Conventional wisdom has long dictated that 4 percent is the most sustainable withdrawal rate from a retirement portfolio, one that enables the investor to not exhaust his savings. However, in recent years, some experts have questioned that long-held benchmark.

Those in the 4 percent camp can take heart: A recent research paper from registered investment advisor Gerstein Fisher upholds the 4 percent rule, albeit with some caveats.

The Gerstein Fisher paper, “Investment Strategy: What’s a Sustainable Portfolio Withdrawal Rate?,” analyzes a hypothetical portfolio divided into 50 percent domestic stocks (as represented by the S&P 500), 40 percent intermediate-term U.S. bonds and 10 percent cash reserves. In this scenario, the investor retires with $1 million in his portfolio, which is expected to support living expenses for 35 years, in combination with other sources of income, such as Social Security, pensions and annuities.

The investor takes monthly withdrawals from the initial portfolio, adjusted for the actual rate of inflation. Six time periods were highlighted (1973-2007, 1974-2008, 1975-2009, 1976-2010, 1977-2011 and 1978-2012), with the withdrawal rates ranging from 4 percent and 8 percent. Taxes were not considered in the analysis, since the portfolio was assumed to be a tax-free retirement account.

After testing each withdrawal rate for all six periods, the Gerstein Fisher researchers concluded that the 4 percent rate was the only one that didn’t deplete the portfolio during that 35-year time span.

However, the researchers stress that many factors can impact this calculation, such as age, health, lifestyle and spending requirements, as well as the size of the portfolio. In fact, sustainable withdrawal rates can bounce between 3 percent of a portfolio for more conservative investors with a longer time horizon to 8 percent. Within that range, investors have a “high probability of not exhausting assets during the specified time horizons.”

Another factor to take into consideration is whether to weight the portfolio toward income generation or yield. With interest rates low, investors may swing in favor of high-dividend-yielding stocks or high-yield bonds. This approach, the Gerstein Fisher researchers warn, entails “excessive risk to investors.”

The paper notes that in the majority of years between 1926 through 2012, the yield on a 50/50 stock/fixed income portfolio either neared or exceeded 4 percent. Further, when yields were less than 4 percent, the market nevertheless did well and the price-to-yield ratio was high. Consequently, liquidating some of the portfolio’s equity holdings for cash to offset any income shortfall “should not be too difficult.”

Therefore, for most retired investors, the best blueprint may be a total return strategy rather than an income-only plan. Such a tactic permits spending both from a portfolio’s cash flows and from the potential increase in the portfolio’s value, the Gerstein Fisher report concludes.

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