A Changed Brain Is Not a Damaged Brain

In 2014 researchers at the Max Planck Institute for Human Development in Berlin scanned the brains of 64 healthy men and asked each how many hours of pornography he watched in a typical week. The more hours, the smaller the right caudate, a region of the striatum that helps the brain learn which actions pay off. Heavier viewers also showed weaker activity in the left putamen when shown sexual images, and weaker links between the caudate and the prefrontal cortex. Headlines announced that pornography shrinks the brain.

The authors were more careful than the headlines. A scan taken at one moment cannot say which came first. Men whose reward circuits respond weakly may need more stimulation to feel the same pull, so a smaller caudate could be a reason for heavy use and not a result of it. The study found an association and nothing more.

The popular story fills the gap with dopamine. Neurons that release it fire most when a reward is better or newer than expected, a signal that Wolfram Schultz described in the 1990s as a prediction error. On this account an endless supply of novelty keeps the signal high and teaches the brain to want more. The account is plausible and has rarely been tested directly in people who watch pornography, so it remains a story and not a measurement.

There is a deeper problem with reading damage into change. The brain changes with everything it does repeatedly. London taxi drivers who spent years learning the city's streets were found in 2000 to have a larger posterior hippocampus than other adults, and the difference grew with their years on the job. Nobody concluded that knowing the streets had injured them. A brain shaped by a habit is simply a brain that has a habit.

A brain shaped by a habit is simply a brain that has a habit.

Clinical work seems to point the other way. In a study of men seeking treatment for problematic pornography use, the ventral striatum responded more strongly to erotic cues than to money, a pattern that resembles what addiction research reports for drug cues. The World Health Organization now lists compulsive sexual behaviour disorder in its classification of diseases, though as a failure of impulse control and not as an addiction.

Yet these are people who already suffer, and cue reactivity is again a snapshot. Psychologists led by Joshua Grubbs have also reported, across several samples, that moral disapproval of pornography predicts the feeling of being addicted to it as strongly as, or more strongly than, the hours actually spent watching. Two men who watch equally often can therefore feel very differently about it, and the one who feels shame is the one who reports losing control.

A scan cannot settle this because it measures activity and not meaning. The same striatal signal accompanies a first kiss, a winning bet, a favourite song and a craving. Whether a pattern signals harm depends on what the person was trying to do and what the habit cost him, and those are judgements about a life and not features of an image.

Better scanners will come, but the difficulty is not technical. To call a brain damaged one must first decide what a healthy one is for. Somewhere between a habit and a harm lies a line that every brain crosses differently, and nobody has yet built an instrument that can see it.