The following is from an article by Professor John Sano which was published by the Intelligencer (Vol. 21, No.3), the journal of the Association of Former Intelligence Officers.
Although often described as the world’s second oldest profession, spying — and specifically human intelligence (HUMINT) — continues to evolve. While the basic tenets of human espionage remain constant, there are a variety of factors, which over time have impacted both the tenets and the parameters of spying. It is not just the “how” of HUMINT, but also the motivations and the methodologies employed. Demographics, technology and cultural expectations all play a role in the shaping of a clandestine service officer.
Read more: 
Sano – The Changing Shape of HUMINT