A young Warren Buffett had 75% of his allocation in GEICO.<p>If you&#x27;re <i>certain</i> something is going to go up in the long term, it makes sense to buy as much of it as you can. You must be certain, and you&#x27;d better be right about being certain.<p>A Buffett-favored aphorism: &quot;Too much of a good thing is wonderful.&quot;<p>If your goal is substantially different performance from a market average, a concentrated position is the ticket. But, unless you&#x27;re really good, the best you&#x27;ll do, on average, is average, in which case you&#x27;d do better by buying an index.