Violent movies can boost testosterone by 30%

The debate over how far films and video games can affect behaviour has taken another turn with the publication of research showing how films can dramatically alter hormone levels.

In the research published in the US journal Hormones and Behavior, men got pumped up with testosterone by the violence in The Godfather II while levels of the female hormone progesterone rose in both sexes when watching a weepie like The Bridges of Madison County.

The sentimental story starring Clint Eastwood and Meryl Streep caused progesterone levels to rise by more than 10% in both men and women, the team at the University of  Michigan found. Women's testosterone levels were unchanged during and after the film while men's testosterone levels fell.

By contrast, The Godfather II prompted testosterone levels in men who already had high levels of the hormone to surge by as much as a further 30%. But both men and women were unaffected hormonally by a documentary about the Amazon rain forest.

'When you're watching movies, your hormones are responding, not just your mind,' said psychology professor Oliver Schultheiss who led the study. 'If you want to learn about someone's personality,' he suggested. 'Look at their video collection.'

Warren Leblanc, the 17 year old who last week pleaded guilty to the murder of a 14-year-old with a claw hammer was reportedly obsessed with a game called Manhunt, developed by Edinburgh-based Rockstar North, in which the player as a death row inmate set lose to fight for his life scores points for violent stabbings and killings. The game, which is rated 18 in the UK, was banned six months ago in New Zealand by censorship officials.

In the US the video entertainment industry is also back in the dock following claims by teenage boys in Tennessee that they were acting out the Grand Theft Auto game when they shot at passing vehicles. Grand Theft Auto is one of the best-selling games of all time. In the US, it carries a mature rating, for aged 17 and over.

• If hormones can be affected by something as tame as a swooning Meryl Streep, what effect does something like Manhunt have? Even their website is terrifying. What do you think?