In the words of famed e conomist John Maynard Keynes, diversification is insurance against ignorance.
著名经济学家John Maynard Keynes说过多样化是战胜无知的保证。
He believed that risk could actually be reduced by holding fewer investments and getting to know them extraordinarily well.
Of course, the man was one of the most brilliant financial minds of the past century so this philosophy isn't sound policy for most investors, especially if they can't analyze financial statements or don't know the difference between the Dow Jones Industrial Average and a Dodo.
These days, widespread diversification can be had at a fraction of the cost of what was possible even a few decades ago. With index funds, mutual funds, and dividend reinvestment programs, the frictional expenses of owning shares in hundreds of different companies have largely been eliminated or, at the very least, substantially reduced.
This can help protect you against permanent loss by spreading your assets out over enough companies that if one or even a few of them go belly-up, you won't be harmed.
In fact, due to a phenomenon is known as the mathematics of diversification, it will probably result in higher overall compounding returns on a risk-adjusted basis.
One thing you want to watch for is correlation. Specifically, you want to look for uncorrelated risks so that your holdings are constantly offsetting each other to even out economic and business cycles.
When I first wrote the predecessor to this piece almost fifteen years ago, I warned that it wasn't enough to own thirty different stocks if half of them consisted of Bank of America, JP Morgan Chase, Wells Fargo, U.S. Bank, Fifth Third Bancorp, et cetera, because you may have owned a lot of shares in several different companies but you were not diversified; that a "systematic shock such as massive real estate loan failure could send shockwaves through the banking system, effectively hurting all of your positions", which is precisely what happened during the 2007-2009 collapse.
Of course, the stronger firms such as U.S. Bancorp and Wells Fargo and Company did just fine despite a period when they had declined 80% on paper peak-to-trough, especially if you reinvested your dividends and were dollar cost averaging into them; a reminder that it's often better to focus on strength first and foremost.