Why are wages upward sloping with tenure?

One of the most stylized facts in labor economics is the finding that wages tend to rise with job duration but what is the role of productivity between this relation? Intuitively, it seems rather unspectacular that experienced workers' earnings are higher than otherwise comparable junior workers', but economic literature offers three competing theories explaining this phenomenon. A unique database from a single professional sports industry, covering the past decade of player performance and wages in the National Basketball Association (NBA) is used to test the superiority of one model over others in explaining players' upwards sloping age-earnings profiles. The empirical results show little evidence of the notion that player wages are solely determined on the basis of their productivity. Findings are rather in accordance with shirking and matching ideas: Returns to tenure are found to be significant but it's magnitude is reduced when the spurious bias - stemming from OLS - is controlled for. The fact that tenure remains considerably large - unaffected of productivity - but is simultaneously mitigated due to job match specific effects, is in harmony with incentive and matching arguments.