"Instincts of the Herd in Peace and War" is the title of an influential book by English surgeon Wilfred Trotter. Based on the ideas of Gustave Le Bon, it was very influential in the development of group dynamics and crowd psychology. It was also cited by Q. D. Leavis in her book "Fiction And The Reading Public."

Wilfred Trotter was an English surgeon, a pioneer in neurosurgery. He was also known for his studies on social psychology, most notably for his concept of the herd instinct, which he first outlined in two published papers in 1908, and later in his famous popular work Instincts of the Herd in Peace and War, an early classic of crowd psychology. Trotter argued that gregariousness was an instinct, and studied beehives, flocks of sheep and wolf packs.