Recent evidence from large-scale field experiments has shown that employers use job candidates' unemployment duration as a sorting criterion. In the present study, we investigate the mechanisms underlying this pattern. To this end, we conduct a lab experiment in which participants make hiring decisions concerning fictitious job candidates with diverging unemployment durations. In addition, these participants rate the job candidates on statements central to four theoretical mechanisms often related to the scarring effect of unemployment: general signalling theory, (perceived) skill loss, queuing theory, and rational herding. We use the resulting data to estimate a multiple mediation model, in which the effect of the duration of unemployment on hiring intentions is mediated by the four theories. The lower hiring chances of the long-term unemployed turn out to be dominantly driven by the perception of longer unemployment spells as a signal of lower motivation. Recent evidence from large-scale field experiments has shown that employers use job candidates' unemployment duration as a sorting criterion. In the present study, we investigate the mechanisms underlying this pattern. To this end, we conduct a lab experiment in which participants make hiring decisions concerning fictitious job candidates with diverging unemployment durations. In addition, these participants rate the job candidates on statements central to four theoretical mechanisms often related to the scarring effect of unemployment: general signalling theory, (perceived) skill loss, queuing theory, and rational herding. We use the resulting data to estimate a multiple mediation model, in which the effect of the duration of unemployment on hiring intentions is mediated by the four theories. The lower hiring chances of the long-term unemployed turn out to be dominantly driven by the perception of longer unemployment spells as a signal of lower motivation.