Labour market extremes: a study of the high and low wage ends of the labour market

This thesis is comprised of three essays studying the high and low wage ends of the labour market. Historically there has been little theoretical analysis of low-paid, non-standard work and so in the first and third essays I develop theoretical models incorporating two types of non-standard work: casual and part-time. The second, co-authored, essay is empirical, using data on football players to study the wage determination of very highly paid workers. In the first essay I study low wage, casual jobs. These jobs provide flexibility for firms to change the size of their workforce cheaply and quickly and for workers to choose whether to supply labour in every period. This flexibility comes at the expense of certainty for both firms and workers. I develop a search and matching model incorporating casual jobs, which I use to evaluate the effect of labour market policies on aggregate outcomes. The equilibrium of the model features the concentration of casual jobs at the bottom of the wage distribution. I find that a ban on casual jobs increases unemployment, but that the average wage of those employed actually increases. In addition, in the model with casual jobs, workers in low wage casual jobs continue to search for a higher quality match that will offer work more frequently. In a model with search frictions, this makes it harder for unemployed workers to match with a firm. This crowding out effect offsets some of the negative effects of a ban. I also consider the effect of a higher minimum wage for casual jobs. I find that the effects are limited. These results are due to an offsetting mechanism: although higher wages lead to higher unemployment, as firms offer more regular jobs, the number of workers called-up to work in any one period increases. In the second essay we turn to the other end of the wage distribution, to study the determinants of superstar wage effects, asking whether productivity or popularity-based explanations are more appropriate. We use longitudinal wage and performance data for workers (players) and firms (teams) from a particular market for sports talent: Major League Soccer in the United States. We find evidence that the top earners, whose annual salaries are mostly not accounted for by their past MLS performances, when compared alongside other footballers, are paid more because they attract significantly higher stadium attendances and thus revenues. There is no evidence that higher residual salary spending by the teams affects their relative performance in football terms, or that the amounts the teams spend on actual talent affect attendances. Taken together, these results suggest that a popularity-based explanation of superstar wage effects is appropriate among the top earners in this labour market. In the final essay I study long-term trends in part-time work. I show that there has been an increase in the part-time share over the past 30 years. There has also been an increase in the part-time work on the extensive margin; part-time workers on average work longer. Despite this, the difference in average hourly pay for part-time and full-time workers (the part-time pay penalty) has steadily decreased. I develop a flexible neoclassical model of the labour market which can explain firms’ and workers’ preferences for part- and full-time work. The equilibrium of the model matches key features of the labour market: full- and part-time workers undertake different tasks; there is bunching of workers at full-time hours; and full-time workers earn higher hourly wages than part-time. The model can be used to disentangle the effects of changes in workers’ preferences and on firms’ production technologies on the relative quantities and prices of part- and full-time labour. I provide an extension of the model which incorporates heterogeneity in workers' preferences, and will enable the study of gender differences in part-time work

The age-wage-productivity puzzle:Evidence from the careers of top earners

There is an inverted u-shaped relationship between age and wages in most labour markets, but the effects of age on productivity are often unclear. We use panel data in a market of high earners, professional footballers (soccer players) in North America, to estimate age-productivity and age-wage profiles. We find stark differences; wages increase for several years after productivity has peaked, before dropping sharply at the end of a career. This poses the question: why are middle-aged workers seemingly overpaid? We investigate a range of possible mechanisms that could be responsible, only finding evidence that tentatively supports a talent discovery theory

Do high wage footballers play for high wage teams? The case of Major League Soccer

Intuition and sports knowledge suggest that the most talented professional footballers play for the best teams, i.e., positive assortative matching based on productivity. We consider Major League Soccer between 2007 and 2017. We estimate a wage equation, finding that player and team fixed wage premiums are negatively correlated. This is a puzzle, especially because our estimates of players' wage premiums do correlate strongly with measures of their performance on the pitch, and there is evidence of positive teammate sorting. However, the estimated wage premiums of MLS teams are highly and negatively correlated with their success in the league and their home game attendances. This is consistent with an explanation whereby part of an MLS team's success comes from its ability to bargain down the price that it pays for talent

Multimodal Imaging Techniques Show Differences in Homing Capacity Between Mesenchymal Stromal Cells and Macrophages in Mouse Renal Injury Models

