Scott Adams (creator of Dilbert) comes up with what may be the world’s simplest portfolio. It sounds like a pretty good one to me.
First, let's assume the hypothetical money is invested entirely for retirement, so we don't need to worry about keeping any of it liquid for college or buying a house. That assumption is just to keep things simple.
Second, we're only talking about investments up to 10 years prior to your planned retirement. When you near retirement, you would typically and gradually convert as much of your stock portfolio into bonds as necessary to get the monthly income you need. That's a more complicated scenario than what I want to discuss here.
I suggest, as a starting point for our discussion, that a perfectly adequate simple portfolio for young(ish) people might involve putting 50% of your money in an ETF from Vanguard (VTI), which captures the entire Wilshire 5000. In other words, you'd buy one financial instrument and own a little bit of just about every public company in the United States. That's all the diversification you can get within one country, and the U.S. is still considered a relatively safe place to invest even if it doesn't have the best growth potential. The fees for the ETF are a low .015% per year, and because ETF managers don't do much buying and selling within the portfolio, it doesn't generate much taxable income to pass along to investors.
I picked 50% to allocate to this investment because I contend that no expert has a good reason for picking a different figure. Some experts might tell you 25% is the right allocation for U.S. stocks, and some might say 75%. I contend that most allocation recommendations of that sort are no more defensible than horoscopes.
For the remaining 50% your investments, let's say you buy the Vanguard Emerging Market ETF (VWO) with a .27% expense ratio. That gives you a play on the best companies in emerging markets around the world, at low cost, with excellent diversity, and low taxes.
Ian Ayers likes the idea too, although he thinks you can improve on the allocation.