Jackiw co-discovered the chiral anomaly, which is also known as the Adler–Bell–Jackiw anomaly. In 1969, he and John Stewart Bell published their explanation, which was later expanded and clarified by Stephen L. Adler, of the observed decay of a neutral pion into two photons. This decay is forbidden by a symmetry of classical electrodynamics, but Bell and Jackiw showed that this symmetry cannot be preserved at the quantum level. Their introduction of an "anomalous" term from quantum field theory required that the sum of the charges of the elementary fermions had to be zero. This work also gave important support to the colour theory of quarks.