Despite a common belief that the attempted assassination of a U.S. politician is a rare event that happens once in a generation, a U.S. historian says it is a tradition that has reared its head whenever the country is in a period of social unrest and radical change.

Joseph Palermo, an expert on political history at California State University in Sacramento, said the struggling U.S. economy, combined with the polarizing hype created by the 24-hour news cycle, means conditions were ripe for the shooting of a congresswoman in Arizona on Saturday.

"People are very quick to try and pretend that the political assassination is very rare," he said in an interview with Postmedia News. "Sadly, the truth is, it's a very common element of any society, especially the United States."...