Professor Sir Tim Hunt had won every honor in his field, from Fellowship of the Royal Society to the Nobel Prize. But last month, the pioneering biochemist was dismissed from his post at University College, London (UCL). Why? Because, speaking at a lunch to celebrate female scientists in Seoul, he had introduced his remarks with a clumsy joke about finding it hard to work with women in the laboratory because he fell in love with them and they cried.

One of the women present, a lecturer called Connie St Louis, complained on Twitter about his "sexism", triggering the usual lynch mob. By the time the professor had returned to London, his career lay about him in broken shards.

The scary thing here is not the Twitter reaction — we are familiar enough with the ugly psychology of mobs. What is truly depressing is the behavior of those directly involved. For it soon emerged that Mrs. St Louis had given only a partial account of events. You would not have gathered from her version that the professor was being ironic, making a little joke before the "now seriously" that led to his main point about female scientists playing an important role in Korea. Plenty of the women present were journalists but, as is the way when a lynch mob forms, they were reluctant to step into its path.

UCL behaved abominably, first ordering the professor to resign quietly to avoid being sacked, and then allowing its ultimatum to become known. It has since emerged that Sir Tim's accuser had made some seriously false claims about her own qualifications, but no one has suggested that she lose her post. As another Nobel prize-winner, Sir Andre Geim, remarked: "No Vice Chancellor would take on an ethnic-minority militant feminist. Those are not humble Nobel laureates who can be forced to resign quietly."

From Shakespeare to The Simpsons, mobs are fickle. One day, they are laying palms before a donkey; the next they are screaming for crucifixion.

We were supposed to leave all that behind us when we adopted representative democracy. Instead of having to infer the mood of the crowd by listening to its most aggressive loudmouths, we had a way to measure opinion empirically: the ballot box. But social media have returned us to a fiercer, cruder age.

The aphorist G.K. Chesterton observed that, when you strip away the Judaeo-Christian morality that we acquired relatively recently, we revert to vindictive savagery. Can anyone on Twitter doubt that he was right?

When a prominent person says something controversial, the pattern is always the same: remove all context, disregard the speaker's past record and demand resignation. There is no volume control knob. No one ever says: "That was badly phrased, but I kind of get what she meant," or "OK, let him apologize and move on." Nothing short of the end of a career will do.

The target of the crowd's rage is wholly arbitrary. Take the current row over the Confederate battle flag in South Carolina. There are arguments for flying or not flying that standard. I've never much cared for it — its revival after World War II owed more to contemporary arguments about desegregation than to any sudden interest in history — but there are sincere people who take a different view. And whatever side you're on, it's hard to see any connection with the abominable murders in Charleston.

And, indeed, no one has claimed that Dylann Roof was inspired by the 13 stars of the Confederacy. Nor has anyone tried to argue that race relations in South Carolina are worse than elsewhere in the South. On the contrary, the first state to secede from the Union has also become the first, since Reconstruction, to elect a black Republican senator. But this was never really about flags. It was about finding something — anything — that would allow people to express their revulsion at the shootings. In no time, Amazon, eBay, Google and the rest had pulled any Confederate-branded merchandise, and Warner Bros stopped making toy cars modeled after the one in The Dukes of Hazzard.

It's always easier to keep your head down. Write about these subjects, as I'm doing now, and you run the risk of being called a sexist or a racist or whatever. But surely we have to take a stand. The next time you see a mob gibbering and shrieking and demanding someone's dismissal, don't hunker down. Speak up. Someone has to, for Heaven's sake.

Dan Hannan is a British Conservative MEP.