ABSTRACT Although exogamy is the worldwide marriage norm, many Middle Eastern populations regularly practice consanguineous marriage. Scholars have posited a number of explanations for this phenomenon, but these theories have not identified a concrete advantage to these marriages sufficient to counterbalance the inbreeding depression and other genetic risks inherent to kin marriages. Drawing on genetic studies and mathematical models, as well as both historical and ethnographic sources, I argue in this article that the Arabian Peninsula's camel Bedouin's dependency on the lactose tolerance allele exerted a selective pressure on marriage strategies that strongly favored consanguineous marriage. For milk-dependent camel Bedouins of the Arabian Peninsula, the advantages of consanguineous marriage did indeed outweigh its risks. In addition, I posit that another common Arabian Peninsula marriage practice, the strong prohibition of marriages between higher-status and lower-status groups, was favored by the same environmental and genetic factors that favored consanguineous marriage.