Years of work experience enter the specification in a quadratic form to allow for the return to be decreasing for high values of experience. This is one of the important empirical regularities originally found by Mincer. The human capital model may explain this regularity. Over their career, individuals are subject to two processes: accumulation of new human capital through learning on the job but also depreciation of the human capital previously acquired. Eventually, the second process dominates leading to a reduction and possibly a reversal of returns to experience.<sup>17</sup> Returns to experience estimates are almost as common as returns to years of schooling estimates since they are frequently jointly estimated in a specification similar to equation [8]. In Montenegro and Patrinos (2014) the estimate for potential experience is centered around 3.1% a year in a large sample of countries. Potential experience is positively correlated with returns to schooling and its variation across countries is large, with some countries registering returns twice as large while others almost zero. The recent literature has also focused on the evolution of returns to experience over time. Kambourov and Manovskii (2008) focus on the US showing a flattening of experience profiles over the birth cohorts 1950-1970. Returns to experience may also change together with other human capital characteristics, such as education [Card and Lemieux (2001)].

An important characteristic of at least some of the human capital accumulation on-the-job is being _specific_ to that job, becoming not easily portable to another job. The first original contributions by Becker, see for example Becker (1962), were emphasizing this argument by distinguishing between _general_ and _specific_ human capital. In this terminology, years of schooling are an example of general human capital while years of experience in a given firm (tenure or seniority) are an example of specific human capital. Typical estimates for a high-income country such as the U.S. are about 11% wage increase for ten additional years of seniority at a given firm [Flabbi and Ichino (2001)]. Between the overall experience accumulated in the labor market and the more specific experience accumulate in a given firms, recent literature has shown the importance of intermediate categories. Kambourov and Manovskii (2009) emphasizes the importance of experience accumulated in a specific occupation or industry. They estimate on US data that 5 years of experience in the same 3- digit occupation increase wages 12% to 20% a year. Sanders and Taber (2012) provide a review of the contributions trying to empirically disentangle the importance of general and specific human capital. They conclude that no human capital accumulation on the job can be fully described as general or specific. Rather, the empirical evidence fits better a model of task-specific human capital which is not fully industry-specific nor occupation-specific. Autor and Handel (2013) estimate that one standard deviation increase in abstract task content within the same occupation leads to a 7% increase in wages.

There is also a large literature claiming that returns to seniority are not related to productivity increases.<sup>18</sup> These contributions challenge the human capital interpretation of the positive coefficients estimated on measures of experience and seniority in equations

> 17 Mincer, however, did not share this human capital interpration of his results: see Heckman et al. (2006) for a recent reconsideration of his arguments.

> 18 The literature started with Medoff and Abraham (1980) who had the idea of using supervisors' evaluations to control for productivity differences in wage equations and continued with Altonji and Shakotko (1987). More recent contributions are Flabbi and Ichino (2001); Dohmen (2004): Buchinsky et al. (2010).

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