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The brains of men who watch lots of pornography tend to be smaller in certain key areas and have fewer connections than those of other men, a new study has found.
The differences appeared in a brain region called the striatum, which is associated with reward and motivation.
"Since the striatum is part of the brain network that is known to respond to sexual cues, one can assume that this reflects a blunting of the reaction to erotic stimuli," study co-author Simone Kühn, a psychologist at the Max Planck Institute for Human Development in Germany, wrote in an email to Live Science.
But although the findings are suggestive, it's not clear that watching porn actually causes men's brains to shrink. Perhaps men who watch a lot of porn are different in some underlying way from those who don't, and that could explain the smaller size, experts said.
Sexual disorder?
With the rise of the Internet, people can now anonymously access pornography at all hours of the day with just the click of a mouse. [Hot Stuff! 10 Unusual Sexual Fixations]
"This has fostered the pervasiveness of pornography use and raises the question of whether it has any effects on brain structure and function," Kühn said.
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Some psychologists argue that porn-watching is harmless, while others claim porn is bad for mental health and can spiral into sex addiction. (The most recent version of the Diagnostic and Statistical Manual of Mental Disorders, the DSM-5, doesn't include sex addiction as a separate disorder.) [Top 10 Controversial Psychiatric Disorders]
A 2013 study in the journal Socioaffective Neuroscience and Psychology, found that people who reported problems controlling their urge to view sexual images did not display brain patterns that are characteristic of people with addictions.
But even behavior that doesn’t rise to the level of an addiction or disorder can still be problematic, said William Struthers, a neuroscientist at Wheaton College in Illinois, who was not involved in the new study.
"We now need to acknowledge that this fits somewhere on a pathological spectrum," with some people affected relatively little by porn, while others may have their relationships and life compromised, he said.
Brain scans
To assess the effect of pervasive porn watching on the brain, Kühn and her colleague, Jürgen Gallinat, a researcher at the Clinic for Psychiatry and Psychotherapy in Germany, placed 64 healthy men in a magnetic resonance imaging (MRI) scanner, and measured the men's responses to sexual and neutral images. On average, outside of the lab, the men watched about four hours of pornography a week, with some of the respondents watching nearly 20 hours a week and others watching none at all.
The avid porn watchers showed lower activation of reward circuitry in the brain (as suggested by lower blood flow to certain brain regions) in response to the sexual images.
When their brains were at rest, they also had smaller grey matter volume in areas of the brain, such as the striatum, associated with reward processing and motivation, compared with men who watched less porn.
Past research, detailed in 2012 in the Journal of Sexual Medicine, suggested watching porn can quiet a brain region that processes visual stimuli called the primary visual cortex.
Questions of causality
It's not surprising, or necessarily worrisome, that frequent porn exposure reduces the brain's reward response to sexual images, said Nicole Prause, a neuroscientist at the University of California at Los Angeles, who co-authored the 2013 study on sex addiction. In fact, that's how the brain becomes habituated to anything, she added.
"They're basically demonstrating a principle of basic learning," Prause told Live Science.
In her lab, Prause and her colleagues monitored brain activity as people smelled chocolate. "When they first start smelling it, their brain is like 'Whoa that's wonderful!'" Prause said. "And then the second time it's like, 'Okay we're kind of done with this now.'"
Furthermore, the imaging the researchers used in the new study to assess brain volume leaves a few open questions, she said.
Both alcoholism and depression shrink the same brain areas that were found to be smaller in porn watchers in the current study. Depressed people are less likely to be in a relationship, and may therefore have fewer sexual opportunities in real life and more time on their hands to look at sexual content. In that instance, it may be the underlying depression, not the porn itself, altering the brain, Prause said.
Still, the researchers did use surveys to try to control for depression and alcoholism, and the brain activity findings are convincing because there's a plausible mechanism — namely, that constant exposure to erotica could make men habituated to sexual rewards, Struthers said.
"The fact that viewing sexually explicit material could potentially become something that is habitual and has negative side effects should not come as a shock to anyone," Struthers said.
The findings were published online Wednesday (May 28) in the journal JAMA Psychiatry.
Follow Tia Ghose on Twitter and Google+. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.
Tia is the editor-in-chief (premium) and was formerly managing editor and senior writer for Live Science. Her work has appeared in Scientific American, Wired.com, Science News and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.