100 Year Thinkers, Ep. 8: The Problem with Modern Portfolio Theory | Robert Hagstrom on How Investing Lost Its Way

In this episode of The 100 Year Thinkers, Robert Hagstrom explains why modern portfolio theory pulled investors away from business analysis and toward portfolio math.

We discuss Markowitz, beta, efficient markets, Warren Buffett, Charlie Munger, business-driven investing, owner earnings, benchmarks, and why thinking like a business owner changes how investors understand risk.

The Warren Buffett Portfolio, 25th Anniversary Edition
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Topics covered:

• Why Hagstrom thinks modern portfolio theory changed investing’s objective

• The difference between volatility, variance and real investment risk

• How Benjamin Graham and John Burr Williams framed risk around intrinsic value

• Why beta became the dominant shorthand for risk

• How the 1973-74 bear market helped institutionalize modern portfolio theory

• Why Berkshire preserved the business owner’s lens

• The “cathedral and casino” distinction between owning businesses and trading stocks

• Owner earnings, return on invested capital and cost of capital

• Why business owners often make better long-term equity investors

• Look-through earnings and building a “mini Berkshire”

• The difference between making money and beating a benchmark

• How benchmarks can distort investor behavior

• Why knowing yourself and your clients matters in portfolio construction

Matt Zeigler and I had the privilege of hosting Robert Hagstrom for a special 100-Year Thinkers Edition of the Excess Returns Podcast.

Available now on Excess Returns Podcast and Talking Billions. 🎧

I’m excited to share this episode with you—it’s reposted here with permission and blessing from the Excess Returns team. Don’t miss it! And follow their work, links below.

https://www.excessreturns.co/

https://cultishcreative.com/

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