In Davis v. Franchise Tax Board (1977) 71 Cal. App. 3d 998, the court considered a challenge to California's income averaging under the Privileges and Immunities clause. Income averaging was an elective method of computing income tax by which an individual who has large income in one year as compared with other years to have the excess taxed at lower rates applying to the average of five years of income. Revenue and Taxation Code section 18243 limited income averaging to taxpayers who were California residents during a five-year period. (Id. at p. 1001.) The provisions were modeled after federal income averaging rules, which denied income averaging to any taxpayer who was a nonresident alien at any time during the five year period. (Id. at p. 1001; see IRC 1303, subd. (a) and (b).) The taxpayers challenging the California statute had been California residents for the years 1969 to 1972, and sought to include 1973 in their income averaging. (Id. at pp. 1000-1001.) Davis reasoned that the purpose of California's income averaging rule was to bring taxpayers with widely fluctuating income into a position of approximate parity with those of relatively stable income. (Davis v. Franchise Tax Board, supra,. at p. 1002.) Furthermore, California income taxes were limited to income from sources within California, and ignored a nonresident's out-of-state income in fixing the nonresident's tax bracket, giving the nonresident the benefit of a tax bracket that may not be based upon his ability to pay. (Ibid.) If income averaging were available to nonresidents, however, "the nonresident could display a fluctuating California income as a cloak for nonfluctuating total income," thus providing a nonresident with an option not available to residents. Therefore, "instead of forcing the nonresident to show out-of-state income as a condition of income averaging, California has chosen to deny the taxpayer the option." (Id. at p. 1003.) Although the taxpayers in Davis were in an "idiosyncratic position," they could not take advantage of California's income averaging rule. (Id. at p. 1004.)