The reward system shrinks with use
Brain scans of 64 men showed that more hours of porn per week correlated with less grey matter in the striatum — a core of the brain’s reward system — along with a weaker response to reward and weaker connectivity to the prefrontal cortex, which governs impulse control. The study measures a correlation, not a proven cause, but the researchers point out that intense stimulation of the reward system may wear on exactly those circuits.
Source: Kühn & Gallinat (2014), JAMA Psychiatry