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How does Vanguard determine the optimal asset allocation for their Target Retirement Funds?
Googling vanguard target asset allocation led me to this page on the Bogleheads wiki which has detailed breakdowns of the Target Retirement funds; that page in turn has a link to this Vanguard PDF which goes into a good level of detail on the construction of these funds' portfolios. I excerpt: (To the question of why so much weight in equities:) In our view, two important considerations justify an expectation of an   equity risk premium. The first is the historical record: In the past,   and in many countries, stock market investors have been rewarded with   such a premium.  ... Historically, bond returns have lagged equity returns by about 5–6   percentage points, annualized—amounting to an enormous return   differential in most circumstances over longer time periods.   Consequently, retirement savers investing only in “safe” assets must   dramatically increase their savings rates to compensate for the lower   expected returns those investments offer. ... The second strategic principle underlying our glidepath   construction—that younger investors are better able to withstand   risk—recognizes that an individual’s total net worth consists of both   their current financial holdings and their future work earnings. For   younger individuals, the majority of their ultimate retirement wealth   is in the form of what they will earn in the future, or their “human   capital.” Therefore, a large commitment to stocks in a younger   person’s portfolio may be appropriate to balance and diversify risk   exposure to work-related earnings (To the question of how the exact allocations were decided:) As part of the process of evaluating and identifying an appropriate   glide path given this theoretical framework, we ran various   financial simulations using the Vanguard Capital Markets Model. We   examined different risk-reward scenarios and the potential   implications of different glide paths and TDF approaches. The PDF is highly readable, I would say, and includes references to quant articles, for those that like that sort of thing.