How does the setting of The Great Gatsby influence the behavior of the characters?

Critics agree that no American novel captures the Roaring 20s as well as The Great Gatsby. The setting is integral to Fitzgerald's theme in regard to the corruption of the American Dream, and the characters are products of their times. In his essay, "Echoes of the Jazz Age," Fitzgerald wrote that during the 1920s, the country went on a "spree." This can be seen vividly in the parties Gatsby threw in the summer of 1922. His guests, most of whom show up without invitation, descend upon his estate, drink, dance, argue, fight, wreck their cars, put on personal performances, and, as Nick observed, "conducted themselves according to the rules of behavior associated with amusement parks." The excess of the times determined this conduct.

The Roaring 20s was also a time when the traditional social structure collided with new money. Great fortunes were made seemingly overnight. Gatsby illustrates this social phenomenon, building his fortune through bootlegging and other criminal activities. Tom tells Daisy that Gatsby's money came from his "drug stores" which sold alcohol, outlawed during Prohibition in the 1920s. 

Those who enjoyed inherited wealth looked with disdain upon those who had wealth without family background. Tom Buchanan feels smug and superior to Gatsby who drives a "circus wagon" and doesn't understand how Gatsby came to know Daisy, "unless [he] brought the groceries to the back door." 

Not everyone in the Roaring 20s, however, enjoyed wealth. Another element of the time can be seen in the Valley of Ashes, a place of grinding poverty and ugliness. These surroundings and her life there motivate Myrtle to engage in her affair with Tom as a way out of the life she has lived with George Wilson for eleven years, her home an apartment over a dingy garage. When she first meets Tom on the train into New York, she goes with him at once, thinking "You can't live forever, you can't live forever."

Whether Fitzgerald's characters came from old money or lived with new money or no money, all were influenced by their time and place in American history.