A hundred years ago they said that masturbating would make you go blind. We've progressed. Today, we're told that watching moderate amounts of pornography will shrink your brain. The claim arrives courtesy of a brain imaging paper published last month in JAMA Psychiatry, a respected medical journal.
Among the global hyperbolic headlines that followed, my favourite was from a German site: "Pea brain: watching porn online will wear out your brain and make it shrivel." Others included "Viewing porn shrinks the brain" (from the reliably untrustworthy Daily Mail) and Watching Porn Linked To Less Gray Matter In The Brain (from Huffington Post).
The study that triggered all this concern was published by a German pair: Simone Kühn, a psychologist, and Jürgen Gallinat, a psychiatrist. They scanned the brains of 64 healthy men (average age 29) in three ways. Note the word healthy. In fact, all the men who participated were free from any psychiatric or neurological disorders. So if they had shrunken brains (we'll come onto that later), it wasn't causing them any major problems.
The first scan was a simple structural brain scan. The second looked at patterns of brain activation when the men viewed sexual or neutral images. The third scan looked at brain activity while the men relaxed in the scanner for five minutes (a so-called resting-state scan). The men also answered questions about how much porn they watch. They averaged four hours per week, and none of them met the criteria for Internet sex addiction according to the "Internet Sex Screening Test".
Here's what's caused all the fuss. The researchers found that hours spent watching porn was negatively correlated with the amount of grey matter in a subcortical region near the front of the brain - the right striatum - that's known to be involved in the processing of reward (as well as lots of other things). In other words, men who said they spent more time watching porn tended to have a smaller amount of grey matter in this part of their brain. Also, the more avid porn viewers showed less activation in their left striatum when they looked at racy images, and they appeared to have reduced connectivity between their right striatum and their left dorsolateral prefrontal cortex.
So, does watching porn shrink your brain? The researchers think it probably does. "One may be tempted," they wrote "to assume that the frequent brain activation caused by pornography exposure might lead to wearing and down regulation of the underlying brain structure, as well as function ...".
One may be tempted, but one should really know better. The most glaringly obvious problem with this study is of course its cross-sectional methodology. It's just as likely that men with less grey matter in their striatum are more attracted to porn, as opposed to porn causing that brain profile. The researchers know this. "It's not clear ... whether watching porn leads to brain changes or whether people born with certain brain types watch more porn,” Kühn told The Daily Telegraph (and yet that paper still ran the headline: "Watching pornography damages men's brains").
A further problem with correlational studies is not just that the causal direction can run either way, but that an unknown or uncontrolled third factor (and others) could be causally involved. In the case of this study, the elephant in the room is personality. Unsurprisingly, personality is linked with media use (including porn consumption) and with brain characteristics. Asking men how much porn they watch is a crude indicator of their extraversion, (lower) conscientiousness and desire for sensation seeking. For instance, men who watch porn in work hours tend to be less conscientious and more impulsive. Last year, a study reported: "Neuroticism, agreeableness, conscientiousness, and obsessional checking all significantly correlated with a latent measure of compulsive behavior upon which use of Internet pornography use also loaded."
Amazingly, although Kühn and Gallinat checked their participants were free from depression and addiction, they otherwise failed to measure their participants' personality traits. Had they done so, they would likely have found strong associations between personality and brain structure and function. Past research has already shown that high sensation seekers have reduced sensitivity to high arousal pictures (including nudity and gore). Other research has documented differences in resting-state brain activity according to personality. Still further research has shown how extraverts, and those more open to experience, are more persuaded by advertising that uses sexual imagery.
By failing to measure or control for personality, the results of this study are virtually meaningless. The men's self-reported time spent watching porn is little more than a rough proxy for their personality profile, including their willingness to diverge details about their private habits. And we already know that key personality traits such as extraversion and sensation seeking are linked with distinct patterns of brain structure and response. By failing to follow up participants over time, the research also provides no evidence that watching porn has any effects whatsoever. Moreover, by also neglecting to measure any other media consumption, then even if before/after evidence were available, we wouldn't know if it were due to porn consumption or to other media activities correlated with that porn use, such as watching violent movies and online gambling (to be fair, the findings did still hold after the researchers controlled for overall levels of internet use).
The researchers have witnessed newspapers spread headlines of brain shrinkage and brain harm, and yet they know that they specifically recruited psychologically and neurologically healthy men. In fact, therein lies the only really meaningful insight from this study. Look at it this way. In a survey of 64 men who answered recruitment adverts for a brain scanning study, it was found that they viewed an average of four hours porn a week. They do so with no apparent ill consequence - screening confirmed no psychiatric, medical or neurological problems. Of course there is a debate to be had about the merits and harms of porn for individuals and society. This study does not make a helpful contribution. Suggested new headline: "Watching moderate amounts of porn won't hurt your brain".
