Constitutional Law: The Rules for the People Who Make the Rules

A legislature passes a law. A citizen says it violates her rights. Who decides? The legislature itself has an obvious interest in saying no. The executive, who enforces the law, has an interest too. So societies have invented an outsider: a judge, or a special court, whose job is to say that the law, though properly passed, is void.

Where that power sits, who may invoke it, and what a ruling does afterwards are the main differences between constitutional systems. In the United States any court can refuse to apply an unconstitutional law. In Germany, only one court can strike it down. In Mexico, a man can go to a federal judge in person and ask for protection, called an amparo, but the ruling will traditionally protect him alone. These are three solutions to the same problem.

This piece describes how constitutional systems work and is not legal advice; constitutions and court rules differ by country and change, and some systems (Mexico's judiciary, for one) have been reformed recently, so check current sources.

Why a constitution: rules about the rule-makers

A constitution does two jobs. It organises power (who makes laws, who enforces them, how courts are chosen, how regions fit in) and it limits power (rights that even a majority cannot simply vote away). Those two jobs require one more feature: it must be harder to change than an ordinary law. A constitution that parliament could rewrite by a simple vote would limit nothing. That is why every serious constitution has special rules for its own amendment, which we return to at the end.

Constitutions come in many sizes. The US Constitution of 1789 has seven articles and 27 amendments. Brazil's 1988 Constitution is a large document that goes into detail on social rights. Mexico's 1917 Constitution, born of the Revolution, was one of the first to include social rights such as labour protections. The United Kingdom has no single codified constitution; its fundamental rules are spread across statutes, court decisions and conventions, and Parliament is sovereign. The question "who has the last word?" gets a different answer in each.

Judicial review: who decides when a law breaks the constitution?

The American story starts with Marbury v. Madison, decided on 24 February 1803 (5 U.S. (1 Cranch) 137). Chief Justice John Marshall, writing for the Court, held that a provision of the Judiciary Act of 1789 gave the Supreme Court a power the Constitution did not, and that a law contradicting the Constitution could not stand. His reason, in one line: "It is emphatically the province and duty of the judicial department to say what the law is." The Constitution does not say in terms that courts may strike down laws; Marshall derived it from the Constitution's status as supreme law, an argument that Alexander Hamilton had already made in Federalist 78. The result is a diffuse model: any court, deciding any ordinary lawsuit, can refuse to apply a statute that violates the Constitution, and the Supreme Court has the last word.

Europe took a different path. The Austrian jurist Hans Kelsen argued that having every judge decide constitutional questions would produce chaos and that the job needed one special court. Austria's 1920 constitution created such a court, the first in a form Kelsen designed; Czechoslovakia's 1920 constitution also provided for one, which first sat in 1921. After the Second World War, Germany (Basic Law of 1949, with the Federal Constitutional Court beginning work in 1951), Italy, and later Spain and Portugal built their own. Under this concentrated model, ordinary judges cannot strike down a statute; if they think one is unconstitutional, they stop the case and refer the question up. When the constitutional court annuls a law, it is gone for everyone. Kelsen called the court a "negative legislator".

Many countries mix the two. Brazil lets any judge set aside an unconstitutional law in a case, while the Supreme Federal Court also hears direct challenges whose results bind everyone. Colombia's Constitutional Court (created by the 1991 constitution) hears abstract challenges and reviews rights decisions from lower courts. France, which long distrusted judges overruling parliament, created the Conseil constitutionnel in 1958 to check bills before promulgation, and since 2010 lets litigants raise constitutional questions about laws already in force. The United Kingdom keeps parliamentary sovereignty: courts can declare a statute incompatible with human rights (Human Rights Act 1998), but the statute remains valid until Parliament changes it.

