David Laibson

Golden Eggs and Hyperbolic Discounting

Hyperbolic discount functions induce dynamically inconsistent preferences, implying a motive for consumers to constrain their own future choices. This paper analyzes the decisions of a hyperbolic consumer who has access to an imperfect commitment technology: an illiquid asset whose sale must be initiated one period before the sale proceeds are received. The model predicts that consumption tracks income, and the model explains why consumers have asset-specific marginal propensities to consume. The model suggests that financial innovation may have caused the ongoing decline in U. S. savings rates, since financial innovation in- creases liquidity, eliminating commitment opportunities. Finally, the model implies that financial market innovation may reduce welfare by providing “too much†liquidity.

An Economic Approach to Social Capital

To identify the determinants of social capital formation, it is necessary to understand the social capital investment decision of individuals. Individual social capital should then be aggregated to measure the social capital of a community. This paper assembles the evidence that supports the individual-based model of social capital formation, including seven facts: (l) the relationship between social capital and age is first increasing and then decreasing, (2) social capital declines with expected mobility, (3) social capital investment is higher in occupations with greater returns to social skills, (4) social capital is higher among homeowners, (5) social connections fall sharply with physical distance, (6) people who invest in human capital also invest in social capital, and (7) social capital appears to have interpersonal complementarities.

Time Discounting for Primary Rewards

Previous research, involving monetary rewards, found that limbic reward-related areas show greater activity when an intertemporal choice includes an immediate reward than when the options include only delayed rewards. In contrast, the lateral prefrontal and parietal cortex (areas commonly associated with deliberative cognitive processes, including future planning) respond to intertemporal choices in general but do not exhibit sensitivity to immediacy (McClure et al., 2004). The current experiments extend these findings to primary rewards (fruit juice or water) and time delays of minutes instead of weeks. Thirsty subjects choose between small volumes of drinks delivered at precise times during the experiment (e.g., 2 ml now vs 3 ml in 5 min). Consistent with previous findings, limbic activation was greater for choices between an immediate reward and a delayed reward than for choices between two delayed rewards, whereas the lateral prefrontal cortex and posterior parietal cortex responded similarly whether choices were between an immediate and a delayed reward or between two delayed rewards. Moreover, relative activation of the two sets of brain regions predicts actual choice behavior. A second experiment finds that when the delivery of all rewards is offset by 10 min (so that the earliest available juice reward in any choice is 10 min), no differential activity is observed in limbic reward-related areas for choices involving the earliest versus only more delayed rewards. We discuss implications of this finding for differences between primary and secondary rewards.

Dynamic Choices of Hyperbolic Consumers

Laboratory and field studies of time preference find that discount rates are much greater in the short-run than in the long-run. Hyperbolic discount functions capture this property. This paper solves the decision problem of a hyperbolic consumer who faces stochastic income and a borrowing constraint. The paper uses the bounded variation calculus to derive the Hyperbolic Euler Relation, a natural generalization of the standard Exponential Euler Relation. The Hyperbolic Euler Relation implies that consumers act as if they have endogenous rates of time preference that rise and fall with the future marginal propensity to consume (e.g., discount rates that endogenously range from 5% to 41% for the example discussed in the paper).

Intertemporal choice – toward an integrative framework

Intertemporal choices are decisions with consequences that play out over time. These choices range from the prosaic--how much food to eat at a meal--to life-changing decisions about education, marriage, fertility, health behaviors and savings. Intertemporal preferences also affect policy debates about long-run challenges, such as global warming. Historically, it was assumed that delayed rewards were discounted at a constant rate over time. Recent theoretical and empirical advances from economic, psychological and neuroscience perspectives, however, have revealed a more complex account of how individuals make intertemporal decisions. We review and integrate these advances. We emphasize three different, occasionally competing, mechanisms that are implemented in the brain: representation, anticipation and self-control.

A Cue-Theory of Consumption

Psychological experiments demonstrate that repeated pairings of a cue and a consumption good eventually create cue-based complementarities: the presence of the cue raises the marginal utility derived from consumption. In this paper, such dynamic preferences are embedded in a rational choice model. Behavior that arises from this model is characterized by endogenous cue sensitivities, costly cue-management, commitment, and cue-based spikes in impatience. The model is used to understand addictive/habit-forming behaviors and marketing. The model explains why preferences change rapidly from moment to moment, why temptations should sometimes be avoided, and how firms package and position goods.

The 6D Bias and the Equity-Premium Puzzle

If decision costs lead agents to update consumption every D periods, then econometricians will find an anomalously low correlation between equity returns and consumption growth (Lynch 1996). We analytically characterize the dynamic properties of an economy composed of consumers who have such delayed updating. In our setting, an econometrician using an Euler equation procedure would infer a coefficient of relative risk aversion biased up by a factor of 6D. Hence with quarterly data, if agents adjust their consumption every D =4 quarters, the imputed coefficient of relative risk aversion will be 24 times greater than the true value. High levels of risk aversion implied by the equity premium and violations of the Hansen-Jagannathan bounds cease to be puzzles. The neoclassical model with delayed adjustment explains the consumption behavior of shareholders. Once limited participation is taken into account, the model matches most properties of aggregate consumption and equity returns, including new evidence that the covariance between ln(Ct+h/Ct) and Rt+1 slowly rises with h.

Most Reported Genetic Associations With General Intelligence Are Probably False Positives

General intelligence (g) and virtually all other behavioral traits are heritable. Associations between g and specific single-nucleotide polymorphisms (SNPs) in several candidate genes involved in brain function have been reported. We sought to replicate published associations between g and 12 specific genetic variants (in the genes DTNBP1, CTSD, DRD2, ANKK1, CHRM2, SSADH, COMT, BDNF, CHRNA4, DISC1, APOE, and SNAP25) using data sets from three independent, well-characterized longitudinal studies with samples of 5,571, 1,759, and 2,441 individuals. Of 32 independent tests across all three data sets, only 1 was nominally significant. By contrast, power analyses showed that we should have expected 10 to 15 significant associations, given reasonable assumptions for genotype effect sizes. For positive controls, we confirmed accepted genetic associations for Alzheimer’s disease and body mass index, and we used SNP-based calculations of genetic relatedness to replicate previous estimates that about half of the variance in g is accounted for by common genetic variation among individuals. We conclude that the molecular genetics of psychology and social science requires approaches that go beyond the examination of candidate genes.

Redefine Statistical Significance

We propose to change the default P-value threshold for statistical significance from 0.05 to 0.005 for claims of new discoveries.

Economic Implications of Extraordinary Movements in Stock Prices

MOST PEOPLE AGREE that stock prices sometimes behave in strange ways. Going beyond this simple observation typically proves more difficult. For at least the past quarter century, economists have been well aware that the variation of stock prices does not nicely match the familiar bell-shaped normal distribution.1 The problem is too many extreme movements. Very large increases or decreases would always be possible even if changes in stock prices were normally distributed, but they would occur only rarely. By contrast, actual stock prices rise or fall by large percentage amounts fairly often-certainly often enough to raise serious doubts that the usual normal distribution provides a useful way to think about how they vary. Economists and other analysts of the stock market have tended to react to this problem in either of two ways. The most common approach is simply to ignore it and go ahead to analyze changes in stock prices as