While still very much a part of the larger realist school of thinking, structural realism differs significantly from classical realism in one crucial way.
Structural realists, like John Mearsheimer, strongly argue that the nature of the international system - the global political structure - is what drives states to behave as they do and, like classical realists, base their theory on a set of key assumption. In contrast to classical realists, whose theory is based on the presumption that people are selfish and egoistic.