Sources of International Law

Sources of International Law

International law does not come from a single legislature, constitution, or comprehensive international code. Unlike domestic legal systems, where statutes, regulations, constitutions, and judicial decisions are generally produced through identifiable governmental institutions, international law develops through several different processes and legal sources.

Understanding these sources is fundamental to understanding international law itself. When lawyers, judges, governments, scholars, or international tribunals ask whether a particular international rule exists, they must determine where that rule comes from and what gives it legal authority.

The most important starting point is Article 38 of the Statute of the International Court of Justice (ICJ). Article 38 identifies international conventions, international custom, and general principles of law as sources that the Court applies, while judicial decisions and the writings of highly qualified publicists are identified as subsidiary means for determining rules of law. The provision has become one of the most influential classifications of the sources of international law.

The Cornell Law School Legal Information Institute’s Wex materials on international law provide a useful introduction to this broader field.

The sources of international law are not, however, as simple as a four-item list. Each category raises difficult questions about legal authority, state consent, evidence, interpretation, hierarchy, and the relationship between different rules.

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What Does “Source of Law” Mean?

A source of law is the basis from which a legal rule derives its authority or can be identified as legally binding.

The concept is particularly important in international law because there is no single institution with general legislative authority over the entire international community.

Suppose two states disagree about whether a particular conduct is prohibited under international law.

A court cannot simply ask:

“What does the international legislature say?”

There is no universal legislature that answers every such question.

Instead, the court may need to determine whether:

• the states are parties to a treaty governing the matter;
• a rule of customary international law exists;
• a general principle of law applies;
• another recognized legal instrument establishes an obligation;
• previous judicial decisions help identify the applicable rule; or
• authoritative scholarly analysis assists in determining what the law is.

The question of sources therefore comes before the application of many international legal rules.

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Article 38 of the Statute of the International Court of Justice

Article 38 is the traditional starting point for studying the sources of international law.

It provides that the International Court of Justice applies:

1. international conventions;
2. international custom;
3. general principles of law; and
4. judicial decisions and the teachings of highly qualified publicists as subsidiary means for determining rules of law.

This classification is sometimes described as the traditional taxonomy of international legal sources.

It is important, however, not to treat Article 38 as a complete constitutional code for the international legal system.

The provision was drafted for the purposes of identifying the law applied by the International Court of Justice. Modern international law has developed considerably since the provision was adopted, and contemporary international legal scholarship recognizes additional questions concerning unilateral acts, acts of international organizations, peremptory norms, and other forms of international legal obligation.

Nevertheless, Article 38 remains indispensable.

It provides the conceptual framework from which much of the study of international legal sources begins.

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1. Treaties and International Conventions

Treaties are among the clearest and most important sources of international law.

A treaty is an international agreement governed by international law that creates legal rights and obligations for its parties.

Treaties may be:

• bilateral, involving two states;
• multilateral, involving many states;
• regional, involving states within a particular geographic area; or
• global, potentially open to participation by states throughout the international community.

Treaties cover an enormous range of subjects.

They regulate matters such as:

• diplomatic relations;
• international trade;
• human rights;
• environmental protection;
• maritime boundaries;
• extradition;
• taxation;
• international transportation;
• refugee protection;
• armed conflict;
• international criminal cooperation; and
• the establishment of international organizations.

A treaty derives its binding force from international law itself.

The principle commonly expressed in the Latin phrase pacta sunt servanda provides that agreements must be performed in good faith. This principle is fundamental to the law of treaties.

The fact that treaties are based on agreement is particularly important in an international system composed of sovereign states.

States ordinarily become parties to treaties through processes established by their constitutional systems and the treaty itself. Once bound, however, they are expected to comply with their international obligations in good faith.

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The Vienna Convention on the Law of Treaties

The law governing treaties is itself a field of international law.

The Vienna Convention on the Law of Treaties of 1969 is one of its central instruments.

It addresses questions including:

• treaty formation;
• consent to be bound;
• reservations;
• interpretation;
• application;
• amendment;
• invalidity;
• termination; and
• withdrawal.

The Convention is particularly important because it provides a structured framework for understanding how treaties operate.

Not every state is a party to every treaty, and the legal effect of particular provisions must therefore be examined carefully. Some treaty rules may also reflect customary international law and consequently have broader significance.

This illustrates an important feature of international legal sources: the same legal principle may sometimes exist simultaneously in treaty law and customary international law.

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Treaties Do Not Automatically Bind Every State

A treaty normally creates treaty obligations for its parties.

