Private eye Martin Hewitt, created by British author Arthur Morrison, is perhaps the first example of the modern style of fictional private detective . By the late 1920s, Al Capone and the Mob were inspiring not only fear, but piquing genuine mainstream curiosity about the American underworld. Popular pulp fiction magazines like Black Mask capitalized on this, as authors such as Carrol John Daly published violent stories that focused on the mayhem and injustice surrounding the criminals, not the circumstances behind the crime. Very often, no actual mystery even existed: the books simply revolved around justice being served to those who deserved harsh treatment, which was described in explicit detail."[3] In the 1930s, the private eye genre was adopted wholeheartedly by American writers. The tough, stylish detective fiction of Dashiell Hammett, Jonathan Latimer, Erle Stanley Gardner and others explored the "mean streets" and corrupt underbelly of the United States. Their style of crime fiction came to known as "hardboiled," which encompasses stories with similar attitudes concentrating not on detectives but gangsters, crooks, and other committers or victims of crimes. "Told in stark and sometimes elegant language through the unemotional eyes of new hero-detectives, these stories were an American phenomenon."[3]

In the late 1930s, Raymond Chandler updated the form with his private detective Philip Marlowe, who brought a more intimate voice to the detective than Hammett's distant, third-person viewpoint. His cadenced dialogue and cryptic narrations were musical, evoking the dark alleys and tough thugs, rich women and powerful men about whom he wrote. Several feature and television movies have been made about the Philip Marlowe character. James Hadley Chase wrote a few novels with private eyes as the main hero, including "Blonde's Requiem" (1945), "Lay Her Among the Lilies" (1950), and "Figure It Out for Yourself" (1950). Heroes of these novels are typical private eyes which are very similar to Philip Marlowe.

Ross Macdonald, pseudonym of Kenneth Millar, updated the form again with his detective Lew Archer, while still writing in what is considered the PI's Golden Age of Detective Fiction, begun by Hammett. Archer, like Hammett's fictional heroes, was a camera eye, with hardly any known past. "Turn Archer sideways, and he disappears," one reviewer wrote. Two of Macdonald's strengths were his use of psychology and his beautiful prose, which was full of imagery. Like other 'hardboiled' writers, Macdonald aimed to give an impression of realism in his work through violence, sex and confrontation; this is illusory, however, and any real private eye undergoing a typical fictional investigation would soon be dead or incapacitated. The movie Harper starring Paul Newman was based on the Lew Archer character.

Michael Collins, pseudonym of Dennis Lynds, is generally considered the author who led the form into the Modern Age. His PI, Dan Fortune, was consistently involved in the same sort of David-and-Goliath stories that Hammett, Chandler, and Macdonald wrote, but he took a sociological bent, exploring the meaning of his characters' places in society and the impact society had on people. Full of commentary and clipped prose, his books were more intimate than his predecessors, dramatizing that crime can happen in one's own living room.

The PI novel was a male-dominated field in which female authors seldom found publication until Marcia Muller, Sara Paretsky, and Sue Grafton were finally published in the late 1970s and early 1980s. Each author's detective was brainy, physical, and could hold her own. Their acceptance, then success, caused publishers to seek out other female authors.

The PI novel today is rich in variety. The strongest characteristic that binds them is that the detective now has a past and a life, while solving cases.

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