The models we have seen so far do not provide satisfying answers to ourcentral questions about economic growth. The models’ principal result isa negative one: if capital’s earnings reflect its contribution to output, thencapital accumulation does not account for a large part of either long-rungrowth or cross-country income differences. And the only determinant ofincome in the models other than capital is a mystery variable, the “effectiveness of labor” (A), whose exact meaning is not specified and whose behavioris taken as exogenous.