Richard D. Kahlenberg, editor of the forthcoming book "Affirmative Action for the Rich: Legacy Preferences in College Admissions," points out that universities in other countries don't give so-called legacy preferences to sons and daughters of their alumni. (Even Oxbridge colleges don't, despite the class-bound history of British education.) So, he asks, why on earth do we do it in America?Broadly speaking, students go to college in search of four things: certification; instruction; reputation; and connections.
In terms of certification, any well-accredited school would do. In terms of undergraduate instruction, the best deal for the money (and perhaps the best deal period) is the small four-year school. (I'm leaving this as an assertion but I'm fairly confident I can argue the point if anyone wants to debate.)
In the next two categories, however, the Ivy League cannot be surpassed, in part because of the legacy system.
Without loss of generality, look at Harvard. The student population of the school consists entirely of two overlapping groups: people who can get into Harvard; people whose parents can get them into Harvard.
The first group is hard-working, ambitious and academically gifted. Assuming the number of need-based legacies is trivial, the second group comes from families that are wealthy (they're paying for a Harvard education) and well-connected (at least one parent went to Harvard).
Putting aside luck, you can put the drivers of success into three general categories: attitude, drive and work habits; talent, intelligence and creativity; reputation and connections. It is possible to succeed with just one of these (hell, I can think of people who made it with none), but there is a strong synergistic effect. A moderate talent who works hard and has connections will generally go farther than a spectacular talent who's lazy and isolated.
Connections are governed by the laws of graph theory. I'm not going to delve too deeply into the subject (since that would require research and possibly actual work on my part), but as anyone who has read even the cover blurbs on Linked or Small Worlds can tell you, adding a few highly connected nodes (let's call them senator's sons) can greatly increase the connectivity of a system.
It would be interesting to model the trade off between picking a well connected legacy over a smarter, harder-working applicant given the objective of producing the greatest aggregate success. Because of the network properties mentioned above, it wouldn't be surprising if the optimal number of legacies turned out to be the 10% to 15% we generally see.
Optimized or not, this mixture is almost guaranteed to churn out fantastically successful graduates regardless of what the schools do after the students are admitted. I'm certain the quality of instruction on the Ivy League schools is very good, but, like most education success stories, the secret here is mostly selection and peer effects.
Update: For a different interpretation (this time with actual data), check out this post at Gene Expression.
Updated update: Why doesn't spell check work in the title field?