How did Bill Cosby suddenly become radioactive?

On Wednesday, the cable network TV Land pulled reruns of “The Cosby Show” from the air, a development that echoed decisions by NBC to drop a sitcom starring Mr. Cosby and Netflix’s announcement that it would postpone the release of his new comedy special.

What’s surprising is the way that rape allegations against Mr. Cosby, which go back decades, have become so damaging to him now.

According to the research of Ari Adut, a University of Texas sociologist, moral scandals like this one arise when a suspected transgression becomes common knowledge.

Even if there is dispersed knowledge about the allegations (as in Mr. Cosby’s case), the information may not trigger a reaction unless it receives what Mr. Adut calls “collective and focused attention.”