3Note Normalized on male log earnings at 0 years of experience being 0 Note gender difference – will return to this laterPart of profile may be cohort effect but not most of itMincer first drew attention to earnings profileWhy did he use experience not age?
4Mincer Specification Earnings Function of the form: Has more or less stood test of time (should perhaps be quartic not quadratic)E=(a-s) so implies interaction between a and s.Earnings profiles for different education groups should be parallel.
5The Returns to Experience by Education (1 is high, 4 is low)
6Mincer’s Interpretation Return to experience is return to general human capitalProductivity rises with experience so wages do as wellWhy should productivity rise with experience?learning-by-doingExplicit investments in trainingWhy do earnings of older workers decline?Depreciation of human capitalNot worthwhile replacing it
7Problems with Mincer Interpretation Mincer explanation probably has some truth in it but is it the whole story?Displaced workers – older workers who lose jobs through plant closures seem to suffer larger earnings lossesEmpirical evidence on whether older workers really have higher productivity a bit weak e.g. Medoff-Abraham QJE 1980
11Search Explanations of Return to Experience If jobs vary in wages then older workers have had more chance to find the good jobsCan think of this as accumulation of search capitalThis can explain earnings losses of displaced workers – a loss of search capitalBut there almost certainly is some earnnigs growth within jobs
12A More Formal ModelAssume there are only two types of job, that we will call bad and goodWages are wg and wbIf fraction of people in good jobs is θ then:So can express everything in terms of θNote ‘true’ returns to experience and job tenure are zero
13Assume that new job offers arrive at a rate λ A fraction α of new jobs are good onesOnly workers in bad jobs will accept new job offers and they will only do it if the new job is a good one - workers in bad jobs will leave for good ones at a rate λαJobs get destroyed at a rate δ – assume everyone gets a new job straight away (simpler)
14Denote by θ(a) the fraction of workers of experience a who are in good jobs. The change in this fraction must equal the inflow of workers into good jobs minus the flow out of it.Using the assumptions on job offer arrival rates and job destruction rates described above we must then have that:In a long-run steady state we will have:
15Can write differential equation as: With solution:i.e. θ(a) will be increasing in a but at a decreasing rate.This implies that average wages will be increasing in a but at a decreasing rate i.e. we can explain the concave earnings function we observe in the data.But what about down-turn for older workers – changing transition rates
18Interpretation (Mincer-Jovanovic) Higher job tenure leads to greater accumulation of job-specific human capitalWorkers manage to extract some share of these returnsThis the return to job tenure
19Other Interpretations May be efficient to offer delayed compensation:Efficiency wages (Lazear, JPE 79)Monopsony – deters quitsReverse causality – those in good jobs are less likely to leaveCan investigate this using the search model we used above – effects quite subtle - a high level of job tenure indicates that:the job has lasted a long time so is likely to be a good onethe job started a long time ago so is less likely to be a good one.
20Empirical Evidence on Returns to Job Tenure and Experience Aim is to get estimate of ‘true’ return to experience and job tenureAn estimate of how much earnings grow on the job is a joint combination of returns to experience and job tenureGet estimate of how much earnings increase on ‘new’ jobs – estimate of the return to experience
21A ModelDenote by wi(a,t) the log wage of an individual with experience a and job tenure t – we will just consider 2 periodsIndividual i enters the labour market with experience 0 and job tenure 0 and with wage – wage is wi(0,0)Can either stay in same job, Si=1 with wage wi(1,1) or move, Si=0, with wage wi(1,0)
22Defining Returns to Experience and Job Tenure Return to experience will be:Return to job tenure will be:Problem is that only observe one wage at date 1 so cannot compute these returns directlyLet’s consider OLS on cross-section data
23The OLS Cross-Section Estimate The OLS estimate of the return to experience involves comparing the average wages among those with age one and tenure zero with the average wages among those with age zero and tenure zero.One way of writing the OLS estimate of return to experience is:
24Can write this as:First term may be what you want to estimate but second is a bias term which is the correlation between the initial wage and whether you move jobs.It is likely that this bias is negative – with panel data one can generally show that it is.
25What about the OLS estimate of the return to job tenure? One way of putting this is that the OLS estimate is the average return to experience ifthe return to experience is uncorrelated with staying in jobsthe initial level of wages is uncorrelated with staying in jobs.What about the OLS estimate of the return to job tenure?
26OLS estimate of return is given by: This again has biases caused by the potential correlation of initial wages and the returns to experience with mobility.How can we do better – panel data?
27Estimates Using Panel data This is essentially the approach of Topel, JPE 91, and Altonji and Shakotko, ReStud, 97 (though these differ in details)Essentially estimate returns using data for whom it is available e.g. for experience use the movers:
28And use the stayers to have: With return to tenure as:These are consistent estimates if there is no correlation between moving and the return to experience.Also this is average treatment effect if no correlation between return to tenure and return to experience.
29Unresolved ProblemsReturns to experience and job tenure likely to be very heterogeneousThere are ‘good’ job moves (quits) and ‘bad’ job moves (fires) with very different returnsMay be that those who have high wage growth remain in their jobNone of existing studies especially convincing