Independence of response force and reinforcement rate on concurrent variable-interval schedule performance.

Pigeons pecked two keys that ran on concurrent variable-interval schedules.

One key always needed a gentle peck. The other key sometimes needed a harder peck.

The team logged how often each key was pecked and how much force each peck used.

Birds spread their pecks in the same ratio as the reinforcement rate, even when one key took more effort.

The log of peck force and the log of reinforcers each predicted choice on their own.

Total force or total reinforcers did not predict choice; only the separate ratios mattered.

Llewellyn et al. (1976) first showed that making one response harder does not drop the overall rate on a single VI schedule. The new study moves the question to two schedules at once and finds the same rule holds.

Anonymous (1995) saw response rate fall when rats had to press harder on a single lever. The pigeon data look opposite, but the setups differ: one schedule versus two. With two keys, birds can slide toward the easier side without stopping work, so total rate stays steady.

Stancliffe et al. (2007) repeated the force test with mice and also saw mixed effects. Together the three papers tell us that harder responses do not punish behavior; they just shift where and how the animal responds.

When you set up choice programs, remember that effort is a separate variable from payoff. A communication button that needs a hard poke may be picked less often even if it earns the same treats. You can balance this by raising the rate of reinforcement for the harder response or by teaching a softer topography. Always measure both force and frequency so you do not miss a hidden bias.