The S&P 500 P/E: how to read valuation without fooling yourself

The PER is the most cited multiple in the world and, at the same time, the most misread. It compares price with earnings —how many years of current earnings you are paying— but a P/E on its own does not tell you whether the market is expensive: it tells you what the market expects. And that only makes sense alongside something else.

What the P/E measures (and what it does not)#

The P/E answers “how much am I paying for each pound of earnings?”. It does not answer “is it expensive?”. For that you need an anchor: rates, expected growth and earnings quality. The same P/E can be cheap or wildly expensive depending on the context.

The historical-average trap#

The most common mistake: comparing today's P/E with its average over several decades. That average blends completely different rate regimes and sector compositions. At low rates the market tolerates higher multiples; with more technology weight, the aggregate P/E rises structurally. The average is a number; the context is the information. The chart shows the pattern (not the real historical series).

How to frame it properly#

• Rates: at lower rates, the market is willing to pay more for the same earnings.
• Margins: if margins are at peaks, base earnings may be fragile and the “real” P/E higher than it looks.
• Composition: an index with more technology weight has a higher aggregate P/E by construction, not because it is “expensive”.

Interpretation#

The P/E is a starting point, not a verdict. Its usefulness appears when it is read in relation to the cost of money and the sustainability of earnings, not against a decontextualised average. It is the same criterion we apply to the rest of the ratios: the number only informs alongside its context.