Đối với các nhà nghiên cứu tiên tiến, họ làm nổi bật những hiểu biết quan trọng và conundrums trong lĩnh vực đề bảo hiểm bao gồm intertemporal tiêu thụ và lý thuyết đầu tư, chi tiêu chính phủ và thâm hụt ngân sách, lý thuyết tài chính và định giá tài sản, | 147 Output Fluctuations Demographics and the Life Cycle Tests on micro-level data can in principle avoid this aggregation problem but have led to ambiguous results. In one important paper Zeldes 1989a presents evidence that a significant fraction of the . families tracked by the time-series cross-section Panel Study on Income Dynamics PSID behave as if they are unable to borrow as much as they would like. Since the PSID covers food consumption only interpretation of Zeldes results requires strong maintained assumptions such as the separability in preferences of food consumption and other forms of consumption. A study of the British Family Expenditure Survey by At-tanasio and Weber 1993 presents results more favorable to the Euler equation for the average consumption of the age cohort bom in the decade 1930-40. Attanasio and Weber 1995 likewise find that data from the . 1980-90 Consumer Expenditure Survey do not contradict the validity of a generalized Euler equation at the household level. Many of the problems that prevent individuals and countries from borrowing as much as they would like in international markets see Chapter 6 also limit to some degree households borrowing even from lenders of the same nationality. Casual observation suggests that borrowing constraints do affect many households especially those with low incomes and little collateralizable Thus Euler equations are unlikely to hold exactly for all individuals. This departure from the theory does not imply however that it yields a poor approximation in practice or that the basic insight that consumers look forward in planning consumption is anything less than an important and even central feature of the macroeconomy. Output Fluctuations Demographics and the Life Cycle The overlapping generations approach captures the essence of the life-cycle theory of consumption and saving imroduced by Modigliani and Brumberg 1954 . According to that view individuals or families with finite
