This article originally appeared on ETF.COM here.

The traditional 60% stock/40% bond portfolio, using publicly available, low-cost mutual funds, performed extremely well over the last 36 years – a period that included two of the worst bear markets in U.S. history.

From 1982 through 2017, a period that begins both with a bull market and peak in interest rates, a portfolio allocated 60% to the S&P 500 and 40% to five-year Treasury bonds returned 10.4% a year with volatility of 10.2%. That 10.4% return was almost 2 percentage points a year higher than the portfolio’s 8.5% return (with volatility of 12%) over the full 90-year period from 1928 through 2017.

Unfortunately, simple mathematics makes clear that today’s investors (including pension plans and endowments) are faced with a harsher reality. Those returns, from what might be called a “Golden Era,” are not likely to be repeated.

The reason is that returns benefited from a pair of favorable tailwinds, neither of which is likely to recur. Meanwhile, the risk of mean reversion continues to exist. The result is that today’s investors are presented with great challenges in terms of achieving their financial goals using traditional investments.

The issues with traditional portfolios

The first big problem is that favorable past performance benefited from a long, steep secular decline in interest rates. We began the 36-year period ending 2017 with the five-year Treasury yielding 14.0%. We finished the period with it yielding just 2.2%.

Not only is today’s low-yield environment creating a drag on portfolio returns, it also creates an asymmetric risk – there is likely limited room for rates to go much lower to boost returns, but a whole lot of room for them to go up.