Cross-cultural comparison is a common method of testing hypotheses regarding the co-evolution of elements of cultures or of the adaptiveness of a cultural practice to some aspect of the environment. It has long been recognized, however, that cultures are not independent but rather may share many cultural elements by virtue of common ancestry and proximity. Attempts to address this issue, known as Galton's problem, range from statistically removing confounding variables to using a standard sample of" independent cultures." We show here that when testing any hypothesis of co-evolution one should not attempt to identify independent cultures or to create them statistically. Rather, cross-cultural comparative studies must be based upon the identification of independent events of cultural change. Once this principle is applied, it becomes apparent that it is in fact groups of closely related cultures that are potentially the most informative for testing cross-cultural hypotheses. Constructing phylogenies of cultures and placing upon them independent instances of cultural elements' arising or changing is an essential part of this task.