The general density dependence model can be applied to explain the founding of specialist firms (those attempting to serve a narrow target market). According to this model, specialist foundings hinge on the interplay between legitimation and competitive forces, both of which are functions of the density (total number) of firms in a particular specialist population. Legitimation occurs as a new type of firm moves from being viewed as unfamiliar to being viewed as a natural way to organize. At low density levels, each founding increases legitimation, reducing barriers to entry and easing subsequent foundings. Competition occurs because the resources that firms seek--customers, suppliers, and employees--are limited, but as long as density is low relative to plentiful resources, the addition of another firm has a negligible impact on the intensity of competition. At high density levels, however, competitive effects outweigh legitimation effects, discouraging foundings. The more numerous the competitors, the fiercer the competition will be and the smaller will be the incentive for new firms to enter the field.
While several studies have found a significant correspondence between the density dependence model and actual patterns of foundings, other studies have found patterns not consistent with the model. A possible explanation for this inconsistency is that legitimation and competitive forces transcend national boundaries, while studies typically restrict their analysis to the national level. Thus a national-level analysis can understate the true legitimation and competitive forces as well as the number of foundings in an industry that is internationally integrated. Many industries are or are becoming international, and since media and information easily cross national borders, so should legitimation and its effects on overseas foundings. For example, if a type of firm becomes established in the United States, that information transcends borders, reduces uncertainties, and helps foundings of that type of firm in other countries. Even within national contexts, studies have found more support for the density dependence model when they employ broader geographic units of analysis--for example, finding that the model's operation is seen more clearly at the state and national levels than at city levels.
The primary purpose of the passage is to
question the validity of an economic model
point out some inconsistencies within an economic model
outline an economic model and suggest revisions to it
describe an economic model and provide specific examples to illustrate its use
explain why an economic model remains valid despite inconsistent research results
D选项：描述一个经济模型并给出具体的例子来说明这个模型的用途。前半部分对，后半部分中，作者也确实提出了一个例子，不过这个例子不是为了说明用途的，而是为了说明为什么要在“international level” 上才能应用这个经济模型。