[4.1] Specification _of Income-Leisure FunctionThe following points must be taken into account in order tofunctional form of the preference function.Firstly, the preference function must fulfill the conditions(3.2.7.1-4).Secondly, in the present stage of analysis it is not favorablepreference function which has an analytical form that apriori requires the specific algebraic sign of (数式) regardlessof the magnitude of nonprincipal earners wage rate, w, principalearners income, I, being given. It can not be said a prioriwhether the relation between the nonprincipal earner's wage rate,w, and the optimal hours of labor supply, h*, is backwardbending when the principal earner's income, I, is given. It is possible for (数式) to be positive in some range of values of wand negative in some other range of w. Hence, specific functionalforms for the preference function which give a positive (数式)and a negative (数式),respectively regardless of the magnitude ofw should be excluded.The algebraic sign of (数式) should be concluded after theparameters of the preference function is estimated, by makinguse of suitable analytical form of preference function. If thealgebraic sign of (数式) is proved to be positive (negative)for the considerable wide range of variation in w, a function whichgives a priori a positive (negative) (数式) regardless of the