The Internet fuels the growth of thought contagions. We have long known that people gravitate to parts of the media that reinforce their beliefs. Liberals listen to NPR; conservatives tune into Rush Limbaugh and Fox News. We also know that being in like-minded company fosters and hardens pre-existing biases. Do a quick Google search and you’re guaranteed to find a liked-minded group. Thought contagions that turn into financial contagions are hardly new. There was tulip mania in 17th-century Holland, the South Sea bubble a century later, and the high-tech bubble at the end of the 1990s. But the Internet is a cruel accelerator of both thought and economic contagions. Just as global air travel made biological contagions more of a worldwide threat, in an Internet-driven, overconnected world there will be many more thought contagions, with far more widespread consequences.