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A Structural Model of Human Capital and Leverage - Duke ...

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is on the market to book ratio. Moving up one decile in terms <strong>of</strong> the market to book ratio results in a drop<br />

in leverage <strong>of</strong> 3.7 percentage points. Deciles <strong>of</strong> labor intensity explain 27.9 percent <strong>of</strong> the variation in market<br />

leverage, compared to a high <strong>of</strong> 36 percent for the market to book ratio.<br />

When all variables are included in the regression, the coefficient on each variable is attenuated, suggesting<br />

the importance <strong>of</strong> controlling for each determinant. Size <strong>and</strong> cash flow volatility lose their significance, while<br />

labor intensity remains one <strong>of</strong> four significant predictors <strong>of</strong> leverage, along with tangibility, market to book,<br />

<strong>and</strong> pr<strong>of</strong>itability.<br />

For completeness, Table 2 presents the same regressions using book leverage as the dependent variable.<br />

The results are very similar, with the most notable difference being that market to book <strong>and</strong> pr<strong>of</strong>itability<br />

are less reliably related to book leverage. This is not surprising because <strong>of</strong> how these variables feed directly<br />

into the firm’s market value. In these regressions labor intensity is second only to tangibility in terms <strong>of</strong> the<br />

significance <strong>of</strong> its relationship to leverage, as measured both by the magnitude <strong>of</strong> its coefficient <strong>and</strong> the R 2 <strong>of</strong><br />

the regression. Moving up one decile in labor intensity results in a drop <strong>of</strong> 2.7 percentage points in leverage,<br />

versus a rise in leverage <strong>of</strong> 2.9 percentage points for a corresponding change in tangibility. Labor intensity<br />

explains 24.7 percent <strong>of</strong> the variation in book leverage, versus the 28.7 percent explained by tangibility. As is<br />

the case for market leverage, labor intensity survives as one <strong>of</strong> four significant predictors <strong>of</strong> capital structure<br />

in a multiple regression.<br />

This evidence suggests that labor intensity is an important factor in a firm’s capital structure decision.<br />

Tangibility, size, market to book, pr<strong>of</strong>itability, <strong>and</strong> cash flow volatility have long been recognized as important<br />

<strong>and</strong> reliable predictors <strong>of</strong> capital structure. The evidence presented in this section suggests that labor intensity<br />

should be included in this list.<br />

4 <strong>Model</strong><br />

In this section I write a model to formalize <strong>and</strong> quantify the dependence <strong>of</strong> capital structure on labor intensity.<br />

The model consists <strong>of</strong> a large number <strong>of</strong> firms which are described by three state variables: net worth (ω),<br />

labor (n) <strong>and</strong> a persistent, idiosyncratic productivity shock (z). In what follows I solve a single firm’s<br />

optimization problem, using primes to denote future values <strong>and</strong> subscripts on functions to denote partial<br />

derivatives. I assume that the shock z has Markov transition function Γ(z ′ |z) <strong>and</strong> takes on values in a<br />

compact set [z, ¯z] with z > 0. Each period the firm chooses next period’s capital (k ′ ), labor (n ′ ), <strong>and</strong> debt<br />

(b ′ ). Investment, hiring, <strong>and</strong> financing decisions are made simultaneously.<br />

8

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