Applying the law of comparative advantage, the household production model of labor supply argues that it is efficient to allocate time of different members of a household between market work and home production according to their comparative advantage [Beeker, A Treatise on the Family, 1981]. This argument can be found in any standard labor economics textbook. However, social norms, customs, fixed monetary costs, and time costs of market work may influence the choice, and the family may decide that at least one member of the household must stay home. Ehrenberg and Smith [Modern Labor Economics, 1991, pp.233-34] argued that "...if a family decides it must have one spouse at home, it will find that its total resources are maximized if the primary household producer is the spouse who is relatively more productive there than in the marketplace." In other words, allocation according to the law of comparative advantage will be efficient. The purpose of this note is to show that their claim is incorrect. This note also derives the rule under which constrained division of labor within the household will be efficient. Consider a two person family (A and B). Each member has 8 hours to allocate between market and household work. Assume that the hourly market productivities of the members are w a = 20, w b = 17 and household productivities are h a = 10, h b = 8, respectively. Both members are more productive in the market than in home production, but A(B) has a comparative advantage in home production (market work). In an unconstrained case, it is efficient for both individuals to participate in the labor market. Now, suppose one of the spouses must stay home full-time, and the family specializes according to the law of comparative advantage. B specializes in the market and can buy $136 worth of commodities (17 x 8), while A could produce $80 (10 x 8) worth of commodities. The total amount of commodities the family may consume is $216. By reallocating their time (A in the market and B in the household), the couple could raise their total consumption from $216 to $224 worth of commodities [(20 x 8) + (8 x 8)]. Hence, in the presence of the constraint that at least one member of the household must stay home full-time, specialization according to the law of comparative advantage may be inefficient. In this case, the objective of the household is to find the second best solution since the first best solution, where both spouses participate in the labor market, is ruled out by the additional constraint. It will be efficient for the household to minimize the cost (compared to the first best outcome) of fulfilling the constraint. The constraint can be satisfied if either A or B stays home fulltime. IfA stays home, the net loss per hour is $10 while that for B is $11. Thus, from a cost minimization point of view, A should stay home. This principle can be generalized in the following way" If one spouse must stay home, it will be efficient if it is the spouse with the smallest net benefit (w~h~) from labor market participation.