In the USA powers are expressly allocated to the Executive; and others allocated to Congress (the Legislature) and to the Supreme Court. Article I Section 6 concludes "No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emouluments whereof shall have been increased during such time; and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office."
Similarly, in France - Article 23 states "Membership of the Government shall be incompatible with the holding of any Parliamentary office..."
According to the House of Commons Library "In the UK, in theory a minister does not have to be a member of either House of Parliament. In practice, however, convention is that ministers must be members of either the House of Commons or House of Lords in order to be accountable to Parliament. From time to time, Prime Ministers appoint non-parliamentarians as ministers. In recent years such ministers have been appointed to the House of Lords." The rational is that "for parliamentary scrutiny to work ministers must have a seat in either chamber."
The key points to remember about the doctrine are
- It's origins - the classification of government powers into Legislative; Executive and Judicial goes back to Aristotle [The Politics]. Montesquieu stressed that "when the legislative powers are united in the same person, or in the same body of magistrates, there can be no liberty...Again, there is no liberty if the power of judging is not separated from the legislative and executive." and
- that the modern interpretation of the doctrine is that a person should not hold power in more than one of the branches of government (compare the US, France and UK)
- that the objective is to create countervailing governmental power centres which check and balance each other and thereby inhibit the abuse of power.
- what the principal institutions are of the Executive; Legislature and Judiciary
- its impacts on
- the Executive and the Legislature
- the Executive and the Judiciary
- the Judiciary and the Legislature
- How the rules have changed in recent years - in the UK it is important to note
- Art 6 ECHR
- Constitutional Reform Act 2005
- creation of the Judicial Appointments Commission
- development of 'Judicial Review'
- Human Rights Act 1998
- House of Commons Disqualification Act 1975
- the sub judice rules of both Houses of Parliament (Commons; Lords)
- How it relates to other key doctrines - the Rule of Law; the Supremacy of Parliament; the doctrine of Ministerial Responsibility.
You may also find the post and mindmap in an earlier post on Separation of Powers useful. It can be accessed here.
There are some excellent explanations and diagrams in the book below