But it all depends on the individual. Diversification, macro-economic factors, the fundamentals of a stock, the flowing metrics of a stock (such as short exposure) , the sectors, investor sentiment , all play in the dynamics of investing. I have put a large percentage on one stock and it paid off, I bought 20000 shares of Ford when it was 1.65 and sold at 15, do the math. So to say one way or another is no good depends on the individual investor, how much you can risk, and the dynamics involved. But yes diversification has the LOWEST risk factor..