Unit-price analysis of opioid consumption by monkeys responding under a progressive-ratio schedule of drug injection.

Scientists let monkeys press a lever to give themselves opioid shots. Each shot cost more and more lever presses. The team changed both the drug dose and the price to see if a simple math rule could predict how much the monkeys would take.

The rule is called unit price: presses divided by dose. If the rule works, consumption should drop the same way every time unit price rises, no matter how the scientists set dose or presses.

Monkeys did take less drug when unit price went up, but the neat line predicted by the rule never showed up. Same unit price gave different amounts of drug depending on whether the change came from bigger dose or bigger lever requirement.

The tidy formula missed the mark. Something else beyond price was steering the monkeys' choices.

Wade-Galuska et al. (2011) ran a similar monkey study and found withdrawal state also breaks the unit-price rule. When the animals were in withdrawal, they kept choosing opioids even at high prices, making demand 'inelastic.' This extends the 1995 warning: both price and internal state matter.

Fine et al. (2005) asked the next question in pigeons. They showed that the sheer number of responses, not unit price, best predicts how fast morphine tolerance grows. This conceptual replication backs the 1995 doubt and points to absolute response requirement as the key variable.

Together the three papers say the same thing: unit price alone is too simple. You also need to track biological state and raw response cost.

If you assess reinforcer value or write treatment plans using only response-to-reward ratios, think twice. Clients might keep working for a preferred item when sick, tired, or stressed even if your 'unit price' says they should quit. Track state, not just math. Next time motivation looks off, check biological events like illness or missed medication before you change the program.