Asset allocation is the theory that any portfolio should have a set of target weights for different asset classes based on time frame and risk tolerance. There are two key principals at work in this theory. The first is that everything goes in cycles and the second is that often when one things is ebbing, the other is flowing. Let's make this very simple and say that bonds return 4% in a bad year, 6% in an average year, and 8% in a good year, and stocks return -5% in a bad year, 10% in an average year, and 15% in a good year.
Stock have cycles and when stocks do well, bonds are more likely to do poorly and vice versa. Let's say we have a portfolio of $100,000 that has a target mix of 60% stocks and 40% fixed income and, therefore, has $60,000 in stocks and $40,000 bonds. Stocks have a good year and bonds have a bad one, and now we have $69,000 invested in stocks and $41,600 in bonds. At this point, we have a total portfolio of $110,600 and an asset mix of roughly 62% stocks and 38% bonds. We began with a target mix of 60-40 but since the equity market fared better than fixed-income market, we are a little off-balance. So how do we fix that? We could sit and wait and watch what happens, or we could shift $2,640 from our equity position to a fixed-income position. Remember, things go in cycles, so we expect that if stocks do well relatively to bonds that sometime in the future, bonds will do well relative to stocks. By shifting $2,640 from our equity position to our fixed-income position, we are essentially selling stocks after they have appreciated (at a high) and buying bonds after they have failed to appreciate (at a low). Look at how the different asset mixes fare, based on a 10-year period that is consistent with historical averages.
Average Returns for Different Weighting Schemes
Different returns are expected for different asset allocations given historical averages
Assuming rebalancing, the expected return of a diversified portfolio is simply the expected return of each of its underlying investments times the allocation weight the investment receives.
The theory can feature different strategies, including strategic asset allocation, tactical asset allocation, and others, but the ideas are the same as the implications for return. A portfolio should consist of a variety of classes of assets to take advantage of zero and negative correlations between those classes, and it should be designed to achieve a target mix of assets that are rebalanced when one grows in relation to another.