Date of Award

Spring 2002

Project Type

Dissertation

Program or Major

Psychology

Degree Name

Doctor of Philosophy

First Advisor

Directors: William M Baum

Second Advisor

Suzanne Mitchell

Abstract

The Ideal Free Distribution (Fretwell and Lucas, 1970), an optimization model from behavioral ecology, predicts that a group of animals will distribute themselves across habitats with unequal resources such that all animals have equal success rates. The six experiments of this dissertation demonstrated that humans conform to the IFD with about the same sensitivity as animals and respond similarly when assumptions of the model are violated. The present study also revealed that cooperation, and its effect on the functional size of the foraging unit, may be an important factor in understanding the distribution of social foragers.

Experiments 1 and 2 showed that the IFD could be applied to groups of humans foraging in spatially separated habitats. The distribution of a group was examined in a simulated panhandling situation, where foragers could obtain nickels by asking for them on two streets with varying ratios of resources. The distributions of panhandlers in Experiments 1 and 2 were comparable to those found with animal subjects.

Experiments 3, 4, and 5 varied the degree to which the model's assumptions (perfect knowledge, equal competitive weight, and lack of interference from other competitors) were met in the panhandling situation. As predicted by findings with non-human animals, violations of these assumptions disrupted the IFD.

Experiment 6 examined the effect of cooperation on the distribution of panhandlers. When a high density of panhandlers foraged cooperatively, they distributed according to the IFD. The same number, working individually, did not. This manipulation showed that consideration of the size of the foraging unit may be important in determining whether a distribution fits the IFD.

Additional data analyses from all six experiments revealed strategies that may provide the mechanism by which a group of animals achieves the IFD. Two effective strategies were identified, although no strong evidence was found for the primacy of one strategy over the other, or for a particular combination of strategies needed to produce the IFD. An examination of the characteristics of individual panhandlers ruled out a number of individual differences as the basis for changes in the distribution of the group.

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