Are there universal characteristics of adult thought? If so, how do they evolve and manifest themselves in cultures throughout the world? Piaget's formulation of formal operations is the best known and most extensively researched answer to the question regarding universal characteristics. Theoretical alternatives to formal operations differ in the specific characteristics proposed as defining adult thought, but they share an assumption about a level beyond formal operations that has come to be characterized as post-formal. Judgment and reasoning constitute a central psychological process in adult thought. The appropriate type of task with which to assess judgment is one in which there is no "correct" solution. Rather, irreconcilable alternatives should be considered and evaluated. A general methodology for investigating the nature of adult thought is proposed within the framework of systems developed by Oser and Reich (1987) and by Wood (1983). A review of a wealth of disparate published reports indicates a consistent suggestion of a distinctively adult level of thought whose manifestation may be influenced by a variety of factors, none of which is presently clearly identifiable. Whether this level is universally attained or whether it is better viewed as a style whose evolution may also be influenced by educational and cultural factors as well as "native endowment" remains to be determined. (YLB)