In Davis v. Michigan Department of Treasury, 489 U.S. 803, 816, 103 L. Ed. 2d 891, 109 S. Ct. 1500 (1989), the Supreme Court invalidated certain Michigan income taxing provisions that taxed retirement benefits paid by the United States Government while exempting retirement benefits paid by state and local governmental entities. The Court determined that the Michigan taxing scheme violated 4 U.S.C. 111 and the intergovernmental tax immunity doctrine. The Davis court summarized the required mode of analysis as follows: Under our precedents, "the imposition of a heavier tax burden on those who deal with one sovereign than is imposed on those who deal with the other must be justified by significant differences between the two classes." Phillips Chemical Co. v. Dumas Independent School Dist., 361 U.S. 376, at 383 1960. In determining whether this standard of justification has been met, it is inappropriate to rely solely on the mode of analysis developed in our equal protection cases. We have previously observed that "our decisions in the equal protection field are not necessarily controlling where problems of intergovernmental tax immunity are involved," because "the Government's interests must be weighed in the balance." Id., at 385. Instead, the relevant inquiry is whether the inconsistent tax treatment is directly related to, and justified by, "significant differences between the two classes." Id., at 383-85. 489 U.S. at 816-17.