The homophilous nature of online social network, a.k.a. “echo chambers”, is thought to be an important factor in the massive spread of misinformation online. However, a convincing test of the link between homophily and the spread of misinformation has been lacking due to confounding factors in observational studies, most notably the greater overall activity and political engagement of individuals in echo chambers. We tested it in a controlled experiment in which participants seek to share only messages they believe to be true, wherein the level of homophily in participant populations was independently varied. The experimental results revealed that contrary to predictions homophilous networks were less likely to propagate both true and false news messages. We reconcile this contrast by showing that the effect of homophily reverses when messages are relatively believable and of moderate political color.