A friend reminded me of some wisdom attributed to Andrew Carnegie that I’ve always loved because it’s counter to what most people believe leads to outsize success or investing returns:

The way to become rich is to put all your eggs in one basket and then watch that basket.

Mark Twain famously said something in the same vein after hearing about Carnegie's remark (source).

This quote resonates with me because it’s what every wealthy person I know personally did. In either company building or investing, concentration (i.e., extreme focus) on one thing is what led to outsize success. If you focus on one thing, you’re more likely to know everything about it and be able to assess it better than others. You’re likely to spot what others have missed, which reduces risk and tilts the probabilities of success in your favor. When everyone else thinks the chance of success is 10%, you realize it’s 60%.

What I’ve also seen is that after someone has achieved outsize success, they embrace diversification as a means of preservation and reducing downside risk.

Said differently, concentration is for building outsize wealth (or a business), and diversification is for preserving that wealth (or that business).