The Australian Government appears to be serious about introducing an R18+ rating for computer games. They have asked for comments here. (Courtesy Tristan.) Unfortunately, you are limited to 1000 characters so I had to cut my 2300-character response down to this:

Is there any scientific study that shows that interactivity increases the impact of mature content? In particular, “actively controlling outcomes by making choices to take or not take action” is not specific to video games — by this logic, choose-your-own-adventure stories should be rated more highly.

Is there evidence that “incentives and rewards” increase impact? Why is drug use banned if it is used as a reward? Nearly everything in a game may be considered a “reward”. Eg, if a game allows the player to pick up drugs, and player finds drugs after beating an opponent, could they be considered a “reward” and trigger an RC rating? The terms “incentive” and “reward” are not defined and are instead deferred to the dictionary. These terms have specific meaning in gaming and are unlikely to be properly represented by a general definition.

With regards to sex in R18+ video games, what does “simulation, yes – the real thing, no” mean? How is sex in a video game ever not a “simulation”?

I originally linked to, and quoted, this passage from the recent United States Supreme Court ruling (PDF) against video game censorship in California:

California claims that video games present special problems because they are “interactive,” in that the player participates in the violent action on screen and determines its outcome. The latter feature is nothing new: Since at least the publication of The Adventures of You: Sugarcane Island in 1969, young readers of choose-your-own-adventure stories have been able to make decisions that determine the plot by following instructions about which page to turn to… As Judge Posner has observed, all literature is interactive. “[T]he better it is, the more interactive. Literature when it is successful draws the reader into the story, makes him identify with the characters, invites him to judge them and quarrel with them, to experience their joys and sufferings as the reader’s own.”