The Marginal Product of Labor Determines a Firm’s Demand Curve for Labor The marginal product of the first janitor is $35. If the wage is above $35, McDonald’s will hire no janitors. If the wage falls to just below $35, McDonald’s will find it profitable to hire one janitor. The marginal product of the second janitor is $30. If the wage falls below $30, McDonald’s will hire its second janitor. If the wage falls below $24, McDonald’s will find it profitable to add a third janitor and so forth.