Failing to diversify is the fund managers' problem. Pension funds shouldn't have all their investment in a single countries' stock market (doesn't matter that most, if not all, of those companies operate worldwide, they are still from the US), not even in equity markets. A wiser investment manager (or individual investor) would put a chunk in different kind of equities, and diversify into fixed rate, precious metals, perhaps real estate... Not bet it all to the volatile S&P 500. "Past performance does not guarantee future results.".