MDMA's increase in serotonin activates neural networks associated with feeling more socially connected and reduces the perception of fear in faces.

“but by also causing big increases in serotonin it activates neural networks that are associated with feeling more socially connected in fact we'll talk about data in a little bit where people have had their brains imaged while under the influence of MDMA and it's very clear that people who have taken MDMA look at faces that ordinarily they would rate as fearful and rate them as less fearful they see faces that are smiling and they rate those smiling happy faces as more positive than they would off the drug the big increases in serotonin create what we call a pro-social effect”