Since the 2000s, U.S. public pension funds have actively shifted their risky investments away from public equities and toward alternative assets like private equity and hedge funds—some much more than others. We explore a range of possible explanations for these trends and argue that beliefs about the risk-adjusted return of alternatives have played a central role. Pension beliefs are shaped by investment consultants, peers, and experience in the 1990s. Other demand-side channels related to risk-seeking motives, spending needs, and fund governance have weaker empirical support. We also consider supply-side factors and agency interpretations of the data.