Increase wages will lead to an increase in Marginal product of labor, since firms hire workers up to the point where market wages equal to marginal productivity of labor of last labor hired. Due to the diminishing Marginal productivity of labor, increase wages will induce firms cut down on number of workers, moving along the labor demand curve, which is price times marginal productivity of labor. Assuming perfect competition in product market, this will mean marginal productivity of labor of last worker hird will be higher.
Simple trick is if you increase labor input, due to dimishing marginal productivity of labor, mpl of last labor hired can only be lower, and if you decrease the labor input, the opposite is the case.
The reference to such question see Labor economics, 3 rd edition, chapter 5 or 4, topics on labor market.