P/E Ratio Explained: Why S&P 500 Valuations Matter

One of the most common terms you'll hear in investing discussions is the price-to-earnings ratio, or P/E ratio. It's simple in concept, widely used in practice, and extremely useful for understanding whether a stock, or an entire market, might be expensive or cheap.

Contents

• What Is a P/E Ratio?
• What Does a High or Low P/E Mean?
• The P/E Ratio and the Whole Market
• Why Valuation Matters for Long-Term Investors
• Why Some Investors Are Wary of Elevated S&P 500 Valuations
• Valuation Is Not Timing
• Frequently Asked Questions

What Is a P/E Ratio?

The price-to-earnings ratio compares a company's share price to its earnings per share (EPS).

In simple terms:

P/E = Share Price ÷ Earnings Per Share

It tells you how much investors are willing to pay for each £1 (or $1) of company earnings.

Example

If a company:

• Trades at £100 per share
• Earns £5 per share annually

Its P/E ratio is: 100 ÷ 5 = 20

That means investors are paying 20 times earnings for the company.

What Does a High or Low P/E Mean?

Low P/E

May suggest:

• The stock is undervalued
• The market has low expectations
• The business is facing challenges
• Or it operates in a slow-growth industry

High P/E

May suggest:

• Investors expect strong future growth
• The company is highly popular
• Or the stock may be priced optimistically

Importantly, a high P/E does not automatically mean something is bad, but it does mean expectations are high.

The P/E Ratio and the Whole Market

P/E ratios aren't just used for individual companies. They're also used to value entire indices, such as the S&P 500.

The S&P 500 represents 500 of the largest publicly traded companies in the United States and is often used as a benchmark for the overall US stock market.

When investors talk about "market valuations," they often mean the average or aggregate P/E ratio of the index.

Why Valuation Matters for Long-Term Investors

Over short periods, stock prices can move for many reasons:

• Interest rates
• Economic news
• Sentiment
• Earnings surprises

But over long periods, returns are heavily influenced by:

• Earnings growth
• Dividends
• And starting valuation

If you buy when valuations are very high, future returns can be lower, because prices may already reflect optimistic expectations.

Historically, markets tend to move in cycles of:

• Expansion
• Euphoria
• Correction
• Recovery

Valuation is one way to understand where we might be in that cycle.

Why Some Investors Are Wary of Elevated S&P 500 Valuations

When the S&P 500 trades at a relatively high P/E compared to historical averages, some investors become cautious for several reasons:

1. Lower Expected Future Returns

If you pay a high price today for earnings, future returns may depend heavily on continued earnings growth.

If growth slows, returns can compress.

2. Increased Sensitivity to Interest Rates

Higher valuations often become more fragile when:

• Interest rates rise
• Bond yields become more attractive
• Discount rates increase

Higher rates can reduce the present value of future earnings, which can pressure high-valuation stocks.

3. Concentration Risk

In recent years, a significant portion of S&P 500 performance has been driven by a relatively small number of large companies.

When valuations are high and concentrated, the index can become more sensitive to:

• Earnings disappointments
• Sector-specific slowdowns
• Market corrections

4. Historical Perspective

Historically, when broad market valuation metrics have been elevated, subsequent long-term returns have tended to be more modest compared to periods when valuations were lower.

This does not predict short-term moves, but it is relevant for long-term planning.

Important: Valuation Is Not Timing

Being cautious about valuation does not mean:

• Selling everything
• Trying to predict crashes
• Avoiding equities entirely

Instead, it often means:

• Managing expectations
• Diversifying
• Maintaining discipline
• Continuing to invest consistently

Markets can stay expensive for long periods.

Valuation is a long-term signal, not a timing tool.

How to Use P/E as an Investor

Here are practical ways to think about it:

1. Use It for Context

Compare current valuations to:

• Historical averages
• Other markets
• Bond yields

2. Focus on Long-Term Returns

If valuations are high:

• Future returns may rely more heavily on earnings growth.

3. Stay Diversified

Valuation concerns can be mitigated by:

• Global diversification
• Exposure to value-oriented assets
• Fixed income allocations
• Cash reserves

The Bottom Line

The price-to-earnings ratio is one of the simplest tools in investing, but also one of the most powerful. It helps investors understand what they are paying, what expectations are embedded in prices, and how future returns might differ depending on starting conditions.

When the broader market trades at elevated valuations, it doesn't mean a crash is imminent.

But it does mean investors should:

• Temper return expectations
• Stay diversified
• And avoid assuming past performance will repeat automatically

In investing, price matters.

Understanding valuation is one way to ensure you're not just buying growth, but buying it at a sensible price. That mindset overlaps closely with the broader discipline behind Bogle's investing philosophy.

Frequently Asked Questions

What is a good P/E ratio for a stock?

There is no universal "good" P/E. Context matters: a P/E of 15 might be cheap for a fast-growing technology company but expensive for a declining retailer. The most useful comparison is against the company's own historical P/E, its sector peers, and the broader market average. The long-run average P/E for the S&P 500 is roughly 15-17.

What is the CAPE ratio and how does it differ from P/E?

The CAPE (Cyclically Adjusted Price-to-Earnings) ratio, developed by economist Robert Shiller, uses average inflation-adjusted earnings over the previous 10 years rather than trailing 12-month earnings. This smooths out the earnings volatility of the business cycle and provides a more stable long-term valuation measure. When people say the S&P 500 looks expensive historically, they often mean the CAPE ratio, which has historically mean-reverted over long cycles.

Does a high P/E always mean a stock will fall?

No. High-P/E stocks can stay expensive for years, particularly when interest rates are low (which makes future earnings more valuable in present-value terms) or when earnings growth is genuinely exceptional. Valuation is a long-term signal about likely future returns, not a short-term timing tool.

Should I stop investing if the S&P 500 P/E is high?

No. Market timing based on valuation has a poor track record for ordinary investors. The practical response to elevated valuations is to manage return expectations, stay diversified across geographies and asset classes, and continue investing consistently. Sitting in cash waiting for a correction often means missing years of compounding returns.

How do I find the current P/E ratio of the S&P 500?

Several free sources publish the current S&P 500 P/E ratio. Multpl.com tracks both the trailing P/E and the Shiller CAPE ratio with historical charts. The Wall Street Journal and Bloomberg also publish current aggregate market valuations.

Further Reading:

Irrational Exuberance - Robert Shiller - Shiller invented the CAPE ratio discussed in this article. His book examines why markets become irrationally expensive and what history suggests happens next - essential context for any investor thinking about long-run valuations. (Affiliate link - we may earn a small commission at no extra cost to you.)

The Little Book of Valuation - Aswath Damodaran - The most accessible treatment of valuation from the world's foremost authority on the subject, covering P/E ratios and how to use them alongside other valuation tools to assess whether a stock or market is cheap or expensive. (Affiliate link - we may earn a small commission at no extra cost to you.)

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