Strip away the tickers and the jargon, and a portfolio is mostly an answer to one question: how much in stocks, how much in bonds? Study after study finds that this single split explains the overwhelming majority of a diversified portfolio’s ups and downs. The fund brands matter a little. The split matters enormously.

So it is worth being unusually clear about what each side of the split actually is.

What a stock is for

A stock is a claim on business profits. Own a broad stock fund and you own a sliver of the earnings of thousands of companies, which is why, over long stretches, stocks have been the growth engine: in the century of US data this site’s charts run on, broad stocks returned high single digits a year in real terms.

The price of that growth is written in the same dataset. Stocks lost roughly a third of their real value in 2008, and in the Depression years they fell by more than half. The engine works; it just refuses to run smoothly.

What a bond is for

A bond is a loan with a schedule. You hand over money; a government or company pays interest and returns the principal. No claim on profits, no participation in the upside — just the schedule.

That dullness is the point. In the same century of data, 10-year government bonds returned a couple of percent a year in real terms — a fraction of what stocks earned, but with drawdowns that mostly stayed in single and low-double digits. And in many (not all) stock crashes, bonds went up as investors fled to safety: in 2008, while stocks fell 37%, 10-year Treasuries gained 20%.

Two honest footnotes. First, bonds are not immune to disaster: 2022 delivered stocks and bonds falling hard together, and high inflation eras quietly bled bondholders for a decade. Second, bond funds wobble more than a savings account; “safer than stocks” is not “safe.”

The dial, illustrated

Think of the split as a dial from 100/0 (all stocks) to 0/100 (all bonds). Every notch trades expected growth for expected calm. Using the historical record as an illustration, not a promise:

  • 100% stocks grew a lump sum the most, and along the way fell by more than half at least once.
  • 60/40 — the classic — captured most of the growth while cutting the worst falls to roughly a third.
  • 30/70 grew modestly and mostly spared its owner the big drops.

You can trace any notch of the dial yourself in the portfolio charts — growth, drawdowns, and how differently each mix treated people who started in different years.

Choosing your number

The wrong way is to pick the highest-returning line on a chart. That line was drawn by people who held through the crashes; buying it does not make you one of them.

The right way runs through the worst day, exactly as in the risk article: take a stock share, halve it, and that is roughly the portfolio fall you should be prepared to sit through. If a 40% drop would break your finances or your nerve, 80/20 is not your allocation, whatever its average return. Age matters too, mostly through time: decades of salary ahead of you argue for more stocks, because there is time to recover and new money keeps buying the dips; money you will spend within a few years argues for bonds and cash, because it has no time to recover.

Then write the number into your one-page policy, set the rebalancing rule that keeps it true, and let the dial sit. The mix you can hold beats the mix you can admire.