A common assumption is that the incentive value of food is determined by weight independently of number of food units. In Experiments 1 and 2, rats trained in a T-maze with four 75-mg pellets in one alternative and a single 300-mg pellet in the other alternative developed a preference for the 75-pellet alternative, which did not diminish with extensive training. When the locations of the 75-mg and 300-mg pellets were reversed, rats reversed their choice of arm. Rats preferred a 300-mg pellet to four 45-mg pellets (Experiment 2) and four 75-mg pellets to four 45-mg pellets (Experiment 3). Thus, rats prefer multiple-to single-food units; this effect is permanent rather than temporary, and the incentive value of food is not determined by weight of food alone or number of food units alone. Rats may count food units, judging a given weight of food as greater the greater the number of units-a failure of conservation common, possibly, to a number of species from birds to humans.

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