不可贸易活动的局部生产率提高最终会自我限制，因为个体服务活动不可能无限制地扩张而不产生对本身不利的贸易条件 — — 即压低自身价格（以及利润 ） 。
Partial productivity gains in non-tradable activities are ultimately self-limiting, because individual service activities cannot expand without turning their terms of trade against themselves – pushing down their own prices (and profitability).