How do Workers Learn? Theory and Evidence on the Roots of Lifecycle Human Capital Accumulation

How do the sources of worker learning change over the lifecycle, and how do these changes impact human capital and wages? Using data from Germany and the US, we document that as workers gain experience their reliance on internal learning (learning from coworkers) decreases, whereas their reliance on external learning (on-the-job training) exhibits an inverted U-shape pattern. Based on this evidence, we build a search model featuring multiple learning sources whose benefits evolve as workers age and accumulate human capital. Quantitative results indicate that internal learning is more important than external learning for early-career human capital, and also leads to key wage gains later in life due to compensation for learning spillovers among coworkers