Multiple job holding as a strategy for skills development

This study investigated the training effect of multiple job holding on the activity of main jobs. First, we developed a dual-labor supply model by adding the training effect of working second jobs. The theory showed that workers with unconstrained hours hold second jobs when they develop skills via the experience of second jobs. To verify the hypotheses from the theoretical model, the causal relationship between holding a second job and the wage rate of a main job was estimated using the Keio Household Panel Survey. Difference generalized method of moments was adopted to remove time-invariant individual effects and endogenous bias. Moreover, the estimations showed heterogeneity of main jobs in terms of length of working hours, tasks, and job turnover. Full-time workers engaged in intelligent tasks and those who did not change their jobs secured training effects from second jobs but only when the comparison group was the workers allowed to hold second jobs by their employers. It was presumed that employers paid to restrict employees’ activities. On the contrary, part-time workers engaged in physical tasks were exhausted by second jobs, which decreased the wage rate of their main jobs.