Banking institutions in the United States are required to hold reservesamounts of currency and deposits in other banksequal to only a fraction of the amount of the bank's deposit liabilities owed to customers. This practice is called fractional-reserve banking. As a result, banks usually invest the majority of the funds received from depositors. On rare occasions, too many of the bank's customers will withdraw their savings and the bank will need help from another institution to continue operating; this is called a bank run. Bank runs can lead to a multitude of social and economic problems. The Federal Reserve System was designed as an attempt to prevent or minimize the occurrence of bank runs, and possibly act as a lender of last resort when a bank run does occur. Many economists, following Nobel laureate Milton Friedman, believe that the Federal Reserve inappropriately refused to lend money to small banks during the bank runs of 1929; Friedman argued that this contributed to the Great Depression.