Countries that play together stay together

Researchers have shown that reciprocity – a cornerstone of how we interact with other people – has also been a significant factor in relations between nations.

A team of researchers at MIT, Harvard, UC Merced, and the Masdar Institute in Abu Dhabi used a new method known as convergent cross mapping (CCM) to analyse interactions between countries from 1995 to 2015. The method predicts a country’s actions based on the actions of a second and evaluates how well the prediction matches actual events. By calculating a “CCM reciprocity” score for country pairs, the researchers determined whether and how strongly the two countries caused one another’s behaviour.¹

The analysis identified 47 country pairs with CCM reciprocity and 15 triads forming reciprocal cycles. By connecting these networks together, the team assembled a global network of reciprocal interactions.

“Although our analysis shows that mutual influence is typically associated with bilateral cooperation, there are notable exceptions,” says Iyad Rahwan, who led the study, citing reciprocal relationships discovered between Israel and Lebanon, Jordan, and Syria. “These relationships are qualitatively very different: Israel has normalized relationships with Jordan, but remains officially at war with both Lebanon and Syria. This shows that reciprocal relationships can also be hostile, but this is still relatively rare in the international system.”

The analysis also revealed qualitatively different interactions between reciprocating countries. “We find that in international relations, countries with reciprocating relationships are ‘faster to anger, faster to forgive’ than non-reciprocating country pairs,” explains Morgan Frank, the study’s first author. This tit-for-tat strategy stabilises cooperation in the international arena.

This research will provide a foundation for future work to explore questions such as the role of indirect reciprocity, the effects of power asymmetries, and the existence of a hierarchy of influence. 

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