This means that a treaty between States A and B does not automatically impose every treaty obligation upon State C merely because State C is part of the international community.

This principle is closely connected to the consensual structure of international law.

However, the situation can become more complicated when a treaty provision reflects an independent rule of customary international law.

For example, a treaty may codify a customary rule that already exists independently of the treaty.

In such a situation, the rule may apply to a state not because that state is a party to the treaty, but because the rule exists as customary international law.

Therefore, when analyzing an international obligation, it is often necessary to ask two separate questions:

Is there a treaty rule?

and

Does an independent customary rule also exist?

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2. Customary International Law

The second major source is customary international law.

Customary international law consists of legal rules that arise from general state practice accepted as law.

Two elements are traditionally required.

State Practice

States must engage in sufficiently general and consistent practice.

Evidence can include:

• diplomatic correspondence;
• national legislation;
• government statements;
• military manuals;
• administrative practice;
• national court decisions;
• treaty practice;
• diplomatic conduct; and
• conduct during international disputes.

Opinio Juris

State practice alone is not enough.

States must generally act in the belief that they are following a legal obligation or exercising a legal right.

This element is known as opinio juris.

A practice may be widespread simply because it is convenient, traditional, politically desirable, or economically efficient. Such a practice does not necessarily constitute customary international law.

The legal belief associated with the practice is therefore crucial.

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Why Customary International Law Matters

Customary international law is particularly important because international law cannot realistically be contained entirely within treaties.

There are thousands of treaties, but no single treaty covers every international legal relationship.

Customary law fills many gaps and can provide rules applicable beyond the parties to a particular treaty.

It also permits international law to develop gradually through the conduct and legal positions of states.

This makes customary international law both flexible and difficult.

A treaty rule can often be identified by reading the treaty.

A customary rule requires an examination of evidence and an assessment of whether the necessary practice and opinio juris exist.

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How Much State Practice Is Necessary?

There is no simple numerical formula.

International courts generally examine whether practice is sufficiently widespread, representative, and consistent.

The importance of particular states may also depend upon the subject matter.

For example, the practice of states particularly affected by a specific legal issue may receive significant attention.

Consistency also matters.

If states repeatedly behave in contradictory ways, establishing a customary rule may become difficult.

The formation of customary international law is therefore an evidentiary and legal process rather than a simple vote.

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Can One State Object to a Customary Rule?

International law recognizes a doctrine commonly known as the persistent objector rule.

Under certain circumstances, a state that consistently and clearly objects to an emerging customary rule while that rule is developing may avoid being bound by the rule.

The doctrine is subject to important qualifications and is not a general escape mechanism from customary international law.

The timing of the objection matters. A state cannot ordinarily wait until a customary rule is firmly established and then simply announce that it never accepted the rule.

The persistent-objector doctrine is one example of the continuing importance of state consent and participation in the development of international law.

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3. General Principles of Law

Article 38 also identifies general principles of law.

These principles are particularly important where treaties and customary international law do not completely resolve a legal question.

General principles may derive from principles common to major legal systems and may assist international tribunals in resolving procedural and substantive legal questions.

Examples sometimes discussed include:

• good faith;
• responsibility for wrongful conduct;
• procedural fairness;
• res judicata;
• the principle that no one should be a judge in their own cause; and
• certain principles concerning reparation.

The precise scope of this source is debated.

It should not be understood as a license for international judges to create whatever rules they believe would produce a desirable result.

Rather, general principles are recognized through legal methodology and must be distinguished from personal moral or political preferences.

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General Principles and the Problem of Legal Gaps

International law inevitably encounters questions for which no treaty provision appears directly applicable and no clear customary rule can immediately be identified.

General principles can help prevent legal proceedings from collapsing simply because an issue is not expressly addressed by a treaty.

They contribute to the coherence of the international legal system.

At the same time, their use must be carefully controlled because an overly expansive interpretation could allow courts to transform broad concepts of justice or fairness into new binding rules without a sufficiently established legal basis.

This is one reason general principles remain an important subject of international legal scholarship.

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4. Judicial Decisions

Judicial decisions are identified by Article 38 as a subsidiary means for determining rules of law.

This wording is important.

International courts do not generally function as legislatures.

A judgment may clarify the meaning of a treaty, identify evidence of customary international law, or explain the application of a general principle. It can therefore have enormous authority in determining what international law requires.

But the judgment itself is not necessarily a universal source of law binding upon every state in every situation.

The International Court of Justice, for example, generally decides cases between the parties before it. The Statute of the Court expressly provides that its decisions have binding force between the parties and in respect of that particular case.

