One summer in college I interned at an investment bank. It was the worst job I ever had.

A co-worker and I survived our days by bonding over a mutual interest in the stock market.

My co-worker was brilliant. Scary brilliant. The kind of guy you feel bad hanging out with because he makes you realize how dumb you are. He could dissect a company's balance sheet and analyze business strategies like no one else I knew or have known since. He was the smartest investor I ever met. 

He went to an Ivy League school, and after college he landed a high-paying gig at an investment firm. He went on to produce some of the worst investment results you can imagine, with an uncanny ability to pile into whatever asset was about to lose half its value.  

This guy is a genius on paper. But he didn't have the disposition to be a successful investor. He had a gambling mentality and couldn't grasp that his book intelligence didn't translate into investing intelligence, which made him wildly overconfident. His textbook investing brilliance didn't matter. His emotional faults led him to be a terrible investor.

He's a great example of a powerful investing truth: You can be brilliant on one hand but still fail miserably because of what you lack on the other.

There is a hierarchy of investor needs, in other words. Some investing skills have to be mastered before any other skills matter at all. 

Here's a pyramid I made to show what I mean. The most important investing topic is at the bottom. Each topic has to be mastered before the one above it matters: 

Hierarchy

Every one of these topics is incredibly important. None should be belittled.

But you can be the best stock-picker in the world, yet if you buy high and sell low – the epitome of bad investing behavior – none of it will matter. You will fail as an investor.

You can be a great stock-picker, but if you only have 20% of your assets in stocks – a poor asset allocation for most investors – you're not going to move the needle.

You can be a super-tax-efficient investor. But if your stock selection is poor, you're not going to have many capital gains to pay taxes on in the first place. And if you're paying too much for advice, tax savings can be irrelevant. 

A common problem for any investor to stumble on is the temptation to solve one problem without first mastering a more fundamental one. It can drive you crazy, because if you've gotten the hang of an advanced topic, you might think that you're on the road to success, but something more basic like investor behavior or asset allocation could still put you on a road to ruin. Just like my old co-worker. 

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Contact Morgan Housel at mhousel@fool.com. The Motley Fool has a disclosure policy.