In government, bicameralism is the practice of having two legislative or parliamentary chambers. Thus, a bicameral parliament or bicameral legislature is a parliament or legislature which consists of two Chambers or Houses.
Although the ideas on which bicameralism is based can be traced back to the theories developed in ancient Greece and Rome, recognisable bicameral institutions first arose in medieval Europe where they were associated with separate representation of different estates of the realm.
The Founding Fathers of the United States eschewed any notion of separate representation for a social aristocracy, but they accepted the prevailing disposition towards bicameralism. However, as part of the Great Compromise between large states and small states, they invented a new rationale for bicameralism in which the upper house would have states represented equally and the lower house would have them represented by population.
In subsequent constitution making, federal states have invariably adopted bicameralism, and the solution remains popular when regional differences or sensitivities require more explicit representation, with the second chamber representing the constitutent states. Nevertheless, the older justification for second chambers – providing opportunities for second thoughts about legislation – has survived. A trend towards unicameralism in the 20th century appears now to have been halted.
Growing awareness of the complexity of the notion of representation and the multifunctional nature of modern legislatures may be affording incipient new rationales for second chambers, though these do generally remain contested institutions in ways that first chambers are not. An example of political controversy regarding a second chamber has been the debate over the powers of the Canadian Senate.
The relationship between the two chambers varies; in some cases, they have equal power, while in others, one chamber is clearly superior in its powers. The first tends to be the case in federal systems and those with presidential governments. The latter tends to be the case in unitary systems with parliamentary systems.
Some political scientists believe that bicameralism makes meaningful political reforms more difficult to achieve and increases the risk of deadlock (particularly in cases where both chambers have similar powers). Others argue strongly for the merits of the 'checks and balances' provided by the bicameral model, which they believe helps prevent the passage into law of ill-considered legislation.
See also: Lower House, Upper House, Unicameralism, List of national legislatures, Tricameralism