Nevertheless, international judicial decisions frequently influence subsequent legal reasoning.

Courts and lawyers may examine earlier judgments because they provide authoritative interpretations of international law.

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The Role of International Courts

Several international courts and tribunals contribute to the development and interpretation of international law.

These include:

• the International Court of Justice;
• the International Criminal Court;
• the International Tribunal for the Law of the Sea;
• regional human rights courts;
• international arbitral tribunals; and
• specialized international dispute-settlement bodies.

Their jurisdiction and legal authority differ.

Consequently, it is important not to assume that every international tribunal has the same powers or that every decision has the same legal effect.

Judicial decisions are particularly valuable as evidence of how established rules are interpreted and applied.

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5. Scholarly Writings

Article 38 also refers to the teachings of highly qualified publicists.

In modern terminology, this generally refers to the writings of respected international-law scholars.

Academic works may include:

• legal treatises;
• scholarly articles;
• commentaries;
• institutional publications;
• expert reports; and
• systematic analyses of state practice and international jurisprudence.

Scholarly writings do not normally create binding international law merely because an influential scholar expresses a particular opinion.

Their role is instead evidentiary and analytical.

A respected treatise may help a court determine whether a customary rule exists, explain competing interpretations of a treaty, or organize extensive state practice.

Scholarship is particularly valuable in international law because the legal system is decentralized and its rules are often scattered among treaties, state practice, judicial decisions, and other materials.

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Primary Sources and Subsidiary Means

The traditional classification can therefore be summarized as follows.

Primary sources

The principal categories identified by Article 38 include:

• treaties and international conventions;
• customary international law; and
• general principles of law.

Subsidiary means

These include:

• judicial decisions; and
• scholarly writings.

The distinction matters because a judicial decision may demonstrate what the law is without necessarily being the original source that created the rule.

Similarly, a scholarly work may convincingly demonstrate the existence of customary law without itself becoming a binding legal rule.

The distinction is therefore between evidence or means of identifying law and the legal rule itself.

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6. Acts of International Organizations

Modern international law also requires attention to the legal significance of acts adopted by international organizations.

The United Nations is the most prominent example.

International organizations can adopt:

• resolutions;
• regulations;
• decisions;
• recommendations;
• declarations; and
• other institutional instruments.

Their legal effect varies considerably.

A resolution of an international organization is not automatically binding international law.

Some decisions may be legally binding under the organization’s constituent instrument. Others may be recommendations or political declarations.

The Security Council, for example, possesses particular powers under the United Nations Charter, and certain Security Council decisions can create binding obligations for UN member states.

The legal effect must therefore be determined by examining the organization’s constitutive instrument and the specific nature of the act.

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Soft Law

The expression soft law is frequently used to describe international instruments that influence international conduct but do not have the same binding legal status as treaties or customary international law.

Examples may include:

• declarations;
• guidelines;
• principles;
• political commitments;
• codes of conduct; and
• certain resolutions of international organizations.

Soft law can nevertheless be extremely influential.

It may contribute to the development of state practice, influence treaty negotiations, guide international institutions, and provide evidence concerning emerging legal norms.

A non-binding instrument can therefore have significant legal and political consequences without itself being a binding rule of international law.

This distinction between legal obligation and normative influence is essential.

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7. Unilateral Acts of States

States can sometimes create international legal obligations through unilateral declarations.

A government may make a public declaration intended to produce legal consequences under international law.

The classic problem is determining whether the statement was genuinely intended to create a legal obligation or was merely a political statement.

International jurisprudence has recognized that unilateral declarations can, in appropriate circumstances, create legal obligations.

This demonstrates that international legal obligations are not always created through bilateral or multilateral agreements.

Nevertheless, unilateral acts must be analyzed carefully. Not every statement made by a government official constitutes a binding international commitment.

The circumstances, wording, authority of the person making the declaration, and intention to be legally bound can all matter.

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8. Peremptory Norms and Jus Cogens

International law recognizes a special category of rules known as peremptory norms of general international law, commonly called jus cogens.

A jus cogens norm is a rule recognized by the international community of states as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of the same character.

This concept introduces an important idea of hierarchy into international law.

Ordinary international legal rules can sometimes be modified by later agreements or applicable legal developments.

A peremptory norm is different.

States cannot simply agree by treaty to violate a valid jus cogens norm.

Examples commonly associated with jus cogens include prohibitions against genocide and slavery, although the precise identification and scope of individual peremptory norms can be debated.

The law of treaties recognizes special consequences for treaties that conflict with peremptory norms.

