A great collection of investment lessons from a great investor. There were no new or revolutionary insights. If you read a lot of investment books you will be familiar with each lesson but it was great to have them written out in a succinct reminder. This is a short book worth reviewing from time to time.
The lessons are:
- Humans can act irrational. We all hold some incorrect opinions, or have wrong information, or are motivated by things other than truth seeking.
- Markets are made by irrational agents with limited information. Inefficiencies can be exploited.
- Investing skill is real and provable (they have long term data on investor decisions). EMH is based on untested assumptions.
- Markets are short term voting, long term weighing machines. He basically invokes this analogy to show a distinction between short term market moves (liquidity driven, buy sell imbalances) and long term movements (economic fundamentals and value of assets).
- The greatest opportunities occur with periods of change.
- "The best portfolio construction combines concentration and diversification". He means this in a very technical sense: analysts should concentrate on their best ideas, and the portfolio pulls ideas from many analysts to be diversified. I would describe this more conceptually as concentrating into your best ideas, but diversifying across the underlying causal drivers. this encompasses his example where analysts are likely using different mental models (different causal mechanisms) but also includes the single analyst/PM who is not building a portfolio from multiple analysts.
- Shorting is different from being long. Shorting involves borrowing, leverage, more competition, performs in spurts, and is asymmetric (infinity downside, limited upside).
- Superior insight comes from combining machine-intelligence with human imagination.
- Understand you are fallible and know the limits of your knowledge. Use leverage carefully (but don't avoid it) and keep an eye on liquidity. "Never be in a position where you lose your flexibility"
- You need to be big enough to cover necessary research and operating costs, but more importantly not so large that you erode your marginal alpha or become inflexible.
- All investing careers end in failure or "memento mori": During periods of exceptional performance you need to remember that you are never as good as you think you are. And when things are going poorly you are never as bad as you think or others say you are.