It's no secret that most mutual funds fail to beat the performance of the S&P 500. And if the pros can't beat the averages, it's not unreasonable to assume that most individual investors can't either. Why? According to Robert Hagstrom, author of The Warren Buffett Portfolio, a big reason is the industry's emphasis on diversification. In the interest of minimising risk, many investors have "become intellectually numb to its inevitable consequence: mediocre results." As a result, they wind up owning too many stocks and churn their portfolios unnecessarily (for example, the average mutual fund holds 100 stocks and turns over 80% of its portfolio annually). In The Warren Buffett Portfolio, Hagstrom shows how Buffett and others use the idea of focus investing to organise a winning portfolio.