Also see the repeal of the Glass Steagall Act:
Following the Great Crash of 1929, one of every five banks in America fails. Many people, especially politicians, see market speculation engaged in by banks during the 1920s as a cause of the crash.
In 1933, Senator Carter Glass (D-Va.) and Congressman Henry Steagall (D-Ala.) introduce the historic legislation that bears their name, seeking to limit the conflicts of interest created when commercial banks are permitted to underwrite stocks or bonds. In the early part of the century, individual investors were seriously hurt by banks whose overriding interest was promoting stocks of interest and benefit to the banks, rather than to individual investors. The new law bans commercial banks from underwriting securities, forcing banks to choose between being a simple lender or an underwriter (brokerage). The act also establishes the Federal Deposit Insurance Corporation (FDIC), insuring bank deposits, and strengthens the Federal Reserve's control over credit.
The Glass-Steagall Act passes after Ferdinand Pecora, a politically ambitious former New York City prosecutor, drums up popular support for stronger regulation by hauling bank officials in front of the Senate Banking and Currency Committee to answer for their role in the stock-market crash.
In 1956, the Bank Holding Company Act is passed, extending the restrictions on banks, including that bank holding companies owning two or more banks cannot engage in non-banking activity and cannot buy banks in another state.
For more information and to see how the Glass Steagall Act was repealed, see Frontline's page: The Long Demise of Glass-Steagall