Vincent W.J. van Gerven Oei arrived on time for the 9 a.m. keynote lecture at the annual International Medieval Congress at the University of Leeds, in England. Titled "The Mediterranean Other and the Other Mediterranean: Perspective of Alterity in the Middle Ages," the lecture kicked off one of the largest annual medieval conferences in the world, which this month featured the theme of "otherness."

The room wasn’t crowded; most attendees — more than 2,400 of them, from 56 countries — were still arriving or recovering from jet lag, so Mr. van Gerven Oei took a seat near the front row. But as soon as the introductions began, he noticed something odd: All of the speakers discussing "otherness" were white, European men.

"Well, that’s awkward," he recalls thinking. "I wonder whether anyone is going to say something about this."

But in introducing the keynote speech, the moderator made a joke about otherness: If audience members thought he was just another old, white man, they should just wait until after his holiday at the beach.

"Whether or not he intended it as a joke, it obviously ridicules the entire importance of race in this debate, as if it was merely a matter of lying in the sun," Mr. van Gerven Oei says. "I was thinking I could do two things: Either I can just get up and leave, and it will be very awkward, or I can tweet about this." ...