Choucri and North (1972; 1975) formulated the first phase of the theory of lateral pressure in qualitative as well as quantitative terms. They noted that, in general, the strength of a country's lateral pressure correlates positively with its capabilities and "power" (a concept that is almost universally used but defined with difficulty). Lateral pressure theory provides a more detailed and nuanced view of the sources of power, the types of leverages, manifestations, and the behaviors that can be inferred. It puts forth specific propositions for why certain types of international behaviors or activities appear to be more prevalent in some countries than others. For reviews of the lateral pressure theory see, for example, Levy 2005; and Schweller and Pollins, 1999).