Why are Harvard's alumni so wealthy?

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Some possible explanations for the discrepancy:

• Harvard selects students with higher expected earnings.
• Harvard selects students from families who are already wealthy (more than other top schools: the wealthy alumni are not self-made).
• Students who aspire to make a lot of money choose Harvard (over other top schools).
• Going to Harvard increases students' earning power (more than other top schools), for example, through better networking opportunities.