Less means more for pigeons but not always

When humans are asked to judge the value of a set of objects of excellent quality, they often give this set higher value than those same objects with the addition of some of lesser quality. This is an example of the affect heuristic, often referred to as the less-is-more effect. Monkeys and dogs, too, have shown this suboptimal effect. But in the present experiments, normally hungry pigeons chose optimally: a preferred food plus a less--preferred food over a more-preferred food alone. In Experiment 2, however, pigeons on a less-restricted diet showed the suboptimal less-is-more effect. Choice on control trials indicated that the effect did not result from the novelty of two food items versus one. The effect in the less-food-restricted pigeons appears to result from the devaluation of the combination of the food items by the presence of the less-preferred food item. The reversal of the effect under greater food restriction may occur because, as motivation increases, the value of the less-preferred food increases faster than the value of the more-preferred food, thus decreasing the difference in value between the two foods.