Whether or not you should buy more stocks, of course, depends on your risk capacity, risk tolerance and current wealth and spending, and not simply on your age. Just because you can buy more stocks doesn't mean that you should. I also agree that a retirement income plan should consider all of a household’s assets, including Social Security benefits.
I draw a line, however, at including the present value of Social Security benefits into the bond portion of an upside portfolio to calculate my asset allocation. That’s why I recommended considering Social Security benefits as a component of the floor portfolio in Three Portfolios and calculating the asset allocation of the risky portfolio separately.
(There is a good discussion on this topic — there usually is — in the Bogleheads forum. You will notice that most posters, despite being members of a forum devoted to the philosophies of Jack Bogle, as am I, aren't really on-board with his position in this case.)
There are several ways in which Social Security benefits are not like a bond, and therein lies the problem.
First, we can't buy or sell Social Security benefits. If we include them in our asset allocation, we can't rebalance that allocation.
Second, for most American households, the present value of Social Security benefits is the largest component of the household's wealth, followed by home equity and then retirement savings. Treating the present value of Social Security as a bond would force most households to invest their entire savings in stocks and still not be able to achieve a reasonable asset allocation.
(The present value of Social Security benefits can be estimated by getting a quote for a life annuity from an insurance company with payouts equal to expected Social Security benefits. In today's interest rate environment, multiplying your expected annual benefits at retirement age by 20 will put you in the ballpark of their present value. For a more accurate valuation, I like Income Solutions at Vanguard.com for a quick online quote.)
Third, and perhaps most importantly, few retirees will value the present value of Social Security benefits as highly as they value an equal amount of stocks and actual bonds at the margin. They might (should) like the benefits better for a large portion of their wealth, but be unwilling to convert all of their stocks and bonds to illiquid Social Security benefits.
Retirees have expressed this preference by refusing to annuitize all of their retirement savings. It's a nearly identical scenario, given that Social Security benefits are an annuity. It stands to reason that they would be equally reluctant to back into an all-annuity position by risking the loss of much of their liquid portfolio.
Stocks and real bonds can be converted to cash to meet emergencies, to spend more in some years than others, or to take advantage of investment opportunities. If you need more money from Social Security benefits, all you can do is wait. Consequently, retirees with most of their wealth in the present value of Social Security benefits should be inclined to take less risk with their relatively small portfolio of stocks and bonds, certainly less risk than a 100%-stock portfolio would entail.
Social Security benefits reduce risk no matter how you treat them in an asset allocation because they reduce spending from a risky portfolio. The probability of a systematic withdrawals portfolio surviving decreases with more spending. Reduce spending from that risky portfolio by spending Social Security benefits, instead, and you can significantly extend the life of your savings.
In fact, reducing spending is usually a more effective way of reducing risk than is adding bonds to your portfolio. As William Bengen's chart below shows (focus on the 30-year line), unless you currently hold more than 70% stocks, adding more bonds doesn't change the SWR rate much. (If the sustainable withdrawal rate decreases, it is because SOR risk has increased, and vice versa, so a falling curve indicates more risk.) Adding bonds below a 30% stock allocation actually worsens risk.
Treating benefits as a bond in the asset allocation requires the purchase of more equities to obtain a desired asset allocation and thereby increases risk.
Here is an example of the two alternatives, considering the present value of Social Security benefits as a bond in the asset allocation as shown in the top chart below, and omitting them from that calculation in the bottom chart.
Assume a household has $250,000 saved in a 401(k). They expect $30,000 a year from Social Security benefits and can buy a similar life annuity today from an insurance company for $600,000. The household might consider their Social Security benefits, then, to be a bond worth $600,000. Let’s further assume that the household would prefer a 50% bond portfolio because they wouldn’t be comfortable with more than a 20% loss of their savings ($50,000) in a bear market crash.
The riskiest portfolio allocation this family can implement for their savings, when considering their benefits a bond, is obtained by investing all $250,000 in stocks. Doing so would still only create a 29% equity portfolio ($250,000 / $850,000), but a 100%-stock upside portfolio. They would have lost over 50% of their cash and bond value ($125,000) in the 2007-2009 market crash and 100% equities is well beyond what Bengen found to be optimal for systematic withdrawals.
Had they ignored Social Security benefits for asset allocation purposes, alternatively, they would hold $125,000 in stocks, $125,000 in bonds and would have lost about 20% of that, or $50,000, in 2007-2009.
How would income be affected in these two scenarios? In both scenarios, the retiree could spend $30,000 a year from Social Security benefits.
In the Social Security bond scenario of the upper chart, again according to Bengen, the systematic withdrawal rate for a 100% equity portfolio would be about 3.6% of $250,000, or $9,000. That would support total annual spending of just $39,000 a year.
In the scenario without a "Social Security bond", represented by the bottom pie chart, he could spend a systematic amount from the risky portfolio of about 4.4%, according to Bengen, with a 50% equity allocation of the spending portfolio, or $11,000, for total spending of $41,000 annually.
I recommend that you calculate your asset allocation both ways. If you’re comfortable with the equity allocation this forces on your upside portfolio, then consider Social Security benefits a bond for asset allocation purposes. If doing so would leave you with an unacceptably high equity allocation inside your upside portfolio, then consider the benefits part of your floor portfolio and manage the upside portfolio separately.
I don’t see an advantage to the Bogle suggestion. It increases risk by increasing your equity allocation. Benefits reduce SOR risk by decreasing spending from your risk portfolio no matter how you consider them in your asset allocation. The present value of your benefits can’t be rebalanced and doesn’t have the liquidity of stocks and real bonds. Total spending is decreased when you consider Social Security benefits a bond because the SWR component of income is reduced by the excessive equity allocation of the spending portfolio, from 4.4% to 3.6% of the upside portfolio in this example.
Placing the benefits in your floor portfolio and ignoring them when calculating your risky portfolio’s equity allocation, as I suggested in Three Portfolios, provides a lot more control of your savings portfolio and, in most cases, exposes you to less risk with more spending.
So, in this case I suggest you take the easy route. Consider your Social Security benefits a safe source of income and address your spending needs net of those benefits separately.
P.S. The Yin and Yang of Retirement Income Philosophies, new from Wade Pfau's blog is great stuff!