Thus, some economists (such as Robert H. Frank and Ugo Pagano) include only goods (like status and power) which are valued specifically because of their relative quality. Hirsch's main contribution is his assertion that positional goods are inextricably linked to social scarcity  – social scarcity relates to the relative standings of different individuals and arises not from physical or natural limitations, but from social factors; for instance, the land in Inter-Provincial Montioni Park is physically scarce, while political leadership positions are socially scarce. The broad theme of Hirsch's book was, he told "The New York Times", that material growth can "no longer deliver what has long been promised for it – to make everyone middle-class". The concept of positional good explains why, as economic growth improves overall quality of life at any particular level, doing "better" than how an individual's grandparents lived does not translate automatically into doing "well", if there are as many or more people ahead of them in the economic hierarchy. For example, if someone is the first in their family to get a college degree, they are doing better. But if they were at the bottom of their class at a weak school, they may find themselves less eligible for a job than their grandfather, who was only a high school graduate. That is, competition for positional goods is a zero-sum game: Attempts to acquire them can only benefit one player at the expense of others. In the case of positional goods, people benefiting from a positional good do not take into account the externalities of their respective sufferers. That is, in the case of "public ... goods, the consequences of this failure implies that an agent consuming the public good does not get paid for other people's consumption; in the case of a positional ... good, the equivalent failure implies that an agent consuming positive amounts is not charged for the negative consumption of other agent's consumption" (Pagano 1999:71). That is, while, in the case of public goods, we have the standard underinvestment problem in their supply, because excluding individuals from externalities that have the "same sign" may turn out to be impossible, by contrast, in the case of positional goods, we have a problem of over-provision, because all agents may try to consume positive amounts of these goods, neglecting to consider the externality on others. For public goods, an undersupply, for positional goods, it signifies an over-supply. In other words, in positional competitions, people work harder to compete and consume more than they would under optimal conditions.