Can the adoption of labor-saving technology lead to social instability and unrest? We examine a canonical historical case, the so-called ‘Captain Swing’ riots in 1830s Britain. Variously attributed to the adverse consequences of weather shocks, the shortcomings of the Poor Law, or the after-effects of enclosure, we emphasize the importance of a new technology – the threshing machine. Invented in the 1780s, it spread during and after the Napoleonic Wars. Using farm advertisements from newspapers published in 66 English and Welsh towns, we compile a new measure of the technology’s diffusion. Parishes with ads for threshing machines had much higher riot probabilities in 1830 – and the relationship was even stronger for machine-breaking attacks. Threshing machines were mainly useful in wheat-growing areas. To establish a causal role for labor-saving technology, we instrument technology adoption with the FAO measure of soil suitability for wheat, and show that this in turn predicts unrest.