Moral Maturity or Rigid Ideology?

In his book Moral Development: Advances in Research and Theory (NY: Praeger, 1986) James Rest of the University of Minnesota and his colleagues describe their work on "moral maturity" using the defining issues test (DIT).  Their assumption that ethics is based on universal ethical principles and their claim that similar levels of scores across several countries evidences universality are simplistic.  Kohlberg, Rest and their colleagues in effect claim that Kant and Mill are morally more mature than skeptics or particularists like Hume, Aristotle or Hegel. They seem to claim that there is a profession of "moral philosophers" who all agree as to what moral maturity might be.  As a member of the Academy of Common Sense, this strikes me as Mickey Mouse. This emphasis on the general at the expense of the particular results in Rest et al.'s claiming that highly educated philosophers are more morally mature than fundamentalist ministers because the complexity of the philosophers' thinking is greater. Moreover, without defining their use of the terms "liberal" and "conservative",  Thoma and Rest claim that liberals are more "morally mature" than conservatives. Rest, et al.  do not indicate whether the study controlled for IQ, socioeconomic status, and "high educational and career orientation," which they elsewhere state have robust correlations with their universalist moral maturity measure.  Might conservatives tend to have a particularistic orientation? Nor do they define  "liberal" and "conservative."   Henry David Thoreau's radical libertarianism might be construed as "conservative."  Was Thoreau morally immature in the opinions of Rest and Kohlberg?