Realists and liberals disagree about a great deal, but both tend to treat states' interests as given: states want security, wealth or power, and the question is how they pursue them. In the 1990s a group of scholars asked a prior question. Where do those interests come from in the first place?

Anarchy has no fixed logic

The best-known answer came from the political scientist Alexander Wendt, who argued in a famous 1992 article that anarchy has no logic of its own; its effects depend on how states come to understand it. Constructivism holds that the interests and identities of states are shaped by shared ideas, norms and practices, not fixed by material power alone. Wendt's best-known illustration contrasts British and North Korean nuclear weapons. Five hundred British warheads worry the United States less than a handful of North Korean ones, because of who the two countries understand each other to be. The missiles are material, but the meaning of weapons depends on the relationship between the states that hold and face them.

For constructivists, this means anarchy can take different forms. In what Wendt called a Hobbesian culture, states treat each other as enemies who might strike at any time. In a Lockean culture they are rivals who compete but accept each other's right to exist. In a Kantian culture, states are friends who no longer expect war between them at all. Which culture holds depends on how states have learned to see one another, and that can change.

Norms and how they spread

A norm is a shared standard of appropriate behavior for actors with a given identity. In 1998 Martha Finnemore and Kathryn Sikkink described a norm life cycle. In the norm life cycle, norm entrepreneurs persuade a critical mass of states, a cascade follows, and finally the norm is internalized and taken for granted. The campaign against the Atlantic slave trade fits the pattern: a small group of activists in the 1780s, then Britain's abolition of the trade in 1807, then treaties and naval patrols, until by the twentieth century slavery was condemned almost everywhere in principle.

Another example is what the scholar Nina Tannenwald calls the nuclear taboo. The nuclear taboo is the strong inhibition against using nuclear weapons that has held since 1945. Constructivists argue that it is not simply a product of deterrence, since nuclear powers have refrained even when fighting opponents who had no nuclear weapons of their own.

Constructivists also distinguish two ways of deciding. Under the logic of consequences, actors calculate costs and benefits. Under the logic of appropriateness, actors ask what someone in their role is expected to do, the way a doctor treats a patient without first calculating the payoff.

The English School and critical theories

A British tradition anticipated part of this argument. In The Anarchical Society (1977), Hedley Bull argued that states form an international society, sharing rules and institutions such as diplomacy and international law, even without a ruler above them. This so-called English School sits between realism and liberalism.

Other approaches question the whole frame. Marxist scholars see world politics through capitalism and class. Feminist scholars ask where women are in international politics and how ideas of masculinity shape war, as FEM explores. Postcolonial scholars point out that the international society of the nineteenth century treated most of the world as colonies rather than members.

Criticisms

Critics say constructivism is better at describing change after it happens than at predicting it. A frequent criticism is that ideas and identities are hard to measure, so constructivist claims are hard to test. Realists add that norms often follow power: the norms that spread are usually the ones strong states favor. And norms are not always good. Racial hierarchy and the right of conquest were once widely accepted norms too. Constructivists accept that last point and even insist on it, since it carries a hopeful corollary: what people built, people can take apart.

Worked example: France and Germany

Between 1870 and 1945 France and Germany fought three major wars. Generations of French schoolchildren learned of Germany as the hereditary enemy, and German generals planned for war in the west. Yet today war between them is as unthinkable as war between two neighboring American states.

Realists explain the change by the Cold War: both countries faced the Soviet Union and sheltered under American power. Liberals point to the European Coal and Steel Community of 1951 and the later European Union, which bound their economies together, and to the fact that both became stable democracies. Constructivists accept these causes but add something more. The Élysée Treaty of 1963, signed by Charles de Gaulle and Konrad Adenauer, created a youth exchange office that has since sent millions of young French and Germans to each other's countries. Joint history textbooks, twinned towns and decades of regular summits changed how each people sees the other. In Wendt's terms, the relationship moved from a Hobbesian culture to a Kantian one.

The strongest evidence comes after 1991. The Soviet threat that realists stressed disappeared, yet neither country began to prepare for war with the other, even though both remain heavily armed and one of them has nuclear weapons. A friendship that outlasts the threat that supposedly created it is hard to explain by power alone.