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used as independent variable in a sample that includes the developed countries in both cases has a coefficient four times as high in absolute value (-0.08) in the itudy of Pry and Lilien (1986), in which factors of production are not introduced, as in the analysis of Hwa (1983) (-0.02), in which the independent variables include the growth of capital and labor. This difference can be interpreted as denoting a negative impact of inflation on the investment rate. Similarly, the weighted growth of exports appears, by reason of its impact on saving and investment, to have a more favorable impact on growth when it is introduced without the investment rate (coefficient of 1.2 to 1.4 in Demeocq and Guillaumont 1984 but less than unity in the studies combining this variable with the investment rate, for example Feder 1983, 1986). Conversely, the instability of exports, which has a positive impact on the investment rate and a negative impact on capital productivity, entails in total a diminished negative impact on growth (Demeocq and Guillaumont 1986).

A second characteristic of this group of models is that it allows the global impact of the economic policy variables on economic growth to be distinguished from that of the environmental variables, both groups acting on growth of productive factors and on productivity. In this perspective the environmental variables receive more attention than in the first category of models and are more diverse: in addition to population, per capita product or existence of oil resources, variables may be introduced representing climate (Landau 1983, Plane 1986), human capital (Agarwala 1984, Guillaumont P. and S. 1986, Landau 1983, Plane 1986), terms of trade (Plane 1983, Wheeler 1983), political instability (Wheeler) and so on.

The significance of population growth in this context needs to be clearly defined: it is no longer a variable regarded as an approximation of growth of employment but one capable of affecting employment, investment rate and productivity simultaneously. It is, moreover, sometimes replaced by natural population growth to the extent that immigration is in part induced by growth or by economic policy (Plane 1986). It should be noted that the population growth rate does not always turn out to be significant, either because its wide-ranging impact can mutually compensate or because the presence of per capita product, with which it is linked, captures part of its impact on economic growth. Similarly, human capital, measured by literacy or school enrollment level, life expectancy or child mortality rate, reflects a certain state of initial conditions or development level: it is then very much a matter of an environmental variable and not of the variation in a productive factor, human capital, which in the case at hand would be difficult to measure. Through these variables analysts seek to identify an explanation of growth that is located more "upstream" in the hierarchy of causes and thereby more qualitative than the analyses that focus on production factors. The limitation on this type of explanation lies, of course, in the impossibility of quantifying the impact of variables that are perhaps more fundamental, such as value systems and motivations.