A nonparametric test of exogeneity

This paper is concerned with inference about a function g that is identified by a conditional moment restriction involving instrumental variables. The function is nonparametric. It satisfies mild regularity conditions but is otherwise unknown. The paper presents test of the hypothesis that g is the mean of a random variable Y conditional on a covariate X. The need to test this hypothesis arises frequently in economics. The test does not require nonparametric instrumental-variables (IV) estimation of g and is not subject to the ill-posed inverse problem that nonparametric IV estimation entails. The test is consistent whenever g differs from the conditional mean function of Y on a set of non-zero probability. Moreover, the power of the test is arbitrarily close to 1 uniformly over a set of functions g whose distance from the conditional mean function is O(n^{-1/2}) where n is the sample size

New meanings from old buildings

The three modest house projects described here are by three fellow travellers — the two authors and David Lea — interested in the Organic side of Modernism. Conversational partners who have worked together in various capacities over many years, they share a common conviction about ‘working with the given

Wealth portfolios in the US and the UK

In this paper, we attempt to explain differences between the US and UK household wealth distributions, with an emphasis on the quite different porfolios held in stock and housing equities in the two countries. As a proportion of their total wealth, British households hold relatively small amounts of financial assets - including equities in stock - compared to American households. In contrast, British households appear to move into home ownership at relatively young ages and a large fraction of their household wealth is concentrated in houseing. Finally, the age gradient in home equity appears to be much steeper in the UK while US households exhibit a steeper age gradient in stock equity. We argue that the higher price housing price volatility in the UK combined with much younger entry into home ownership there are important factors accounting for the relatively small participation of young British householders in the stock market. We show it is important to acknowledge the dual role of housing - providing both wealth and consumption services - in understanding wealth accumulation differences between the US and the UK. Institutional differences, particularly in housing markets, that affect the demand and supply of housing services, turn out to be important in generating portfolio differences between the two countries. In particular, these differences in housing price risk imply steeper life-cycle accumulations in housing and less steep accumulation in stock equity over the life cycle in the UK

House Price Volatility and Housing Ownership over the Lifecycle

We develop and test a model on the effects of spatial housing price risk on housing choice. Housing price risk can be substantial but, unlike other risky assets which people can avoid, most people want to eventually own their home thereby creating an insurance demand for housing ownership early in life. With increasing demographic needs over the life cycle, our model predicts that people living in places with higher housing price risk should own their first home at a younger age, should live in larger homes, and should be less likely to refinance. These predictions are shown to hold using comparable panel data from the United States and United Kingdom. (JEL D12, D91

The central engines of radio-quiet quasars

Two rival hypotheses have been proposed for the origin of the compact radio flux observed in radio-quiet quasars (RQQs). It has been suggested that the radio emission in these objects, typically some two or three orders of magnitude less powerful than in radio-loud quasars (RLQs), represents either emission from a circumnuclear starburst or is produced by radio jets with bulk kinetic powers 10^3 times lower than those of RLQs with similar luminosity ratios in other wavebands. We describe the results of high resolution (parsec-scale) radio-imaging observations of a sample of 12 RQQs using the Very Long Baseline Array (VLBA). We find strong evidence for jet-producing central engines in 8 members of our sample.Comment: 7 pages, 8 figures. Accepted for publication in MNRA

Universal scaling relations in molecular superconductors

Scaling relations between the superconducting transition temperature $T_{\rm c}$, the superfluid stiffness $\rho_{\rm s}$ and the normal state conductivity $\sigma_0(T_{\rm c})$ are identified within the class of molecular superconductors. These new scaling properties hold as $T_{\rm c}$ varies over two orders of magnitude for materials with differing dimensionality and contrasting molecular structure, and are dramatically different from the equivalent scaling properties observed within the family of cuprate superconductors. These scaling relations place strong constraints on theories for molecular superconductivity.Comment: 4 pages, 4 figure

Extremely red galaxy counterparts to 7C radio sources

We present RIJHK imaging of seven radio galaxies from the 7C Redshift Survey (7CRS) which lack strong emission lines and we use these data to investigate their spectral energy distributions (SEDs) with models which constrain their redshifts. Six of these seven galaxies have extremely red colours (R-K>5.5) and we find that almost all of them lie in the redshift range 1<z<2. We also present near-infrared spectroscopy of these galaxies which demonstrate that their SEDs are not dominated by emission lines, although tentative lines, consistent with H-alpha at z=1.45 and z=1.61, are found in two objects. Although the red colours of the 7CRS galaxies can formally be explained by stellar populations which are either very old or young and heavily reddened, independent evidence favours the former hypothesis. At z~1.5 at least 1/4 of powerful radio jets are triggered in massive (>L*) galaxies which formed the bulk of their stars several Gyr earlier, that is at epochs corresponding to redshifts z>5. If a similar fraction of all z~1.5 radio galaxies are old, then extrapolation of the radio luminosity function shows that, depending on the radio source lifetimes, between 10-100% of the near-IR selected extremely red object (ERO) population undergo a radio outburst at epochs corresponding to 1<z<2. An ERO found serendipitously in the field of one of the 7CRS radio sources appears to be a radio-quiet analogue of the 7CRS EROs with an emission line likely to be [OII] at z=1.20. The implication is that some of the most massive elliptical galaxies formed the bulk of their stars at z>5 and these objects probably undergo at least two periods of AGN activity: one at high redshift during which the black hole forms and another one at an epoch corresponding to z~1.5.Comment: 20 pages, 10 figures, accepted for publication in MNRA