With the S&P 500 hovering just below all-time highs, investors face a familiar but unresolved dilemma. Traditional valuation metrics suggest caution, yet market dynamics have changed dramatically over the past decade. From AI-driven growth to liquidity and rate expectations, this analysis explores whether today’s prices are excessive — or simply the cost of participating in a transformed market.

S&P 500 Index since the beginning of 2025 (1D chart)

Let's start with the simplest concept, but one that people often use incorrectly: the P/E. The price-to-earnings ratio, in the case of an index, tells you how many dollars investors are willing to pay for one dollar of expected earnings of the companies in the index. It sounds trite, but there's a whole market psychology encoded in it. When the P/E is high, the market typically believes in either high future earnings growth or low interest rates (or both). When the P/E is low, the market either doesn't believe in earnings or demands a high risk…