Effects of shock intensity and duration on the frequency of biting attack by squirrel monkeys.

Thomas et al. (1968) worked with squirrel monkeys. They gave each monkey a brief electric shock to the tail. The team changed two things: how strong the shock was and how long it lasted. They watched how often the monkey bit a rubber tube right after each shock.

Each monkey got many shocks across sessions. The researchers tracked bites per minute. They also tested what happened as time passed since the last shock.

Stronger shocks made the monkeys bite more. Longer shocks also made them bite more. When the shock doubled in strength or length, biting nearly doubled too. The effect faded fast: within five minutes, biting dropped to near zero.

The relation was clean and straight-line. The authors plotted intensity times duration against bites. The points fell on a simple upward slope.

Hake et al. (1967) used the same monkeys and shock box one year earlier. They showed that stronger shock suppresses lever pressing. The new study flips the coin: the same shock parameters increase biting. Same cause, opposite effect, because the response class differs.

Kelly (1973) later kept the intensity-times-duration product but switched the task to lever-press avoidance. Response rates rose with the same product. The multiplicative rule holds across biting and lever pressing, showing the variable is general.

Morse et al. (1966) and Harrison et al. (1975) showed that extinction also sparks aggression in pigeons. Shock and extinction sit at opposite ends: one adds pain, the other removes reward. Both can evoke attack, so you need to watch for aggression whenever the environment shifts sharply.

If you use aversive events or extinction, map possible aggressive outlets first. Provide a safe target, like a chew tube, before the procedure starts. Record intensity and duration of any aversive you deliver; even small increases can double attack risk. Finally, give the client a cool-down period—effects fade within minutes, so wait before asking for delicate tasks.