Over the last decades, fixed-term employment has raised in the European Union. Fixed-term employment provides flexibility to firms in front of workload change, but it is costly to the workers: fixed-term workers face a higher unemployment risk than open-ended workers. Moreover, fixed-term employment also causes an inefficiently high turnover. As a response, European governments have carried out many reforms of employment protection legislation. Prominent questions remain, though. In particular, how do fixed-term contracts impact unemployment and welfare? What are the main risks associated with the implementation of these reforms?

To tackle these issues, I extend the classic model of Mortensen and Pissarides to add open-ended and fixed-term contracts. On the job creation side, firms post vacancies while unemployed workers search for a job. Vacancies and unemployed workers meet according to a matching function. New firm-worker matches differ in productivity. They optimally choose between going back to search or starting production under a fixed-term contract or an open-ended contract. On the job destruction side, firms lay off open-ended workers that become unprofitable and pay firing costs. Fixed-term matches only split with an exogenous probability at zero cost.

The future value of a match is uncertain to searching firms and workers. Two polar cases arise. If the initial productivity of the match is low, the firms and the worker go back to search. On the contrary, if the initial productivity is high, the firm and the worker lock up the match to make the most out of it; the immediate gains overcome the potential firing costs in the future. The option of offering a fixed-term contract strikes a balance. It provides a production opportunity for the best rejected matches and a quick return to searching for a more productive match.

Two opposite mechanisms shape the equilibrium. On the one hand, higher firing costs encourage substitution towards fixed-term contracts. On the other hand, higher firing costs discourage the destruction of open-ended matches, which reduces unemployment and job creation of both open-ended and fixed-term contracts. As such, the response of unemployment and welfare is non-monotonous. I calibrate the model on the French labor market and find that small changes in firing costs cannot jointly enhance employment and welfare. The optimal reform consists in a large cut in firing costs and the ban of fixed-term contracts.

Post-reform dynamics are costly in the short run. Consider a 50 % cut in firing costs combined with the ban of fixed-term contracts. On impact, it leads to an increase in unemployment and decrease in open-ended employment. Unemployment remains above its initial value for at least one year before eventually reaching a lower value. Similarly, open-ended employment stays below its initial value in the short-run. Overall, an in-depth reform may improve open-ended employment and unemployment in the long-run at the expense of an adverse transition.

WP n°2020-03 - Financial Cycles with Heterogeneous Intermediaries

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By Nuno Coimbra (Paris School of Economics) and Hélène Rey (London Business School, CEPR and NBER)

Executive Summary
In the aftermath of the great financial crisis, the question of excessive risk-taking by banks and other financial intermediaries became very important in the minds of policy makers and academics alike. Understanding its causes and origins is essential in designing adequate regulations, since risk is also a fundamental and inherent aspect of the financial sector. One important concern is understanding whether monetary policy affects risk-taking in the financial sector and also when and how the two are linked. Did the period of historically low rates before the crisis worsen financial stability? Is the current low interest rate environment also leading to the build-up of systemic risk?

This paper looks at how competition between different financial intermediaries can generate a strong connection between policy rates and financial stability. Interest rates affect the composition of the financial sector and how assets are distributed between riskier and less risky financial intermediaries.

When interest rates become very low, riskier financial intermediaries find it very cheap to grow using leverage and competition can push safer, more conservative ones out of risky financial markets. For example, a cautious bank who wanted to provide mortgages during the boom might find it hard to compete with a neighboring bank which is willing to provide No-Income-No-Job-no-Assets (NINJA) mortgages at very cheap rates. As the name suggests, these were very risky mortgages where the lender did not even require the borrower to provide any information on their job, income or wealth. The easier it is for the riskier bank to find cheap funds to provide these mortgages, the harder it will be for the conservative bank to find clients for his own mortgages. This means that low interest rate environments can lead to lower financial stability through its effect on the composition of the financial sector.

A macroeconomic model with heterogeneous financial intermediaries helps clarify the full effect of interest rates on the composition of the financial sector. When interest rates fall, there are two competing effects. The first, already explained above, means that competition for financial assets drives up asset prices and decreases expected returns, which can lead to a decrease in the market share of safer banks and financial intermediaries. This leaves the financial sector more concentrated into riskier, highly-levered intermediaries generating low risk-premia and higher systemic risk. The second effect works through the cheaper cost of liabilities. If the cost of liabilities falls, then more conservative intermediaries might find it now worthwhile to increase their holdings of risky assets and so the market becomes less concentrated and financial stability is improved. The first effect tends to dominate when interest rates are already low to begin with, while the second one dominates for environments with higher rates. So an expansionary monetary policy worsens financial stability in low interest rate environments, but can actually improve it when rates are high to begin with.

In conclusion, when interest rates are not very low, central banks who need to stimulate the economy do not face any trade-off in terms of financial stability. But when interest rates are already low, further cuts might come at a cost in terms of systemic risk and central banks would do well to recognize this.

WP n°2019-02 - From Microeconomic Favoritism to Macroeconomic Populism

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By Gilles Saint-Paul (Paris School of Economics, ENS-PSL and NYU-AD)

Executive Summary
Why would people support macroeconomic policies that are likely to lead to sovereign crises, balance of payments crises, and the like? A rational explanation is based on favoritism – an institutional feature of society implying that some social groups have better access to public goods than others. A favored group that bears a low fraction of the costs of a crisis but benefits in the short-run from unsound policies is likely to support fiscal indiscipline. This paper formalizes the role of favoritism for public spending, indebtedness, and crisis in an illustrative model based on Saint-Paul et al. (2017), and studies support for political parties implementing it, so-called «populists». It argues that favoritism shaped the recent history of French pension reforms and confirms its effect on macroeconomic policy across a panel of countries.


