This is a tricky topic to talk about. So please read the caveat at the bottom of this post!
The wealthiest people in the world have, on average, made their fortunes on ONE stock… and that is usually the stock of the company they themselves founded and ran in the form of a small business that just kept expanding. But of course for every small business that becomes the next GOOGLE, there are countless more that fail and go under. This is the ultimate example of why you won’t get rich quickly by diversifying AND how you could lose everything quickly by NOT diversifying.
Understanding this debate is understanding the trade off that you want to make between risk and reward. You take on an astronomical amount of risk by buying only one company’s stock, but if that company is indeed the next GOOGLE well then it was worth it, wasn’t it? :)
Similarly, a lot of successful investors advocate buying only a handful of stocks (maybe 10-20) that they KNOW and BELIEVE in and expect will grow faster than the rest of the economies in which they are domiciled. They understand it will be a rockier ride than holding every stock in the world, but expect to be rewarded over the long term for the increased amount of risk they expose themselves to.
By properly diversifying, you virtually guarantee you will never get rich overnight, but you also virtually guarantee you won’t lose your shirt either. Conversely, by under-diversifying you open up the possibility of making a lot money very fast… or losing it!
For new investors – you will want to diversify as much as possible, and across multiple levels of diversification as well – see “What is Diversification?”. (At least until you get your feet wet and experience a full market cycle.)