A procession of cultural changes, often referred to as “modernization,” is initiated as a society undergoes economic development. But cultural change continues to be rapid in societies that industrialized several generations ago. Much of the change in both developed and developing societies is a progressive abandonment of the norms, values, and beliefs that encourage behavior consistent with the pursuit of genetic fitness. The kin influence hypothesis suggests that these changes are part of a cultural evolutionary process initiated by the replacement of largely kin-based communities with social groups consisting largely of non-kin. Kin have an interest in encouraging one another to behave in ways consistent with the pursuit of reproductive success, and a high level of social exchange between kin will tend to maintain norms that prescribe such behaviors. When social exchange between kin is reduced, these norms begin to relax. Cross-national comparisons of measures that reflect attitudes and behavior support the hypothesis by showing that cultural differences between countries can be substantially explained by their position on a cultural continuum that begins with social networks widening so that they become less kin-based.