Favoritism generates fiscal indiscipline if the decisive voter is favored relative to the mean in crisis times. When the government’s fiscal capacity is insufficient to cover its obligations and society enters a fiscal crisis, people’s access to their entitlement of publicly provided goods must be rationed. Under favoritism, this adjustment is mostly burdened on unfavored groups. By pursuing unsound fiscal policies, the favored decisive voter can engineer future crisis and manages to have the public good on average financed by others. For example, increasing the level of public debt implies more rationing in crisis times but relatively less so for favored groups who also benefit from increased private consumption possibilities through higher debt. Absent crisis, Ricardian equivalence holds and debt has no effect on society. Thus, the incentive to raise more debt stems from states of fiscal crisis only. It becomes stronger the greater the probability of a crisis. Similarly, favoritism increases public spending. Since favoritism need not be a structural property of society, the paper then studies how favoritism arises as an outcome of collective choice between either a populist or a technocrat. The populist implements favoritism regardless of fiscal and macroeconomic conditions. The technocrat sticks to anonymity and rations access to publicly provided goods only in a crisis. It is shown that the support for the populist is greater, the greater greater the likelihood of a crisis.


The recent history of French pension reforms is used as an example for this paper’s mechanisms. In particular, it was rational for French public sector employees to support the reduction in the retirement age from 65 to 60, implemented by the Mitterand administration following his 1981 presidential victory, despite overwhelming evidence that it was fiscally unsustainable. Civil servants, having their own special pension system, had good reason to anticipate that subsequent adjustments were likely to hit other social groups proportionally more. Indeed, the first attempt to balance the accounts of the pension system, the 1993 Balladur reforms, made it more difficult for private sector employees only to retire at the age of 60 by raising the duration of their contributions from 37.5 to 40 years. Formal statistical evidence in favor of the model’s predictions is provided by merging four country-level databases, the IMF’s World Economic Outlook for macro indicators, the Institutional Profiles Database (IPD) for indicators of favoritism at the institutional level, the Database of Political Institutions (DPI) for indicators of party ideology, and the CRAG-Bank of Canada database of sovereign defaults to get proxies for fiscal crises. Overall, the results support the theory. Unequal treatment from administrations, a proxy for favoritism, is more likely to generate high debt, high public expenditures, and high deficits, as well as (indirectly through debt) sovereign default. Furthermore, adverse fiscal conditions such as high public debt, high deficits, and low fiscal capacity are more likely to lead to a populist government.

WP n°2019-01 - Trading ambiguity: a tale of two heterogeneities

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By Sujoy Mukerji (Queen Mary University of London), Han N. Ozsoylev (Koç University) and Jean-Marc Tallon (Paris School of Economics, CNRS)

Executive Summary
The financial literature largely assumes that investors know the distribution of asset returns. In most real-world situations, however, decision makers are uncertain about the data-generating process underlying asset returns. This has important implications for portfolio choice, because investors may prefer portfolio allocations that are robust across the set of return distributions believed to be possible. Novel to the literature, the paper shows that such ambiguity potentially explains several puzzling cross-sectional regularities.

Two heterogeneities are key: First, the heterogeneity in the uncertainty about the mean of an asset’s return distribution. This uncertainty parameter encapsulates the ambiguity of assets. It is high, for example, for stock returns of new-technology companies whose risks have not yet been fully learned. Second, investors differ, additionally to their risk aversion, in their tolerance for ambiguity. Together, these heterogeneities give rise to a parsimonious extension of the standard mean-variance framework (referred to as robust mean-variance) in which investors face a three-way trade-off between expected return, variance, and ambiguity. The paper considers, in turn, the implications for portfolio choice, equilibrium prices and returns, and trade upon the arrival of public information.

More ambiguity-averse investors are shown to hold less of the more ambiguous assets. This finding not only confirms the failure of the classical mutual funds theorem (Tobin, 1958) in the presence of heterogeneous ambiguity-aversion, but the direction of departure is also empirically compelling. Indeed, conservative investors are commonly encouraged to hold more bonds, relative to stocks. Such financial planning advice is inconsistent with standard mean-variance investors (asset allocation puzzle), but can be accommodated in this framework.

Turning to equilibrium prices, the authors show that despite the failure of the mutual funds theorem, a single-factor pricing formula emerges. As in the standard consumption asset pricing model (CAPM), the single factor is the excess return of the market portfolio. However, the CAPM beta is adjusted by the extent to which the ambiguity of the asset return is correlated with the ambiguity of the market portfolio. Two uncertainty premia explain the cross-section of expected returns: a risk premium and an ambiguity premium. The latter has the potential to explain the size and value premia documented by Fama and French (1992, 1993). High book-to-market firms, which tend to be in financial distress, and small-cap firms, due to their over-reliance on external financing, likely carry a high ambiguity premium.

In the dynamic extensions of the model implications of public signals for trading volumes are analysed. Earning announcements or aggregate uncertainty shocks are shown to induce trading if and only if agents are heterogeneously ambiguity averse. Such trade occurs because public signals change the return-risk-ambiguity trade-off, making investors seek a different allocation across ambiguous assets depending on their different ambiguity tolerances. Trade results from uncertainty sharing considerations and leads to no or very small price movements, which is consistent with the empirical literature.

The paper concludes by proposing strategies to estimate the ambiguity of individual assets returns. Since ambiguity about the return distribution is taken to be the uncertainty about the mean of the return distribution, a measure can be obtained from a Bayesian estimate of this parameter.