A Note on the Theme of Too Many Instruments*

The difference and system generalized method of moments (GMM) estimators are growing in popularity. As implemented in popular software, the estimators easily generate instruments that are numerous and, in system GMM, potentially suspect. A large instrument collection overfits endogenous variables even as it weakens the Hansen test of the instruments’ joint validity. This paper reviews the evidence on the effects of instrument proliferation, and describes and simulates simple ways to control it. It illustrates the dangers by replicating Forbes [American Economic Review (2000) Vol. 90, pp. 869–887] on income inequality and Levine et al. [Journal of Monetary Economics] (2000) Vol. 46, pp. 31–77] on financial sector development. Results in both papers appear driven by previously undetected endogeneity.     

Diagnostics cannot have much power against general alternatives

Model diagnostics are shown to have little power unless alternative hypotheses can be narrowly defined. For example, the independence of observations cannot be tested against general forms of dependence. Thus, the basic assumptions in regression models cannot be inferred from the data. Equally, the proportionality assumption in proportional-hazards models is not testable. Specification error is a primary source of uncertainty in forecasting, and this uncertainty will be difficult to resolve without external calibration. Model-based causal inference is even more problematic.     

Stock market perceptions of the motives for mergers in cases reviewed by the UK competition authorities: an empirical analysis

A number of studies have considered the motivation of managers to follow a merger strategy. However, as far as we are aware none has looked at the influence of competition regulation on merger motives using stock market data and event study techniques. Data drawn from 63 merger cases in the UK between 1989 and 2003 are examined for the stock market's perceptions of what motivated managers to pursue their initial merger bid. The findings suggest that the Synergy and Hubris dominate as motivations for mergers and that, unintentionally, competition policy may help to reduce the number of mergers motivated by Managerialism. Copyright © 2008 John Wiley & Sons, Ltd.     

Homeownership and Child Welfare

Recent studies have concluded that homeownership is beneficial to children. This result is important because it is used to justify large government subsidies that encourage homeownership. We reexamine the results of two of the most prominent of these studies using the Panel Study of Income Dynamics, Public Use Microsample, and National Longitudinal Survey of Youth data. We extend this research by controlling for residential mobility, wealth, dwelling type and vehicle ownership, as well as by using a differences in differences methodology to deal with possible treatment effects bias. We find that the beneficial effects of homeownership previously measured are substantially reduced or eliminated by controlling for these factors. We confirm these results using data from the Early Childhood Longitudinal Study.     

Public Sector Employment and the Skill Premium: Sweden versus the United States 1970–2012

Swedish census data and tax records reveal an astonishing decline in the aggregate skill premium of 30 percent between 1970 and 1990, with only a modest recovery in the next couple of decades. In contrast, the US skill premium rose by around 24 percent over those four decades. A theory that equalizes wages with marginal products can rationalize these disparate outcomes when we replace commonly used measures of total labor supplies by private sector employment. The dramatic decline in the skill premium in Sweden is the result of an expanding public sector that has disproportionately hired unskilled labor.     

The Political Economy of the Global Environment

Issues of the ‘global commons’ have secured a prominent place in environmental discourse. The temperature-regulating functions of the global atmosphere and radiation control functions of stratospheric ozone offer clear examples of true public goods. Other environmental assets, such as biodiversity and forests, are treated as if they are public goods, but in reality are complex mixtures of private goods, local public goods and global public goods. The approach to the provision and protection of these goods has tended to focus on the development of international agreements, such as those at the Rio de Janeiro ‘Earth Summit’ in 1992. But do these agreements contain the relevant incentives to conserve the global commons? Much depends on one's view of human nature. The Scottish economic tradition suggests that unless incentives focus on ‘self love’, as postulated by Hume and Smith, improvements will, at best, be marginal. A richer array of policy measures is obtained by analysing the potential for ‘global bargains’, trades that improve the environment whilst making each party better off. The souls of Hume and Smith surely approve recent developments in practical global bargains.