In the labor market Wages will have a direct relationship to the productivity of labor empoloyed. Employers (Demand side) will hire workers to maximize profit.

In deriving the demand for labor it is important to remember that the basic productivity of labor is subject to change with the price of capital (a complement) and with the level of technology in the economy.

If the wage falls, the investment of capital may change (for example) and then the productivity of a unit of labor decreases (capital per effective worker decreases see Solow Growth Model). So the shift along the old MRP curve will not reflect the full adjustment in labor demand, it will in fact be less elastic because of the decreased incentive to invest in complimentary capital.