(rather than gross) national product and investment, sY is the rate of growth of
the capital stock. Let y = Y/L denote output per worker and let k = K/L
denote capital per worker. Let n denote the rate of growth of the labor force.
Finally, let a "^ " over a variable denote its exponential rate of growth. Then the
behavior of the economy can be summarized by the following equation:
The first line in this equation follows by dividing total output by the stock
of labor and then calculating rates of growth. This expression specifies the
procedure from growth accounting for calculating the technology residual.
Calculate the growth in output per worker, then subtract the rate of growth of
the capital-labor ratio times the share of capital irlconle in total income from the
rate of growth of output per worker. The second line follows by substituting in
an expression for the rate of growth of the stock of capital per worker, as a
function of the savings rate s, the growth rate of the labor force n, the level of
the technology A(t), and the level of output per worker, 4'.
Outside of the steady state, the second line of the equation shows how
variation in the investment rate and in the level of output per worker should
translate into variation in the rate of growth. The key parameter is the
exponent p on labor in the Cobb-Douglas expression for output. Under the
neoclassical assumption that the economy is characterized by perfect competition,
p is equal to the share of total income that is paid as conlperlsation to labor, a number that can be calculated directly from the national income
accounts. In the sample as a whole, a reasonable benchmark for @ is 0.6. (In
industrialized economies, it tends to be somewhat larger.) This means that in
the second line of the equation, the exponent (-@)/(I - @) on the level of
output per worker y should be on the order of about - 1.5.
We can now perform the followirlg calculation. Pick a country like the
Philippines that had output per worker in 1960 that was equal to about 10
percent of output per worker in the United States. Because 0. is equal to
about 30, the equation suggests that the United States would have required a
savings rate that is about 30 times larger than the savings rate in the Philippines
for these two countries to have grown at the same rate. If we use 2/3
instead of .6 as the estimate of @, the required savings rate in the United States
would be 100 times larger than the savings rate in the Philippines. The
evidence shows that these predicted saving rates for the United States are
orders of magnitude too large.
A key assumption in this calculation is that the level of the technology A(t)
is the same in the Philippines and the United States. (The possibility that A(t)
might differ is considered below.) If they have the same technology, the only
way to explain why workers in the Philippines were only 10 percent as
productive as workers in the United States is to assume that they work with
about 0.1'/('-~) or between 0.3 percent and 0.1 percent as much capital per
worker. Because the marginal product of capital depends on the capital stock
raised to the power -@, the marginal product of an additional unit of capital is
O.l-P/"-P' times larger in the Philippines than it is in the United States, so a
correspondingly higher rate of investment is needed in the United States to get
the same effect on output.
Figure 2 plots the level of per capita income against the ratio of gross
investment to gross domestic product for the Heston-Summers sample of
countries. The correlation in this figure at least has the correct sign to explain
why poor countries on average are not growing faster than the rich
countries-that is, a higher level of income is associated with a higher investment
rate. But if @ is between 0.6 and 0.7, the variation in investment between
rich and poor countries is at least an order of magnitude too small to explain
why the rich and poor countries seem to grow at about the same rate. In
concrete terms, the share of investment in the United States is not 30 or 100
times the share in the Philippines. At most, it is twice as large.
Of course, the data in Figures 1 and 2 are not exactly what the theory calls
for, but the differences are not likely to help resolve the problem here. For
example, the display equation depends on the net investment rate instead of
the gross investment rate. Because we do not have reliable data on depreciation
for this sample of countries, it is not possible to construct a net investment ratio.
A reasonable conjecture, however, is that depreciation accounts for a larger
share of GDP in rich countries than it does in poor countries, so the difference
between the net investment rate in rich and poor countries will be even smaller than the difference between the gross investment rates illustrated in the figure.
The display equation also calls for output per worker rather than output per
capita, but for a back-of-the-envelope calculation, variation in income per capita
should be close enough to variation in output per worker to show that a simple
version of the neoclassical model will have trouble fitting the facts.