4 Dangerous Assumptions That Could Hurt Your Retirement Plan
4 Dangerous Assumptions That Could Hurt Your Retirement Plan · Morningstar

As inveterate watchers of sitcom reruns (and a real-life Felix/Oscar combination), my sister and I loved The Odd Couple while we were growing up. One of our favorite episodes featured a courtroom sequence in which Felix (Tony Randall) berates a witness to "never assume," and proceeds to use the chalkboard to demonstrate what happens when you do. More years later than I care to admit, the mere mention of the word "assume" makes me smile.

But assumptions aren't always a laughing matter, and that's certainly true when it comes to retirement planning, where "hope for the best, plan for the worst" is a reasonable motto. Incorrect--and usually too rosy--retirement-planning assumptions are particularly problematic because, by the time a retiree or pre-retiree realizes her plan is in trouble, she may have few ways to correct it; spending less or working longer may be the only viable options.

What follows are some common--and dangerous--assumptions that individuals make when planning for retirement, as well as some steps they can take to avoid them.

Dangerous Assumption 1: That Stock and Bond Market Returns Will Be Rosy
Most retirement calculators ask you to estimate what your portfolio will return over your holding period. It may be tempting to give those numbers an upward nudge to help avoid hard choices like deferring retirement or spending less, but think twice.

To be sure, stocks' long-term gains have been pretty robust. The S&P 500 generated annualized returns of about 10% in the 100-year period from 1915 through the end of last year, and returns over the past 20 years have been in that same ballpark. But there have been certain stretches in market history when returns have been much less than that; in the decade ended in 2009, for example--the so-called "lost decade"--the S&P 500 actually lost money on an annualized basis.

The reason for stocks' weak showing during that period is that they were pricey in 2000, at the outset of the period. Stock prices aren't in Armageddon territory now, but nor are they cheap. The Shiller P/E ratio, which adjusts for cyclical factors, is currently at 27, versus a long-term mean of 17. Morningstar's price/fair value for the companies in its coverage universe is a not-as-scary 1.04, meaning that the typical company is 4% overpriced relative to our analysts' estimate of intrinsic value. But that slight overvaluation surely isn't a bullish signal, either.

What to Do Instead: Those valuation metrics suggest that prudent investors should ratchet down their market-return projections somewhat just to be safe. Morningstar equity strategist Matt Coffina has said that long-term real equity returns in the 4.5% to 6% range are realistic. Vanguard founder Jack Bogle's forecast for real equity returns is in that same ballpark.