Porotic hyperostosis, is a pathological condition that affects bones of the cranial vault, and is characterized by localized areas of spongy or porous bone tissue.^[1] The diploë, or spongy tissue within the bones of the cranium, swells and the tissue of the outer surface becomes thinner and more porous in appearance.^[2][3]

This condition was widely accepted as a result of anemia, which is typically due to an iron deficient diet,^[4] but several lines of evidence suggest that the accelerated loss and compensatory over-production of red blood cells seen in hemolytic and megaloblastic anemia are the most likely proximate causes of porotic hyperostosis.^[5]

In anthropology, the presence of the condition has been considered evidence that a past population suffered chronic or episodic malnutrition. Anthropologists examine bones of past populations to learn about their lifestyles. A sub-discipline known as paleonutrition has focused on the presence of porotic hyperostosis, among other nutritional disorders. A high incidence of the disease indicates the population adapted poorly to its environment or was under nutritional stress.^[4] A low level of iron in the blood is also a defense against pathogens, so a high incidence of the disease in a population could also indicate an attempt to fight off an infectious disease.^[4] From this perspective, porotic hyperostosis could be viewed as a biological attempt to adapt to the environment, rather than an indicator of malnutrition.^{[citation needed]}