You're right, the asset allocation is one fundamental thing you want to get right in your portfolio. I agree 110%. If you really want to understand asset allocation, I suggest any and all of the following three books, all by the same author, William J. Bernstein.
They are excellent – and yes I've read each. From a theory perspective, and being about asset allocation specifically, the Intelligent Asset Allocator is a good choice. Whereas, the next two books are more accessible and more complete, covering topics including investor psychology, history, financial products you can use to implement a strategy, etc. Got the time? Read them all.
I finished reading his latest book, The Investor's Manifesto, two weeks ago. Here are some choice quotes from Chapter 3, "The Nature of the Portfolio", that address some of the points you've asked about. All emphasis below is mine.
The good news is [the asset allocation process] is not really that hard: The investor only makes two important decisions:
- The overall allocations to stocks and bonds.
- The allocation among stock asset classes.
Rather, younger investors should own a higher portion of stocks because they have the ability to apply their regular savings to the markets at depressed prices. More precisely, young investors possess more "human capital" than financial capital; that is, their total future earnings dwarf their savings and investments. From a financial perspective, human capital looks like a bond whose coupons escalate with inflation.
The most important asset allocation decision is the overall stock/bind mix; start with age = bond allocation rule of thumb. [i.e. because the younger you are, you already have bond-like income from anticipated employment earnings; the older you get, the less bond-like income you have in your future, so buy more bonds in your portfolio.]
He also mentions adjusting that with respect to one's risk tolerance. If you can't take the ups-and-downs of the market, adjust the stock portion down (up to 20% less); if you can stomach the risk without a problem, adjust the stock portion up (up to 20% more).
[in reference to a specific example where two assets that zig and zag are purchased in a 50/50 split and adjusted back to targets] This process, called "rebalancing," provides the investor with an automatic buy-low/sell-high bias that over the long run usually – but not always – improves returns.
The essence of portfolio construction is the combination of asset classes that move in different directions at least some of the time.
Finally, this gem on pages 88 and 89:
Is there a way of scientifically picking the very best future allocation, which offers the maximum return for the minimum risk? No, but people still try. [... continues with description of Markowitz's "mean-variance analysis" technique...] It took investment professionals quite a while to realize that limitation of mean-variance analysis, and other "black box" techniques for allocating assets.
I could go on quoting relevant pieces ... he even goes into much detail on constructing an asset allocation suitable for a large portfolio containing a variety of different stock asset classes, but I suggest you read the book