SEMAINE D'ÉTUDE SUR LE ROLE DE L’ANALYSE ECONOMETRIQUE ETC. 23C 
Hence (13) expresses equality, at all times #, of the marginal 
productivity of capital (in producing capital, say) to the growth 
rate A — a prescription known as the golden rule of accumula- 
tion (© 
1G 
6. EXISTENCE AND CHARACTERISTICS OF OPTIMAL PATHS 
We now return to the original problem that allows x, and z, 
to vary in time and recognizes the restriction (10 d) of a histo- 
rically given initial capital stock, and look about for a suitable 
maximand. We admit to an ethical preference for neutrality 
as between the welfare of different generations. After some 
hesitation, we tentatively and arbitrarily resolve another ethical 
conundrum by interpreting this « timing neutrality » in a per- 
capita sense. That is, we assume first of all that labor force 
and population grow in proportion. Furthermore we thus imply 
(") PHELPS [1061 
+1 Koopmans - pag. 15