Amparo: rights you can take to court yourself

Latin America's characteristic invention is a fast, simple court action for someone whose constitutional rights are being violated. It began in Mexico. Manuel Crescencio Rejón designed the first version in the 1841 constitution of the state of Yucatán, and Mariano Otero carried it to the federal level in the Acta de Reformas of 1847. It was repeated in the federal constitutions of 1857 and 1917 (article 107 in the current one). Otero's rule, the Otero formula, says an amparo ruling protects only the person who asked for it. The law may remain on the books, and a neighbour whom it also hurts has to file her own case.

The idea spread. Argentina's Supreme Court recognised amparo by decision in the Siri case (1957), followed the next year by Kot; a statute followed in 1966, and the 1994 constitutional reform wrote it into article 43. Colombia's 1991 constitution created the tutela (article 86), a rapid action that any person can bring before any judge. Chile has a recurso de protección for rights violations. Spain and Germany reached a similar place from Kelsen's direction: Spain's recurso de amparo and Germany's Verfassungsbeschwerde let individuals bring complaints to the constitutional court after other courts have ruled. The United States has no stand-alone equivalent; individuals raise constitutional claims inside ordinary lawsuits, in criminal defences, in habeas corpus petitions, or in civil-rights suits against officials.

Compare the effect of a ruling. Under the Otero formula, 100 people may be hurt by a law, but if only 20 sue, 20 are protected. Under Kelsen's model, once the constitutional court annuls the law, everyone is protected. Mexico's 2011 reforms opened a route to a general declaration of unconstitutionality after repeated rulings, with a demanding voting threshold, so the formula is no longer absolute. The trade-off is real: relative effects respect the legislature and the individual case, but leave people unprotected unless they can afford to sue; general effects protect everyone but give a court the power to remove a law.

Rights and their limits: tiers of scrutiny and proportionality

No right is absolute. Free speech yields to defamation law; property to taxation. So the question in a rights case is always: when is a restriction justified? Systems have developed two main structures for answering.

US courts apply tiers of scrutiny. The more important the right (or the more suspect the classification), the harder the government's job. Under rational-basis review, a law survives if it is rationally related to a legitimate aim. Under intermediate scrutiny, the aim must be important and the means substantially related to it. Under strict scrutiny, the aim must be compelling and the law narrowly tailored, which in practice means no less restrictive way would do. The famous footnote 4 of United States v. Carolene Products (1938) is usually credited with hinting that laws burdening certain rights and minorities deserve closer inspection.

Germany, and many courts influenced by it (including Canada's, after R. v. Oakes in 1986, and the European Court of Human Rights), use proportionality. The court asks four questions in order: is the aim legitimate? is the measure suitable to achieve it? is it necessary, meaning is there a less restrictive way that would work as well? and is it proportionate in the strict sense, meaning do the benefits outweigh the harm to the right? The German Federal Constitutional Court's Lüth decision (1958) is a classic starting point for the idea that rights are not only shields against the state but also values that shape the whole legal order. Proportionality has a built-in balance; tiers have a built-in sorting step. Critics of proportionality say it lets judges substitute their own weighting for the legislature's; critics of tiers say the sorting is arbitrary and hides the real weighing. Both camps are serious.

Splitting power: separation and federalism

Constitutions also fight tyranny by dividing power. Montesquieu's The Spirit of the Laws (1748) argued that liberty is safest when the powers to make, enforce and judge laws are held by different hands. James Madison, in Federalist 51 (1788), turned it into a design principle: "Ambition must be made to counteract ambition." Different bodies, with different interests, check each other.

Constitutions arrange the pieces differently. In a presidential system, an elected president and the legislature have separate mandates and can end up in opposition; the US, Mexico, Argentina, Brazil, Chile and Colombia are examples. In a parliamentary system (Germany, Spain), the government depends on the confidence of the legislature, so there is less separation but fewer deadlocks. France is semi-presidential. And power is divided vertically, too: federal states (the US, Mexico, Argentina, Brazil, Germany) give regions constitutional powers that the centre cannot simply take away; unitary states (France, Chile, Colombia) delegate power downwards. Spain sits between, with autonomous communities.

