We've all done it. Walked out of a cinema after watching Rambo and started knifing the air/a nearby bollard/your mates. Or, after a hefty stint on Grand Theft Auto, yearned for a world where it was kind of OK to steal someone's car, find a small arsenal of weapons and lay waste to an entire city. But a new study in America has found that playing violent video games not only increases the likelihood of an immediate rise in violent behaviour, it also has an effect in the months to come.

An investigation published this month in the US medical journal Pediatrics set out to see if normal children in both America and Japan grew more violent after playing violent games. They already knew this was the case in the short term, but very few studies had followed up their analysis over months to see the long-term effects. Guess what? Six months down the line, those same kids were still acting more violently than before the test.

Now, one important thing to note is the study's definition of ‘violence'. It's pretty broad. While you or I may think of violent games as those that depict guns, blood, guts and death, the guys at Pediatrics expand the definition to mean any behaviour where an individual inflicts harm on another who presumably doesn't want to be harmed. All those years you were innocently offing Koopa Troopas in Mario by jumping on their heads? Well those guys didn't want to be harmed. You were being violent.

Is this telling us something we already kind of knew? Perhaps. But at least now we've got it in writing from proper science bods. So next time you see a kid playing a game where they are forced to inflict harm on seemingly innocent enemies, it's perhaps best to stay out of their way for a couple of months - or at least make sure they can't get into the knife drawer.