The article is by Joel Achenbach, who cites this study:
Social psychologist Muzafer Sherif took 22 Boy Scouts and separated them into two groups camping at Robbers Cave State Park in Oklahoma. Only after a week did they learn that there was another group at the far end of the campground.What they did next fascinated the research team. Each group developed irrational contempt for the other. The boys in the other group were seen not just as rivals, but as fundamentally flawed human beings. Only when the two groups were asked to work together to solve a common problem did they warm up to one another.
Ironically, the top comments over there are about how Republicans are fundamentally flawed human beings.