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International audience; Language use differs between truthful and deceptive statements, but not all differences are consistent across people and contexts, complicating the identification of deceit in individuals. By relying on fact-checked tweets, we showed in three studies (Study 1: 469 tweets; Study 2: 484 tweets; Study 3: 24 models) how well personalized linguistic deception detection performs by developing the first deception model tailored to an individual: the 45th U.S. president. First, we found substantial linguistic differences between factually correct and factually incorrect tweets. We developed a quantitative model and achieved 73% overall accuracy. Second, we tested out-of-sample prediction and achieved 74% overall accuracy. Third, we compared our personalized model with linguistic models previously reported in the literature. Our model outperformed existing models by 5 percentage points, demonstrating the added value of personalized linguistic analysis in real-world settings. Our results indicate that factually incorrect tweets by the U.S. president are not random mistakes of the sender.

Temporary incentives are offered in anticipation of persistent effects, but these are seldom estimated. We use a nationwide randomized experiment in the Philippines to estimate effects three years after the withdrawal of two incentives for health insurance. A premium subsidy had a persistent effect on enrollment that is more than four fifths of the immediate effect. Application assistance had a much larger immediate impact, but less than a fifth of this effect persisted. The subsidy persuaded those with higher initial willingness to pay to enroll and keep enrolling, while application assistance achieved a larger immediate effect by drawing in those who valued insurance less and were less likely to re-enroll.

Despite widespread exposure to substantial medical expenditure risk in low-income populations, health insurance enrollment is typically low. This is puzzling from the perspective of expected utility theory. To help explain it, this paper introduces a decomposition of the stated willingness to pay (WTP) for insurance into its fair price and three behavioral deviations from that price due to risk perception and risk attitude consistent with prospect theory, plus a residual. To apply this approach, we elicit WTP, subjective distributions of medical expenditures and risk attitude (utility curvature and probability weighting) from Filipino households in a nationwide survey. We find that the mean stated WTP of the uninsured is less than both the actuarially fair price and the subsidized price at which public insurance is offered. This is not explained by downwardly biased beliefs: both the mean and the median subjective expectation are greater than the subsidized price. Convex utility in the domain of losses pushes mean WTP below the fair price and the subsidized price, and the transformation of probabilities into decision weights depresses the mean further, at least using one of two specific decompositions. WTP is reduced further by factors other than risk perception and attitude.

This dissertation deals with the theory of decision making under uncertainty. It aims at describing and modeling decisions in order to disentangle beliefs and attitudes towards uncertainty. It is made of four main contributions. The first one is theoretical and characterize risk and ambiguity attitude through tradeoff relations. Indeed, expected utility and some of its generalization represents risk and ambiguity aversion through decreasing marginal utility. The tradeoff relation is defined on consequences and allows us to compare concavity of different utility functions under expected utility when probabilities need not be known. The other parts of the dissertation are based on nonexpected utility models including probability weighting functions. The second contribution is an experimental study of how decision makers combine experts' probability judgments. A new choice-based method is proposed and applied to the comparison of two typical situations. In the first one, the experts give an imprecise evaluation of the risk and in the second one, they disagree and each of them gives his/her own evaluation. The third work is based on uniform sources of uncertainty, i.e. set of events that pertain to a similar mechanism of uncertainty and on which a probability measure exists. In an experiment, such probabilities are elicited. Then willingness-to-bet on events having the same subjective probabilities but from different sources are obtained. The last work is specifically dedicated to the technique for eliciting subjective probability. It observes that this technique is more robust than several well-known techniques and provides new evidence regarding its feasibility., La thèse porte sur la théorie de la décision individuelle en situation d'incertitude. Elle vise à comprendre, décrire et représenter les décisions, en différenciant ce qui provient des croyances du décideur de ce qui se rattache à son attitude face à l'incertitude. Elle est principalement composée de quatre contributions complémentaires. La première est théorique et caractérise l'attitude face au risque et à l'ambiguïté via l'utilisation de relations d'arbitrage portant sur les conséquences, dans la lignée des modèles de type utilité espérée, représentant l'aversion au risque et/ou à l'ambiguïté par la décroissance de l'utilité marginale. Le reste de la thèse s'appuie sur des modèles généralisant l'utilité espérée où les probabilités sont transformées par le décideur. La deuxième contribution est alors expérimentale et s'intéresse au décideur disposant d'avis d'experts lui indiquant le risque encouru. Il est proposé une méthode, basée sur l'observation des choix, pour étudier comment le décideur combine les avis à sa disposition. Cette méthode est appliquée pour comparer des situations où les experts donnent une évaluation imprécise du risque à des situations où leurs évaluations du risque encouru sont conflictuelles. Le troisième travail introduit le concept de source uniforme d'incertitude, c'est-à-dire d'ensemble d'événements générés par un même mécanisme d'incertitude et pour lesquels il existe une mesure de probabilité subjective. Une expérience est conduite dans laquelle de telles probabilités subjectives sont obtenues. Est ensuite étudié le consentement à parier des individus sur des événements de probabilité (subjective) équivalente mais provenant de sources différentes. La dernière contribution revient sur la méthode d'obtention des probabilités subjectives et la compare (théoriquement) aux autres méthodes. Sa faisabilité et ses limites sont ensuite étudiées dans une nouvelle expérience.