General Theory on Structural Constraints: Investment Traps, Expectation Formation, and the Conditional Nature of Policy Effects

This paper proposes a structural diagnostic framework in macroeconomics to explain why monetary easing and conventional macroeconomic policies have ceased to generate sustained growth in advanced economies. Rather than interpreting this phenomenon as a failure of policy implementation or as a temporary demand shortfall, the paper conceptualizes it as a transition toward a stage in which investment opportunities themselves become structurally constrained. The core contribution of the framework is the introduction of the structural frontier coefficient Φ, a theoretical variable representing latent constraints generated by demographic conditions, institutional environments, and capital structures. Φ is not directly observable and is not intended to be precisely measured. Instead, it serves as an ordinal indicator of an economy’s relative position with respect to its capacity to absorb productive investment. Accordingly, this paper does not aim to numerically predict growth rates, inflation, or policy effects, but rather to provide a diagnostic framework that clarifies under which structural conditions existing macroeconomic theories and policy instruments are effective, and under which conditions they are neutralized. Within this framework, when Φ exceeds a critical threshold θ, standard models such as the Solow growth model and New Keynesian frameworks operate effectively. When Φ falls below θ, however, monetary expansion fails to generate real investment and instead manifests primarily as asset price inflation or capital outflows. This regime is defined as an investment trap, while phenomena such as liquidity traps and secular stagnation are interpreted as approximate manifestations observed within this structurally constrained domain, rather than as fundamental causes. The paper further hypothesizes that, as economies mature, the elasticity of substitution among structural components declines sharply. Under such conditions, the most binding constraint effectively governs investment feasibility, producing a bottleneck structure. To capture this property, a minimum operator is introduced not as a production or forecasting function, but as a diagnostic approximation to identify the dominant structural constraint. By rigorously separating structural variables from outcome variables, the framework avoids circular reasoning and offers an identification strategy based on asymmetric responses of investment allocation, employment, growth, and inflation to monetary policy. Time-series and cross-country evidence, including the case of Japan, is shown to be consistent with the central prediction that declining structural frontiers qualitatively neutralize the growth channel of monetary policy. The contribution of this paper lies not in prescribing specific policies, but in providing a general diagnostic framework that systematically excludes structurally ineffective policy options prior to policy selection.