What is the historical and social background of Dr Jekyll and Mr Hyde, especially in relation to the theme of duality?
Historically, Stevenson wrote during the Victorian period, the imperialistic reign of Queen Victoria. The Strange Case of Dr. Jekyll and Mr. Hyde reflects the duality that was inherent in London life--indeed in all England's large cities--and revealed in the religious and philosophical ideas prevalent in England at the time, which, in some regards, produced the social dualities arising from the crushing incursion of country people to the cities (and complicated by the startling advances in technology and science, such as geological studies and Darwinism). The incursion of country people came in an effort to reclaim the sources of income they had lost because of industrialization, which made handlooms and spinning wheels obsolete.
The historical conditions of life in London that affected Stevenson's story included living conditions for the upper classes and the poor classes. The upper classes were focused on cushioning life with home-comforts and with pronounced divisions between who thought and did what, for instance, who worked and who didn’t; who was educated where and who wasn’t. Part of this insularization involved elaborate and strictly enforced modes of etiquette--a breach could cause gossip, social shunning, even irreparable scandal.
The lower classes had been forced from "cottage" industries in textiles and foods, and such, into cites where they focused on trying to survive the conditions in teeming "row houses." It was these living conditions, housing 80 percent of England’s population, that caused sanitation related diseases like cholera and typhus. How people lived was a dramatic testament to the duality that surfaced in the Victorian period
The middle and upper classes emotionally and psychologically shut the existence of the struggling teeming thousands out from their minds and focused on extreme moral certitude (i.e., following rigid moral ideas), etiquette, and religious form. The extremity of these moral and religious ideologies was the result of the newly sprung teachings of Benthamism and evangelicalism, another indication of the historical duality influencing Stevenson's writing.
Benthamism was, in brief, a hedonistic philosophy for the modern Victorian person. It taught that self-interest was a person greatest good and that happiness came through the pursuit of pleasure and the avoidance of pain. As eNotes points out, this philosophy obliterated the concept of moral conscience and the impulses of compassion, charitableness, mercy, kindness self-sacrifice, generosity, and love. Ironically, if there is no true moral conscience, there must be rigid moral rules of conduct and etiquette to impose an artificial order over the true order inspired by conscience and a philosophy of virtue.
The dualistic contrast to this was the new religious ideology of evangelicalism. This focused on a continual self-examination to search for spiritual and moral imperfections and for the moment-by-moment condition of the spiritual soul. This again was a necessary substitute for true spirituality and moral conscience. The reflection of these dualistic historical, cultural, scientific, and ideological conditions are all evident in the thematic dualism of Jekyll and Hyde.