We develop a model of endogenous skill-biased technical change in developing countries. The endogenous response to a rise in skill supply counters the traditional substitution effect and dampens its role in reducing wage inequality. The model re-enforces consensus estimates of the elasticity of substitution between more/less educated workers by reconciling dispersed existing estimates. It also rationalizes estimates that were hitherto deemed implausible or model-inconsistent. We produce new estimates for developing countries with a novel global panel (finding values at or just above 2) and with Latin American data that facilitates analysis of dynamics (which reduce estimates to 1.7-1.8). We therefore shed new light on a parameter that is crucial for inequality, growth, and other key macroeconomic questions.