شواهد تجربی در مورد شغل و سرمایه انسانی خاص صنعت

This paper presents instrumental variables estimates of the effects of firm tenure, occupation specific work experience, industry specific work experience, and general work experience on wages using data from the 1979 Cohort of the National Longitudinal Survey of Youth. The estimates indicate that both occupation and industry specific human capital are key determinants of wages, and the importance of various types of human capital varies widely across one-digit occupations. Human capital is primarily occupation specific in occupations such as craftsmen, where workers realize a 14% increase in wages after five years of occupation specific experience but do not realize wage gains from industry specific experience. In contrast, human capital is primarily industry specific in other occupations such as managerial employment where workers realize a 23% wage increase after five years of industry specific work experience. In other occupations, such as professional employment, both occupation and industry specific human capital are key determinants of wages.

A large literature has examined the sources of wage growth over the lifecycle, with considerable attention devoted to determining the relative importance of employer tenure and overall labor market experience in determining wages. According to this view of the human capital accumulation process skills are either firm specific or transferable across all jobs, but skills are not occupation or industry specific.1 A different view of the human capital accumulation process is presented by Neal, 1995 and Parent, 2000, who both find that industry specific human capital is a key determinant of wages, while firm-specific human capital contributes little to wage growth. More recently, Kambourov and Manovskii (2009a) have challenged the view that human capital is primarily industry specific, finding that after controlling for occupation tenure both industry and firm tenure have little importance in determining wages.2 Their results suggest that previous estimates of large returns to industry experience were driven primarily by the omission of occupation specific work experience from wage regressions, a variable that is highly correlated with industry experience. This paper presents new evidence on the specificity of human capital by estimating the returns to firm, occupation and industry specific work experience using data from the 1979 Cohort of the National Longitudinal Survey of Youth (NLSY). These explanatory variables are endogenous, so the wage equation is estimated using the instrumental variables approach developed by Altonji and Shakotko (1987). The empirical results demonstrate that the conclusions drawn about the specificity of human capital hinge on the treatment of within-firm occupational mobility. When within-firm occupational mobility is ruled out, the estimates confirm Kambourov and Manovskii's (2009a) finding that human capital is primarily occupation specific. However, this paper presents new empirical evidence on the validity of within-firm occupation changes by exploiting a change in the NLSY occupation coding scheme that was designed to more accurately detect within-firm occupation changes. Beginning in 1994 the NLSY occupation coding scheme changed so that within-firm occupation changes were allowed to occur only if workers directly reported a change in the type of work done on their job. In contrast, before 1994 NLSY respondents re-reported their occupation for all jobs, and were not directly asked whether or not they had switched occupations within their current firm. A comparison of pre and post 1994 data suggests that within-firm occupation changes do in fact reflect true changes in occupation, and are not simply the result of measurement error. When these within-firm occupational transitions are allowed, the empirical results point to a role for both industry and occupation specific human capital in determining wages.3 This paper also expands on the existing literature by allowing the returns to human capital to vary across occupations, departing from previous work which has constrained the parameters of the wage equation to be the same for all occupations. This is a key extension of existing research because there is no reason to believe that the technology of skill production is the same across all occupations. In fact, the results show that the specificity of skills accumulated at a job varies widely across one-digit occupations. For example, craftsmen accumulate skills that are primarily occupation specific, experiencing a wage gain of 14% after five years of occupation experience. On the other hand, human capital is primarily industry specific for managers, who experience a wage increase of 23% after five years of industry experience. In contrast, professionals accumulate skills that are both occupation and industry specific, as they realize wage increases of 22% after five years of occupation experience and 14% after five years of industry experience. Finally, sales workers do not experience wage gains from either occupation or industry experience, but they realize large wage gains as they accumulate general work experience. The differences in the returns to human capital across occupations are large and statistically significant. Restricting these effects to be equal across occupations leads to misleading estimates of the effects of occupation and industry specific human capital on wages. Whether wage growth over the career is due to the accumulation of skills that are specific to firms, occupations, industries, or completely general and transferable between all jobs is a fundamental question about the wage determination process. In addition, the finding that human capital is both occupation and industry specific has implications for a number of areas of current research. For example, it suggests that studies of career choice and career mobility should define careers using both occupation and industry codes since workers acquire skills that are specific to both occupations and industries.4 The specificity of human capital is also relevant for macroeconomic studies of wage inequality and aggregate productivity. For example, Kambourov and Manovskii (2009b) develop a theoretical model which shows that rising occupational mobility can explain a large fraction of the observed increase in wage inequality if human capital is largely occupation specific. The specificity of human capital is also relevant when studying the impact of job displacement, a topic which has been the subject of a large amount of research by economists as well as a considerable amount of policy discussion. The value of firm, occupation, and industry specific skills relative to the value of general skills is an important determinant of the cost of displacement since the transferability of a worker's skills to a new job is a key determinant of the wage loss accompanying job displacement.

This paper presents instrumental variables estimates of the effects of firm tenure, occupation experience, industry experience, and general work experience on wages for a sample of young men from the NLSY. Multiple specifications of the wage equation are estimated, and the results show that estimates of occupation and industry experience effects are quite sensitive to the treatment of within-firm occupational mobility. When within-firm occupation switches are ruled out, it appears that human capital is primarily occupation specific. However, this paper provides new evidence on the validity of within-firm occupation switches by exploiting a change in the NLSY occupation coding scheme designed to increase the reliability of reported within-firm occupation switches. This new data, combined with empirical evidence from a simple model of worker mobility suggests that workers make actual occupation switches within firms. When within-firm occupation switches are allowed, the IV wage equation estimates show that wages increase with both occupation and industry experience. Additional evidence supporting the industry and occupation specificity of human capital is provided by an analysis of the relationship between firm tenure and wage changes accompanying mobility between firms. High tenure workers who switch occupation or industry when switching firms experience larger wage losses than those who do not switch occupation or industry, which suggests that workers accumulate skills that are both occupation and industry specific. The results presented in this paper show that workers accumulate skills that are specific to both occupations and industries, while truly firm-specific skills contribute little to the growth of wages over the career. A key feature of the human capital accumulation process is that the value of occupation experience, industry experience, and general work experience varies substantially across occupations. Constraining the effects of human capital on wages to be the same across occupations leads to misleading estimates of the effects of occupation and industry specific human capital on wages. Skills are primarily occupation specific in some occupations (craftsmen and service), industry specific in others (managers), and both occupation and industry specific in others (professional). On the other hand, general skills are the primary determinant of wage growth in other occupations (sales and clerical). These conclusions about the importance of both occupation and industry experience are quite different from those reached in the recent literature, which has found that either occupation or industry experience affects wages, but not both types of experience.