Many rez Indians would vehemently disagree with this finding. One elder said to me, "It seem like they go to school just to learn how to steal more.”
Indeed, on most Indian reservations I am aware of, having an advanced degree does not make a person more ethical.
It may not be that Kohlberg's theory does not apply. Possibly we are not providing the right kind of education and training. One of the motivations for developing our ethical training program was to focus education more on ethics. Of course business management, accounting, information technology and finance are all important fields but, if "they just learn how to steal more" this education has not been of any benefit to the tribe.