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The debate over various definitions of sustainability has for the most part been conducted within the framework of traditional welfare economics. Discussion has centered on technical issues imbedded within the functional forms of various optimization models, especially the coefficient of the elasticity of substitution and the social discount rate. Two more basic problems are: (1) intractable theoretical difficulties within welfare economics call into question the results of traditional models of sustainability regarding intergenerational welfare and (2) equating per capita consumption with welfare contradicts empirical evidence suggesting that the link between happiness and wealth/income is relatively weak. Alternative approaches to measuring well-being are being developed and these have great potential to move the sustainability debate forward.

The debate between advocates of weak and strong sustainability has, for the most part, focused on the substitutability between natural capital and human-made capital of various sorts (see the summary by Pezzey and Toman, 2002). A great deal of work has explored the conditions for optimizing intergenerational social welfare but little attention has been given in the sustainability literature to the intractable difficulties inherent in making Pareto consistent welfare comparisons. Weak sustainability is firmly rooted in the New Welfare Economics (NWE) that dominated economic theory from the late 1930s until the 1990s (Bowles and Gintis, 2000 and Suzumura, 1999). The emphasis of NWE is on achieving efficiency in allocating economic outputs and inputs through substitution and seeking potential Pareto improvements (PPIs). Weak sustainability is based on the work of Solow, 1974 and Solow, 1993 and Hartwick, 1977 and Hartwick, 1996 concerning the allocation through time of an exhaustible resource. The basic idea is that social welfare (defined as the sum of individual utilities) should be non-declining through time.1 Welfare is (explicitly or implicitly) equated with consumption, broadly defined, so sustainability across generations is assured by maintaining the total stock of capital used to generate economic goods, broadly defined. In the weak sustainability framework, substitution is not only permitted, it can be a moral imperative: if the net present value generated by transforming natural capital into human-made capital is greater than the net present value generated by leaving natural capital intact, then this transformation should be done (Beckerman, 1994 and Solow, 1993). Otherwise, the inefficient use of capital will mean that future generations will be needlessly worse off.2 Advocates of strong sustainability argue that traditional neoclassical models overestimate the possibilities of substitution between natural and manufactured capital including the related problems of complementarity, irreversibility, pure uncertainty and discontinuous change (Daly, 1995, Gowdy, 2004, McDaniel and Gowdy, 2000 and Ng and Wills, 2002). The debate over strong sustainability has, for the most part, also taken place within the framework of NWE. The question of the substitutability of manufactured for natural capital can be reduced to a purely empirical question within neoclassical economics as to the elasticity of substitution between different kinds of capital. Weak sustainability, and strong sustainability as it relates to capital substitutability, boils down to applying the Second Fundamental Theorem of Welfare Economics and “getting the prices right”. But the problems with NWE models of sustainability run much deeper than disagreements over which prices to use and the degree of substitutability between human-made and natural capital. NWE has foundered on the attempt to make social welfare judgments without making interpersonal comparisons of utility. This calls into question a central concern of economics during the last 50 years, that is, the identification of the most “efficient” economic policies to increase the output of goods and services. The theoretical difficulties with neoclassical measures of potential Pareto improvements, and the abandonment of NWE by leading neoclassical theorists, are critically important for the sustainability debate. If the NWE framework cannot be used as a guide to evaluate welfare changes over time, what framework should take its place? Fortunately, theoretical and empirical research is quickly filling the void left by NWE. Economists are going back to Bentham to address the question: “What makes people happy?” (Dixon, 1997, Easterlin, 2001, Frey and Stutzer, 2002, Kahneman et al., 1997, Layard, 2003, Ng, 1997, Ng, 2003 and Schwarz and Strack, 1999). Accepting the necessity of interpersonal comparisons of well-being, what economic policies should be put in place to increase the greatest good for the greatest number? Instead of using consumption as an indicator of well-being, economists are directly estimating human “welfare” in all its complexity. This body of work has the potential to move the sustainability debate out of the quagmire of theoretical difficulties associated with the NWE. Replacing the perfectly rational Homo economicus with realistic models consistent with known facts from anthropology, neuroscience and psychology is a logical step toward improving economic science (Camerer and Loewenstein, 2003). Among the most important findings of the happiness literature are these: (1) traditional economic indicators such as per capita NNP are poor measures of welfare, (2) utility depends on interpersonal comparisons and relative position, (3) all humans have common, identifiable biological and psychological characteristics related to their well-being. These observations have direct bearing on the sustainability debate and have the potential to guide intergenerational welfare and environmental preservation policies.

The first major difficulty with measures of weak sustainability is the theoretical impossibility of making welfare comparisons without resorting to interpersonal comparisons of utility (Chipman and Moore, 1978 and Suzumura, 1999). In spite of this, the Kaldor–Hicks approach is followed by most current applications of economic theory to the sustainability problem. By this approach, it is assumed that, as long as some index of total welfare is increased, it is possible to make some people happier without making anyone else less happy. But contemporary microeconomic theory tells us that, even if all externalities, including intergenerational ones, are corrected the NWE still offers no theoretically consistent basis to evaluate changes in the well-being of different contemporary individuals, much less changes in the well-being of different generation. The second major difficulty with weak sustainability is the use of per capita consumption as a proxy for welfare. Findings from happiness surveys, and other evidence about well-being and environmental attitudes, suggest some very different sorts of policies than those based on sustaining per capita consumption. The argument that economists could concern themselves with efficiency and ignore distribution has proved to be theoretically unsupportable. This calls into question the preoccupation of economists with increasing net economic output in the view that this will, at least potentially, lead to greater social welfare. Most important for the sustainability debate is the implication of contemporary welfare theory that more economic growth is not the key to sustain well-being or environmental integrity. But how much economic growth do we need to provide money for the material things that contribute to making people happy? Can we “develop” without growing? Can one country stop growing economically or would this amount to unilateral disarmament, leaving that country behind in the technology, capital investment and entrepreneurial dynamism necessary to successfully compete in a capitalist economy? What policy changes would the transition to a happiness economy require? Finally, and perhaps the most serious issue, how is human welfare related to sustaining the life support systems of the planet? How directly can welfare, however broadly defined, be related to preserving ecosystems and non-human life forms necessary for long-run human survival? The failure of NWE has spurred the search for a more scientifically valid foundation for economic theory and policy. In fields as varied as development economics, game theory and finance new models are being developed based on actual human behavior in its institutional and environmental contexts. A common theme in these new models is “consilience”, that is, “the linking of facts and fact-based theories across disciplines to create a common groundwork for explanation” (Wilson, 1998, p. 8). Integrating economics with anthropology, biology and psychology can lead not only to better economic policies for immediate human welfare, it can also lead to a better understanding of how humans fit in with the rest of the natural world. Such an understanding is essential if Homo sapiens' presence on the planet is to be something more than a fiery but short-lived phenomenon.