Do Ivy League Universities Favor Applicants From Elite Prep Schools?

The elite Ivy League universities of the U.S. power elite (like Harvard, Yale, Princeton, Columbia, etc.) have apparently historically favored, in an undemocratic way, applicants to admission to these universities who are graduates of the U.S. power elite's elite prep school system. As Richard Zweigenhaft and G. William Domhoff noted in their 1991 book Blacks in the White Establishment? A Study of Race and Class in America:

"...By the early 1980s, `only' 34 percent of the incoming freshmen at Harvard and 40 percent at Yale and Princeton were from prep schools...It remains...a distinct advantage for an applicant to an Ivy League school to attend an elite prep school. Two studies have shown that students from the...private secondary schools continue to have an advantage over public school graduates when it comes to admission to Harvard. In one of these studies, David Karen, a doctoral student in sociology at Harvard, noted that the Harvard admission staff places applications from certain boarding schools in special colored folders to set them apart from other applications. Karen found that applicants from these schools were more likely to be accepted for admission, even when he controlled for parental background, grades, SAT (Scholastic Aptitude Test) scores, and other characteristics...."