Well, it's betting on some other factors as well, like the portion of the growth that publicly held firms will capture (versus more of the growth going to new startups or privately held firms). A stock portfolio is by necessity non-diversified on that measure, though large investors can diversify by putting money into VC, angel, and private-equity funds, among other things. It's probably also geographically non-diversified, since some of the fastest-growing markets are hard to buy into (e.g. China is very picky about what kinds of shares it lets foreign investors buy).