Why do fertility subsidies consistently fail in China? Why do consumptionand fertility collapse globally despite intact household book wealth? This paper proposes the Family Long Cycle Hypothesis (FLCH), extending thedecision-making unit of the life cycle hypothesis from an individual to an intergenerational family network, and expanding the budget constraint from a singlelifetime resource constraint to a dual constraint of “total resources + liquiditystructure”. The core mechanism is: intergenerational “blood-transfusion” homepurchase locks in network liquidity without changing household net assets, completing a balance sheet morph of “book wealth unharmed, decision-making paralyzed” at the moment of purchase. This liquidity depletion spikes effective riskaversion, forming a multiplier effect with income uncertainty, causing fertility—the irreversible commitment with the longest duration—to enter the corner solution region first. Within this region, the elasticity of fertility decisions to costsubsidies is strictly zero, but they remain highly sensitive to liquidity repairand uncertainty reduction. Consequently, this paper proves the fundamentalmechanism difference between consumption subsidies (cash rewards, childcarefee waivers) and capital transfers (mortgage principal write-down, unconditionalcash transfers), and proposes three effective policy directions: reducing incomeuncertainty, repairing family liquidity, and raising the reference income of thebottom 90% of the population. The model nests the standard life cycle hypothesis as a special case and is distinguishable from the competitive saving hypothesis on six pairs of opposing predictions. The theory also explains theasymmetric “fast-falling, slow-rising” adjustment of housing prices: the drop isdriven by defensive behavior (business cycle scale), while the recovery is constrained by intergenerational liquidity reconstruction (intergenerational scale of10–20 years).