An excellent medical student, he decided to specialise in surgery and was appointed Surgical Registrar at University College Hospital in 1901 and Assistant Surgeon in 1906.

He was also a member of the Council of the Royal Society that conferred their Honorary Membership on Professor Freud, whom he had met earlier at psychoanalytic gatherings, and whom he attended after his move to England.

Trotter's popular book, The Instincts of the Herd in Peace and War is an analysis of group psychology and the ability of large numbers of people to be swayed through an innate tendency.

In it he popularised in English the concept, first developed by French sociologist, Gustave Le Bon, of an instinct over-riding the will of the individual in favour of the group.

Trotter's writings about herd mentality, which began as early as 1905 and were published as a paper in two parts in 1908 and 1909 are considered by some to represent a breakthrough in the understanding of group behaviour, long before its study became important in a variety of fields, from workplace relations to marketing.