6 Appendix
The following OLG model illustrates a situation in which the economy must
have a bubble. Suppose that the financial wealth of a country, W, is composed
of the present value of rents, F, and a bubble, B. The flow counterpart
of F is a dividend f. Total output in the economy is made of these rents and
some endowment, totalling y and growing at a rate g. These goods are nonstorable
and consumption is proportional to financial wealth (hence, there is
a non-ricardian feature): ct = θWt.
Equilibrium in the goods market requires that:
θWt = yt
Now suppose that dividends grow a rate g−ρ g.
Replacing W by its components and dividing by the propensity to consume,
we find that:
Ft + Bt = yt
θ
to imply:
Bt ≥ max ½yt
θ − ft
ρ , 0
¾
It follows that if the share of income from rents is not too large, the economy
must have a bubble in equilibrium.