Question posed to ChatGPT4o:

> Why is the effect of tariffs said to go as the square of the tariff rate?

Longish reply returned. Next prompt:

> Provide web links to standard references to the above?

"Deadweight Loss From Taxes is Proportional to the Square of the Tax Rate" by David Henderson on Econlib:

    This article provides a detailed explanation and proof that the deadweight loss from a tax increases with the square of the tax rate. Henderson illustrates how doubling the tax rate quadruples the deadweight loss. 

"Tariffs – Principles of Microeconomics" by BCcampus Open Publishing:

    This resource discusses the impact of tariffs on market equilibrium, illustrating how tariffs create deadweight loss by reducing the quantity of imports and increasing prices, leading to inefficiencies in the market. 

"Trade Restrictiveness and Deadweight Losses from US Tariffs" by Douglas A. Irwin:

    This paper analyzes the deadweight losses associated with U.S. tariffs, emphasizing that the welfare loss from a tariff is proportional to the square of the tariff rate, highlighting the significant inefficiencies introduced by higher tariffs.

EDIT: Dropped reference to Wikipedia, because quoted extract conflates power law vs. exponential growth.