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9. Obligations Erga Omnes

Another important concept is the idea of obligations erga omnes.

An erga omnes obligation is an obligation owed toward the international community as a whole rather than merely toward one particular state.

The concept was famously discussed by the International Court of Justice in the Barcelona Traction case.

Certain fundamental obligations have been recognized as possessing this character because their protection is considered to be an interest of the international community as a whole.

The concept should not be confused with jus cogens.

The two doctrines are related but address different questions.

Jus cogens concerns the hierarchical status of a norm and whether states may derogate from it.

Erga omnes concerns toward whom an obligation is owed.

The distinction is subtle but important.

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The Relationship Between Treaties and Custom

Treaty law and customary international law frequently overlap.

A treaty may:

1. create a new rule applicable to its parties;
2. codify an existing customary rule; or
3. contribute to the development of a new customary rule.

This means that the existence of a treaty provision does not necessarily end the legal inquiry.

A court may need to determine whether the same rule exists independently as customary international law.

This is particularly important because customary law may apply more broadly than the treaty itself.

The interaction between treaty and custom is therefore one of the central features of international legal development.

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The Principle of Pacta Sunt Servanda

The Latin expression pacta sunt servanda means, broadly, that agreements must be kept.

In international law, it expresses the principle that treaties in force are binding upon their parties and must be performed in good faith.

Without such a principle, treaties would have little practical value.

International cooperation depends upon the expectation that states will honor the commitments they undertake.

At the same time, the principle does not mean that every purported international agreement is automatically valid or that treaties can never be terminated, suspended, or challenged.

The law of treaties establishes circumstances and procedures governing these questions.

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Does State Consent Create All International Law?

State consent is fundamental to international law, but the relationship is more complicated than the proposition that “nothing is law unless every state expressly agrees.”

Treaties are strongly based upon consent.

Customary international law, however, can develop through general state practice and opinio juris without each state signing an instrument expressly accepting the rule.

Some international obligations may also arise from other recognized legal sources.

Furthermore, jus cogens norms cannot simply be displaced by an agreement between states.

Consequently, modern international law combines consent-based and community-based elements.

This tension between state sovereignty and collective legal obligations is one of the deepest theoretical questions in international law.

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The Hierarchy of International Legal Rules

International law does not have a perfectly simple hierarchy comparable to the hierarchy of laws in a domestic constitutional system.

Nevertheless, some forms of priority exist.

The most important example is jus cogens.

Peremptory norms occupy a special position because states cannot derogate from them.

The United Nations Charter also contains provisions concerning the relationship between Charter obligations and other international agreements. In particular circumstances, Charter obligations have priority.

Outside these specific situations, determining which international rule prevails can require detailed analysis of treaty provisions, customary law, chronology, specificity, and other principles.

Therefore, it is misleading to imagine international law as a simple pyramid in which every type of rule has a universally predetermined rank.

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Sources of International Law and International Legal Research

Understanding the sources of international law is also essential for legal research.

When researching an international legal question, a lawyer should not simply search for a statement that “international law says X.”

Instead, the lawyer should ask:

What is the legal source?

Is the rule contained in a treaty?

If so, which states are parties?

Is the rule customary international law?

If so, what evidence establishes state practice and opinio juris?

Is there an applicable general principle?

Has an international court interpreted the rule?

Does an international organization’s instrument have binding legal effect?

Is the relevant material merely soft law or political guidance?

Does a domestic court interpret the international rule differently for purposes of domestic enforcement?

These questions distinguish serious international legal analysis from general commentary about international affairs.

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A Practical Method for Identifying an International Legal Rule

When approaching a new international law problem, a useful sequence is:

Step One: Identify the Relevant Actors

Determine whether the dispute involves states, international organizations, individuals, corporations, or other actors.

Step Two: Identify Applicable Treaties

Determine which treaties govern the relationship and whether the relevant states or other parties are bound by them.

Step Three: Examine Customary International Law

If treaty law does not provide the complete answer, examine state practice and opinio juris.

Step Four: Consider General Principles

Determine whether a recognized general principle helps resolve the legal question.

Step Five: Examine Judicial Decisions

Look at relevant judgments and advisory opinions for interpretations of the applicable rules.

Step Six: Examine Scholarly Authority

Consult authoritative scholarship to understand competing interpretations and the evidence supporting particular propositions.

Step Seven: Check for Special Rules

Determine whether international organizational law, unilateral acts, jus cogens, erga omnes obligations, or another specialized doctrine affects the analysis.

This methodology helps prevent one of the most common errors in international law: treating a political statement, academic opinion, treaty provision, and customary rule as though they possessed identical legal status.

