Diversification is considered to be the only free lunch in investing. (If you are new to this concept, check out Introduction To Diversification, The Importance Of Diversification and A Guide To Portfolio Construction.) In a nutshell, by investing in a range of assets, you reduce the risk of one investment's performance severely hurting the return of your overall investment. You could think of it as financial jargon for "don't put all of your eggs in one basket".
In the middle of the 13th century, Venetian bankers began to trade in government securities. In 1351 the Venetian government outlawed spreading rumors intended to lower the price of government funds. Bankers in Pisa, Verona, Genoa and Florence also began trading in government securities during the 14th century. This was only possible because these were independent city-states not ruled by a duke but a council of influential citizens. Italian companies were also the first to issue shares. Companies in England and the Low Countries followed in the 16th century.
According to one interpretation of the efficient-market hypothesis (EMH), only changes in fundamental factors, such as the outlook for margins, profits or dividends, ought to affect share prices beyond the short term, where random 'noise' in the system may prevail. The 'hard' efficient-market hypothesis does not explain the cause of events such as the crash in 1987, when the Dow Jones Industrial Average plummeted 22.6 percent—the largest-ever one-day fall in the United States.