The US presidential veto shows the mechanics. The Constitution (Article I, section 7) lets the president veto a bill; Congress can override with a two-thirds vote in each chamber. Notice that the 60-vote threshold that often decides what passes in the Senate is not in the Constitution at all. It is a Senate rule, which the Senate can change.

Changing the constitution: how hard should it be?

The amendment rule is a constitution's way of describing how much it trusts the present. If it is too easy to change, the document protects nobody. If it is too hard, it freezes the arrangements of a past generation and pressure builds up elsewhere, often in judicial interpretation.

The US Constitution requires two-thirds of both houses of Congress (or a convention called by two-thirds of the states), then ratification by three-quarters of the states: 38 of 50. Only 27 amendments have passed since 1789, the first ten in 1791. Germany needs two-thirds of the Bundestag and two-thirds of the Bundesrat votes (Basic Law, Article 79(2)); and Article 79(3) puts some principles, such as human dignity and the federal, democratic and social character of the state, beyond amendment altogether. Brazil requires three-fifths of the members of each house in two rounds of voting (Article 60), and also protects a core of "stone clauses" from change. Mexico requires two-thirds of the legislators present in Congress and then approval by a majority of the state legislatures (Article 135). Spain's ordinary route needs three-fifths of each chamber, and a stricter route with a referendum exists for the most basic provisions.

Courts can also police the limits. India's Supreme Court held in Kesavananda Bharati (1973) that Parliament's power to amend does not extend to destroying the constitution's "basic structure". And when a country tries to replace its constitution altogether, it can fail: in Chile, voters rejected a proposed new constitution in a plebiscite on 4 September 2022, with 61.87% voting against.

What a graduate just sees

• Ask who has the last word on a constitutional question, and what their ruling does to the law afterwards.
• "Diffuse" (any judge) and "concentrated" (one court) are two designs for judicial review, and many countries mix them.
• An amparo-style remedy brings the constitution to the individual; the price is that relative effects leave the law standing for everyone else.
• A right is a claim plus a test for when it can be limited, and the test (tiers or proportionality) shapes outcomes.
• Separation of powers is built from veto points. Count them to predict how hard it is to change anything.
• An amendment rule shows how much a constitution trusts the present. Easy rules invite rewriting; hard rules invite creative interpretation.
• Rules of a legislature (like the 60-vote Senate threshold) are not the same as rules of the constitution.

Where it shows up next

• Administrative Law (35/09): the constitutional limits on agencies, and the courts that check them.
• Criminal Law & Procedure (35/07): the rights of the accused are mostly constitutional rights.
• International Law & Human Rights (35/10) and European Union Law (35/11): when a regional court or treaty sits above the national constitution, or a constitution makes a treaty equal to it.
• Comparative Politics (35/13): how electoral rules and the executive-legislature relationship shape stability.
• In the world: every news story about a court striking down a law, a government refusing to obey a ruling, or a country drafting a new constitution.

If you want to go deeper

• 📘 Textbook. Erwin Chemerinsky, Constitutional Law: Principles and Policies (Wolters Kluwer/Aspen; use the latest edition). A standard US law-school text.
• 📘 Textbook. Vicki C. Jackson and Mark Tushnet, Comparative Constitutional Law (Foundation Press). The standard comparative casebook, including Kelsenian courts and proportionality.
• 📘 Gentler. Akhil Reed Amar, America's Constitution: A Biography (Random House, 2005), a readable story of the US text; and Mark Tushnet, Advanced Introduction to Comparative Constitutional Law (Edward Elgar) for the wider world.
• 🌐 Free course. MIT OpenCourseWare 17.245 Constitutional Law: Structures of Power and Individual Rights (Spring 2013), covering judicial review, federalism and separation of powers through US cases. For the texts of the world's constitutions, the Constitute project publishes them in searchable form.