According to director James DeMonaco, the famous road-rage incident awakening the idea for The Purge happened like this: He and his wife were driving on the freeway when a drunk driver cut them off nearly killing them. After both cars stopped, the other driver's lack of remorse enraged DeMonaco enough to engage in a fistfight and police eventually had to get involved. After the incident was over, DeMonaco's wife turned to him and commented how great it would be to have one free murder a year. He felt bad admitting this because she's normally a "sweet woman".
Director James DeMonaco was asked why the film began with footage of multiple purges taking place outdoors - yet he opted to have all purging occur inside one family's home. He responded, "we only had 19 days to shoot and $2.7 million to work with". If he ever got a chance to do a second one, he vowed it would be more like New-York 1997 (1981).
Ethan Hawke is an old friend of producer Jason Blum and director James DeMonaco, and was happy to appear in The Purge to give their low-budget movie some star-power. As usual in Blum's productions, cast and crew work for scale and receive a percentage of the profits, if any. Hawke reportedly received only $3000 upfront, but due to the movie's worldwide success and the back-end profits, he has probably received more money for this film than for any other in his career.
On the prospect of a real Purge, director James DeMonaco once said "...honestly I'd probably just go to Canada and hide."
The homicidal ladies in white dresses were based on Charles Manson's killer/followers. Director James DeMonaco claims obsession with Manson from an early age.