Secondary reinforcement and rate of primary reinforcement.

Herrnstein, R. J. (1964). Secondary reinforcement and rate of primary reinforcement. Journal of the Experimental Analysis of Behavior, 7, 27-36.

Four pigeons were trained to peck at either of two response-keys. Pecking at either key occasionally produced a secondary reinforcer. Then, in the presence of the secondary reinforcer further pecking occasionally produced the primary reinforcer, food. The relative rate at which each pigeon pecked to obtain a secondary reinforcer equalled the relative rate of primary reinforcement in its presence.