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Geophagy, clay or dirt eating, is a worldwide phenomenon of long historical precedence. Historically perceived as an irrational behavior, today, growing evidence supports the contention that earth eating has adaptive value. Case studies from Africa and the United States illustrate the practice. A literature review shows that three primary explanations have been offered to account for the presence of geophagy in human populations. The first argues that geophagy has medicinal value as an anti-diarrheal and as a dietary supplement in vitamin and mineral poor environments. The Second contends that geophagy is a culturally transferred practice. The third states that earth is eaten as a means to detoxify certain plant species and inaease diet breadth. This paper suggests that geophagy can be best understood within the broader context of human behavior when viewed from both a cultural and biological perspective.