“…This results are supportive of signalling hypothesis that claims the declining return to education is due to the inability of the highly educated to persistently maintain their productivity-unrelated earnings advantage over the less educated (Groot & Oosterbeek, 1994;Spence, 1973). Many evidences supporting the returns for workers fall because they might be dissatisfied with their current job and dropped their productivity level in the firm (Green & Zhu, 2010;Murillo, Rahona-López, & Salinas-Jiménez, 2012;Verhaest & Omey, 2006 For the second phase, 2007-2012, the estimation result in Table 4 showed that the trend of average private returns increased for all certificates except for LCE in 2007-2012. The results indicated that the employers might pay for certificates if they adequately signal worker productivity; that is, if they act as proper sheepskins.…”