Constitutions can be defined as a set of rules that aim at regulating the channels of access to principal government positions, the allocation of powers among different branches of government, and the rights of citizens. Most constitutions also include rules establishing procedures for their own amendment and the conditions under which constitutional provisions can be suspended.
Nearly all countries in the contemporary world have written constitutions, often identified as the "fundamental law." Even for these countries, however, it would be misleading to restrict the constitution to a single document so named. Some of the rules that create the structures of government and delimit their authority are also contained in statute law (such as laws establishing the jurisdiction and powers of governmental departments or independent agencies) and in judicial decisions (such as the rulings of a constitutional court) that are not codified in a single document. In addition, there are always unwritten conventions that regulate the behavior of representatives and citizens, particularly in areas where written rules are silent or unclear. Moreover, most parts of a constitution can be composed of unwritten conventions, as is the case of the United Kingdom, New Zealand, and Israel.
Constitutions generally attempt to prevent the arbitrary use of state power. But there is a wide variation in the degree to which state authorities effectively abide by the constitution. Rulers are more likely to observe a constitution that emerged out of a democratically elected body representing a plurality of political forces. They are also likely to comply with the constitution when citizens agree on the authority of the constitution as a set of impartial procedures for the resolution of conflicts. This consensus, however, is often lacking when societies are divided by overlapping cleavages of an economic, religious, or ethnic nature.
Constitutions that are at least minimally enforced are essential for the existence and legitimacy of democratic regimes. Citizens would not be free to criticize the government and keep its decisions in check without basic constitutional rules guaranteeing freedom of expression and providing remedies against arbitrary state action. Truly competitive elections could not exist without constitutional rules guaranteeing freedom of assembly and organization.
There is great variation in the way constitutions organize a democratic regime. Constitutional democracies can be presidential, if the chief of government is elected by the people for a fixed term, or parliamentary, if he or she is elected by the assembly and responsible to the legislative majority. Legislative assemblies can be unicameral or bicameral. States can be unitary or federal. Most contemporary constitutions establish independent courts responsible for interpreting the constitution. Constitutional courts, however, vary in organization, composition, and powers. Finally, while the majority of constitutions include amendment rules that attempt to make constitutional reforms more difficult to pass than ordinary laws, these rules can be relatively flexible or extremely rigid.
In the twenty-first century constitutions are implicitly or explicitly central to some of the most important research fields in social sciences. Constitutions and their various designs are considered to have a crucial impact on the stability and quality of democracy, on economic policy and economic performance, and on the rate of policy change in political regimes.
