While the findings of Kubik and others clearly attest to the essential Africanness of many essential aspects of blues expression, studies by Willie Ruff and others have situated the origin of "black" spiritual music inside enslaved peoples' exposure to their masters' Hebridean-originated gospels. African-American economist and historian Thomas Sowell also notes that the southern, black, ex-slave population was acculturated to a considerable degree by and among their Scots-Irish "redneck" neighbors. Additionally, there are theories that the four-beats-per-measure structure of the blues might share its origins with the Native American tradition of pow wow drumming.