A 67-year-old friend told me he won’t file for Social Security (SS) benefits until he turns 70 because “it will pay off as long as I live to at least 81”. Okay, so benefit levels increase by about 8% for each year they’re deferred after your “full retirement age” (probably about 66 for him), and he has no doubt he’ll live more than the extra 11 years. Yes, his decision will “pay off” in a “break-even” sense if he lives that long: he’ll collect more incremental dollars of benefits beyond his 70th birthday than he’ll lose during the three-year deferral (but actually, he’d have to live till he’s 81.5 to break even). But that does not mean his decision is “optimal”.

Good things come to those who wait. I’ll simplify here just a bit, but let’s say an 8% increase in benefits is uniform for every year deferred beyond age 62. (It’s actually a bit more than that after full retirement age, but it’s less than 8% in some years prior to full retirement age.) 8% is a very good, “safe” return, assuming you don’t mind putting your faith in the government to make good.

The Reaper approaches: Unlike your personal savings, SS benefits end at death (a surviving spouse would continue to receive the higher of your respective benefit payments). That means the “safe” 8% return is eroded by diminishing life expectancy with each passing year. For example, average life expectancy at age 62 is 25.4 years, but it falls to 24.5 years at age 63. That’s a decline of 3.5% in the number of years one can expected to receive those higher, deferred benefits. At ages 69 and 70, remaining life expectancy is 19.6 and 18.8 years, respectively. Therefore, waiting the extra year to age 70 means a 4.1% decline in future years of benefits. So rather than a safe, 8% return, subtract about 4%. You’re looking at roughly a 4% uncertain return for deferral of benefits between age 62 and age 70. If you have health issues, it’s obviously worse.

Opportunity Cost: It would be fine to take an expected 4% annual return for deferring SS benefits if you had no immediate use for the extra funds. But you could take the early benefits and invest them! If you’re still working, you could possibly save a like amount of funds from your employment income tax-deferred. So taking the early benefits would be worthwhile if you can earn at least 4% on the funds. Sure, investment returns are uncertain, but over a few years, a 4% annualized return (which I’ll call the “hurdle” rate) should not be hard to beat.

The same logic applies to an already retired individual who would withdraw funds from savings to afford the deferral of SS benefits. Instead, if he or she takes the benefits immediately, leaving a like amount invested, any return in excess of about 4% will have made it worthwhile. But of course, all of this is beside the point if you really just want to retire and the early benefits allow you to do so. You value the benefits now!

But what about taxes? Investment income will generally be taxed, and it’s possible the incremental benefits from deferred SS benefits won’t be. That might swing the calculus in favor of waiting a few extra years to file. And taking benefits early, while still employed, might mean a larger share of the early benefits will be taxed. If 80% of your benefits are taxed at a marginal rate of 25%, state and federal, you’re out 20% of your early benefits. Also, if you expect to be in a lower tax bracket in the future (good luck!), or if you plan to move to a low-tax state at some point in the future, deferring benefits might be more advantageous.

On the other hand, if you’re subject to tax on a portion of your early benefits, you’re likely to be subject to tax on benefits you defer as well. If you’re SS benefits and investment income are both taxed, the issue might be close to a wash, but that hurdle return I mentioned above might have to be a bit higher than 4% to justify early benefits.

Optimal? So what is an “optimal” decision about when to file for SS benefits? For anyone in their 60s today who has not yet filed for SS benefits, it depends on your tolerance for market risk and your tax status.

—You can likely earn more than the rough 4% annual hurdle discussed over a few years in the market, so taking benefits as early as 62 might be a reasonable decision. That’s especially true if you already have some cash set aside to ride out market downturns.

—If you are an extremely conservative investor then you are unlikely to achieve a 4% return, so the “safe” return from deferring SS benefits is your best bet.

—If you believe your tax status will be more favorable later, that might swing the pendulum in favor of deferral, again depending on risk tolerance.

—If you are afraid that failing health and death might come prematurely, filing early is a reasonable decision.

—If you simply want to retire early and the benefits will enable you to do that, filing early is simply a matter of personal time preference.

So my friend who is deferring his SS benefits until age 70 might or might not be optimizing: 1) he is supremely confident in his long-term health, but that’s not something he should count on; 2) he might be an extremely cautious investor (okay…); and 3) he’s still working, and he might expect his tax status to improve by age 70 (I doubt it).

I plan to retire before I turn 65, and I think I’ll be happy to take the benefits and leave more of my money invested. As for Social Security generally, I’d be happy to take a steeply discounted lump sum immediately and invest it, rather than wait for retirement, but that ain’t gonna happen!