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Why the Sources of International Law Matter

The question of sources is not merely academic.

Suppose a state is accused of violating an international obligation.

Before determining whether the state has violated international law, it is necessary to establish that a legal obligation exists.

The source determines:

• whether the rule is legally binding;
• which actors are bound;
• the geographical or substantive scope of the obligation;
• how the rule can be interpreted;
• whether exceptions apply;
• whether another rule has priority; and
• what legal consequences may follow from a violation.

The identification of the source therefore determines much of the subsequent legal analysis.

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Key Takeaways

The sources of international law are the recognized foundations from which international legal rules derive their authority or through which those rules can be identified.

The traditional framework is found in Article 38 of the Statute of the International Court of Justice.

The principal categories are:

• Treaties and international conventions;
• Customary international law;
• General principles of law;
• Judicial decisions, as subsidiary means for determining rules of law; and
• Scholarly writings, also as subsidiary means.

Modern international law also requires attention to:

• acts and decisions of international organizations;
• soft law;
• unilateral declarations of states;
• peremptory norms (jus cogens);
• obligations erga omnes; and
• the interaction between treaties and customary international law.

The most important lesson is that not every international instrument, statement, or practice has the same legal status.

A treaty, a customary rule, a judicial decision, a United Nations resolution, and a scholarly opinion may all be important in international legal analysis, but they do not necessarily derive their authority in the same way or produce the same legal consequences.

Understanding that distinction is the foundation of competent international legal research.

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Frequently Asked Questions

What are the main sources of international law?

The traditional sources are treaties, customary international law, and general principles of law. Judicial decisions and scholarly writings are recognized as subsidiary means for determining rules of law under Article 38 of the ICJ Statute.

What is Article 38 of the ICJ Statute?

Article 38 identifies the legal materials applied by the International Court of Justice when deciding disputes. It is widely regarded as the foundational classification of the sources of international law.

Are treaties a source of international law?

Yes. Treaties create binding international legal obligations for their parties, subject to the rules governing treaty formation, validity, interpretation, application, and termination.

What is customary international law?

Customary international law consists of legal rules arising from general state practice accepted as law. Both state practice and opinio juris are important in establishing a customary rule.

What is opinio juris?

Opinio juris is the belief that a particular practice is followed because international law requires, permits, or recognizes it. It distinguishes customary legal practice from conduct followed merely because it is customary, convenient, or politically desirable.

Are international court decisions sources of international law?

Judicial decisions are generally treated as subsidiary means for determining and interpreting international legal rules rather than as legislation creating universally binding rules. Their authority depends upon the court, its jurisdiction, the applicable legal framework, and the nature of the decision.

Are United Nations resolutions international law?

Not automatically. Some decisions of UN organs can have binding legal effect under the UN Charter, while many resolutions, declarations, and recommendations are not themselves binding rules of international law. Their legal significance must therefore be determined individually.

What is soft law?

Soft law refers to international instruments or standards that may influence international conduct and legal development without necessarily creating binding legal obligations. Declarations, guidelines, principles, and certain resolutions can fall into this category.

What is jus cogens?

Jus cogens refers to peremptory norms of general international law from which no derogation is permitted. States cannot simply agree by treaty to override a valid peremptory norm.

What does erga omnes mean?

An obligation erga omnes is an obligation owed to the international community as a whole. The concept concerns the beneficiaries of an obligation and should not be confused with the hierarchical concept of jus cogens.

Does international law depend entirely on state consent?

No. State consent is fundamental, particularly in treaty law, but international law also develops through customary international law, general principles, and other recognized legal mechanisms. Certain fundamental norms cannot simply be displaced by state agreement.

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Conclusion

The sources of international law reveal the distinctive character of the international legal system.

There is no single global legislature producing a comprehensive international code. Instead, international law emerges from treaties, customary practice, general principles, and a network of judicial and institutional processes through which legal rules are identified and interpreted.

Treaties express formal agreement. Custom reflects legally significant patterns of state conduct. General principles contribute to the coherence of the legal system. Judicial decisions and scholarship assist in identifying and interpreting the law. International organizations, unilateral acts, soft-law instruments, and peremptory norms add further layers to an already sophisticated system.

For anyone studying international law, the most important habit is therefore to ask not merely “What is the rule?”, but “What is the source of the rule, why is that source legally authoritative, and whom does the rule bind?”

That question transforms international law from a collection of diplomatic practices and international documents into what it properly is: a complex and evolving legal system with identifiable sources, legal methodologies, institutions, and consequences.