In 1919, Sidis was arrested for participating in a socialist May Day parade in Boston that turned violent.
He was sentenced to 18 months in prison under the Sedition Act of 1918. Sidis' arrest was featured prominently in newspapers, as his early graduation from Harvard had garnered him considerable local celebrity.
During the trial, Sidis stated that he had been a conscientious objector to the World War I draft, he was a socialist, and an atheist.