The best investing advice I ever read was to focus on looking for opportunities in small- and mid-cap stocks. This idea came from Peter Lynch's book "One Up on Wall Street," which was profoundly influential to my investing approach.

Lynch is widely considered a legend based on his performance managing Fidelity's Magellan Fund. In his book, he explains how individual investors can outperform Wall Street professionals. Lynch points out that large institutional investors, managing billions in funds, are limited to around 10-15% of tradable stocks because of the sheer size of their portfolios. They simply can't invest in smaller companies without moving the market or taking on significant risks.

This dynamic creates an opportunity for boutique investment managers. Smaller companies often grow faster than large ones, albeit with more volatility. This is because they're starting from a lower base, which allows for higher growth in percentage terms. It's much easier for a $1 billion company to double in size than it is for a $100 billion company. Smaller companies, therefore, have more room for expansion in their markets. Their smaller size also makes them nimble, allowing them to quickly adapt to new conditions. Finally, with less analyst coverage and fewer other investors, there's a higher chance of finding undervalued companies that the market has not yet fully recognized.