economicus should be extended, so that positional goods are included in theories of individual consumption and social concerns are considered among the basic motivations for individual economic behaviour. The triad of economic goods – private, public and positional goods – can be defined in terms of individual and total consumption. Private goods are characterized by the fact that they are consumed only by single individuals. The exclusion of others from positive amounts of consumption is impossible in the case of public goods. Instead, when some individuals consume positional goods, other individuals must be included in the consumption of related negative quantities. A pure positional good can be defined as a good of which a certain amount of positive consumption by one agent is matched by an equally negative amount of consumption by another agent. That is, in the case of positional goods, individuals' consumption levels have opposite signs. However, in the case of certain positional goods, such as Olympic medals, one may talk of new positional products created out of nothing; such products do not result in negative externalities, especially if even last place at the Olympics is viewed as prestigious enough to bring positive utility to the competitor. The distinction among private, public and positional goods brings different rules for deriving total demand. In a diagrammatic view, total demand of a private good is the horizontal sum of individual demands. For a public good, instead, total demand is the Samuelsonian vertical summation of individual demands. Finally, for positional goods, the optimal level of consumption does not coincide, as it does in the case of private goods, with the intersection of any individual marginal rate of substitution curve with the marginal cost curve since an externality emerges for the consumption of other. Thus, it is necessary to first calculate the total marginal rate of substitution and, consequently, find the intersection with the marginal cost curve. As in the case of public goods, the total marginal rate of substitution is calculated by the summation of individual marginal rates of substitution. But in the case of positional goods, one marginal rate of substitution is subtracted since there is negative consumption. Therefore, the total marginal rate of substitution is the difference between the two individual marginal rates of substitution. See also Conspicuous consumption Expenditure cascades Status symbol Veblen good References Goods (economics) Welfare economics