Watching pornography may shrink the brain and dull responses to sexual stimulation, researchers have suggested.

Scientists have found for the first time that regularly viewing sexual images could be physically harmful.

Researchers from the Max Planck Institute in Berlin found that a part of the brain which activates when people feel motivated or rewarded, shrinks and works less efficiently.

So people who watch a lot of porn are likely to need increasingly graphic material to achieve the same sexual stimulation.

The scientists also believe that people who already have a smaller ‘striatum’ – the reward part of the brain – may be more likely to use pornography.

“It's not clear, for example, whether watching porn leads to brain changes or whether people born with certain brain types watch more porn,” said Dr Simone Kühn.

“We found that the volume of the so-called striatum, a brain region that has been associated with reward processing and motivated behavior was smaller the more pornography consumption the participants reported.

“Moreover we found that another brain region, that is also part of the striatum that is active when people see sexual stimuli, shows less activation the more pornography participants consumed.”

For the study, Dr Kuhn and her colleague Jurgen Gallinat from Charite University, also in Berlin, recruited 64 healthy men between the ages of 21 and 45 years and asked them questions about their porn-watching habits.