Relative and absolute density of reinforcement as factors influencing the peak shift.

Hawkes et al. (1974) worked with four pigeons in a lab chamber. The birds pecked a key on two different VI schedules that alternated every few minutes.

The team varied how much food each side gave. They tracked where the birds pecked when tested with new key colors spread between the trained shades.

Peak shift showed up in every density condition. The birds always pecked most at the key that looked slightly like the richer side.

The best predictor was simple: whichever component had the higher payoff grabbed most of the birds’ responses. No contrast effects were needed.

Green et al. (1975) had just reported a local surge in response rate right before a switch to extinction. The new data seem to clash — no surge appeared here. The difference is timing: the 1975 paper watched moment-to-moment rates inside one session, while the 1974 study looked at choice across test stimuli after training.

Van Hemel (1973) showed that contrast appears in the first 40 s after food. The 1974 paper agrees that early responding is sensitive, but adds that the stimulus linked to richer reinforcement still wins even when overall contrast is absent.

Smith et al. (1975) found no contrast on the main key, only shorter pecks after switches. Together these papers say: contrast-like patterns can live in response topography or timing, yet peak shift marches to the drum of reinforcement density alone.

If you run stimulus discrimination programs, watch which S+ carries the denser schedule. That component will pull responding toward similar novel stimuli, with or without any visible contrast. When you see a shift, check the reinforcement history first — it may save you from adding extra procedures you don’t need.