The question of whether mesenchymal stromal cells (MSCs) home to injured kidneys remains a contested issue. To try and understand the basis for contradictory findings reported in the literature, our purpose here was to investigate whether MSC homing capacity is influenced by administration route, the type of injury model used, and/or the presence of exogenous macrophages. PROCEDURES: To assess the viability, whole-body biodistribution, and intra-renal biodistribution of MSCs, we used a multimodal imaging strategy comprising bioluminescence and magnetic resonance imaging. The effect of administration route (venous or arterial) on the ability of MSCs to home to injured renal tissue, and persist there, was assessed in a glomerular injury model (induced by the nephrotoxicant, Adriamycin) and a tubular injury model induced by ischaemia-reperfusion injury (IRI). Exogenous macrophages were used as a positive control because these cells are known to home to injured mouse kidneys. To assess whether the homing capacity of MSCs can be influenced by the presence of exogenous macrophages, we used a dual-bioluminescence strategy that allowed the whole-body biodistribution of the two cell types to be monitored simultaneously in individual animals. RESULTS: Following intravenous administration, no MSCs were detected in the kidneys, irrespective of whether the mice had been subjected to renal injury. After arterial administration via the left cardiac ventricle, MSCs transiently populated the kidneys, but no preferential homing or persistence was observed in injured renal tissue after unilateral IRI. An exception was when MSCs were co-administered with exogenous macrophages; here, we observed some homing of MSCs to the injured kidney. CONCLUSIONS: Our findings strongly suggest that MSCs do not home to injured kidneys

Murine models of renal ischemia reperfusion injury: An opportunity for refinement using noninvasive monitoring methods

BACKGROUND: Renal ischemia reperfusion injury (R‐IRI) can cause acute kidney injury (AKI) and chronic kidney disease (CKD), resulting in significant morbidity and mortality. To understand the underlying mechanisms, reproducible small‐animal models of AKI and CKD are needed. We describe how innovative technologies for measuring kidney function noninvasively in small rodents allow successful refinement of the R‐IRI models, and offer the unique opportunity to monitor longitudinally in individual animals the transition from AKI to CKD. METHODS: Male BALB/c mice underwent bilateral renal pedicle clamping (AKI) or unilateral renal pedicle clamping with delayed contralateral nephrectomy (CKD) under isoflurane anesthetic. Transdermal GFR monitoring and multispectral optoacoustic tomography (MSOT) in combination with statistical analysis were used to identify and standardize variables within these models. RESULTS: Pre‐clamping anesthetic time was one of the most important predictors of AKI severity after R‐IRI. Standardizing pre‐clamping time resulted in a more predictably severe AKI model. In the CKD model, MSOT demonstrated initial improvement in renal function, followed by significant progressive reduction in function between weeks 2 and 4. Performing contralateral nephrectomy on day 14 enabled the development of CKD with minimal mortality. CONCLUSIONS: Noninvasive monitoring of global and individual renal function after R‐IRI is feasible and reproducible. These techniques can facilitate refinement of kidney injury models and enable the degree of injury seen in preclinical models to be translated to those seen in the clinical setting. Thus, future therapies can be tested in a clinically relevant, noninvasive manner

Non-invasive imaging reveals conditions that impact distribution and persistence of cells after in vivo administration

Background: Cell-based regenerative medicine therapies are now frequently tested in clinical trials. In many conditions, cell therapies are administered systemically, but there is little understanding of their fate, and adverse events are often under-reported. Currently, it is only possible to assess safety and fate of cell therapies in preclinical studies, specifically by monitoring animals longitudinally using multimodal imaging approaches. Here, using a suite of in vivo imaging modalities to explore the fate of a range of human and murine cells, we investigate how route of administration, cell type and host immune status affect the fate of administered cells. Methods: We applied a unique imaging toolkit combining bioluminescence, optoacoustic and magnetic resonance imaging modalities to assess the safety of different human and murine cell types by following their biodistribution and persistence in mice following administration into the venous or arterial system. Results: Longitudinal imaging analyses (i) suggested that the intra-arterial route may be more hazardous than intravenous administration for certain cell types; (ii) revealed that the potential of a mouse mesenchymal stem/stromal cell (MSC) line to form tumours, depended on administration route and mouse strain; and (iii) indicated that clinically tested human umbilical cord (hUC)-derived MSCs can transiently and unexpectedly proliferate when administered intravenously to mice. Conclusions: In order to perform an adequate safety assessment of potential cell-based therapies, a thorough understanding of cell biodistribution and fate post administration is required. The non-invasive imaging toolbox used here can expose not only the general organ distribution of these therapies, but also a detailed view of their presence within different organs and, importantly, tumourigenic potential. Our observation that the hUC-MSCs but not the human bone marrow (hBM)-derived MSCs persisted for a period in some animals, suggests that therapies with these cells should proceed with caution

The age-wage-productivity puzzle: evidence from the careers of top earners

There is an inverted u-shaped relationship between age and wages in most labour markets and occupations, but the effects of age on productivity are often unclear. We use panel data on productivity and salaries in a market of high earners, professional footballers (soccer players) in North America, to estimate age-productivity and age-wage profiles. We find stark differences between these profiles; wages continue to increase for several years after productivity has peaked, before dropping sharply at the end of a career. This discrepancy poses the question: why are middle-aged workers seemingly overpaid relative to their contemporaneous productivity? The richness of our dataset allows us to investigate a range of possible mechanisms that could be responsible, including institutional factors, unobserved elements of productivity, and a talent discovery theory, by which teams pay younger players less because their productivity is more uncertain. We find some evidence that tentatively supports this last mechanism