"In practice, believers of asset allocation have a choice: either use historical data to project future returns or use their own estimates. Since the latter reeks of the alchemy of market timing (which most serious investors and nearly all academics agree has all the credibility of a perpetual motion machine -- it just doesn't work), this limits the believer of asset allocation to using historical data. In fact, most of the asset allocation software used by professionals and non-professionals (and, dare we say, robo-advisors) employs this approach."