Since when are international relations a "social science"?

"One major divide in international relations, as well as in other social sciences, is between those who believe in structure and those who believe in agency." (David A. Graham, "Why Putin Turned Against the U.S." Former ambassador Michael McFaul on what really motivated Russia to invade Ukraine," The Atlantic

Question: Since when have international relations become a "social science"? International relations are far too complex (perhaps "frivolous" is a better word) to be reduced to being a "social science" ...

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