Look at the article. Even just the abstract. It claims that changes at Northwestern's campus meant that a type of student who previously didn't join frats start

Look at the article. Even just the abstract. It claims that changes at Northwestern's campus meant that a type of student who previously didn't join frats started to join frats. By looking at those students, you attempt to control for "interest in joining a frat" as a variable, and look at the outcomes for people with equivalent interest who either did or did not join a frat.