On the Size-Dependence of the Inclination Distribution of the Main   Kuiper Belt
  We present a new analysis of the currently available orbital elements for the known Kuiper belt objects. In the non-resonant, main Kuiper belt we find a statistically significant relationship between an object's absolute magnitude (H) and its inclination (i). Objects with H<6.5 (i.e. radii >~170 km for a 4% albedo) have higher inclinations than those with H>6.5 (radii >~ 170 km). We have shown that this relationship is not caused by any obvious observational bias. We argue that the main Kuiper belt consists of the superposition of two distinct distributions. One is dynamically hot with inclinations as large as \~35 deg and absolute magnitudes as bright as 4.5; the other is dynamically cold with i<~5 deg and H>6.5. The dynamically cold population is most likely dynamically primordial. We speculate on the potential causes of this relationship. 
H.F. Levison and S.A. Stern