Few things are truly universal. But while people across the world speak different languages, eat different foods and even feel different emotions, millions across the world watch porn. Despite being so widely consumed, porn is maligned as the source of society’s ills. It’s even been labelled a public health hazard by politicians in Utah.

Porn has transformed over the past few decades, due to the availability of the internet and faster web connections. It is also becoming more immersive than ever before. Take virtual reality. Earlier this year, researchers from Newcastle University in the UK pointed out that VR changes the experience of porn from detached observer to protagonist. They warned that this has the potential to blur the line between reality and fantasy, perhaps damaging relationships and encouraging harmful behaviour.

But what does the evidence actually say about how porn may or may not be affecting people? Can research provide any answers? The truth is that it is a difficult question for scientists to study. The nature of porn dictates that researchers must either rely on people self-reporting their porn habits, or show it to them in laboratory settings that are unnatural. (And no doubt, slightly awkward, too.)

That said, there is a growing body of literature that can provide hints. BBC Future reviewed what researchers have concluded so far:

SEXUAL VIOLENCE

The fundamental question surrounding porn – which resurfaces every time a violent crime involves the perpetrator’s porn use – is whether it has the power to encourage, normalise or even trigger acts of rape and sexual violence.

This possibility has been explored for decades. In the 1970s, for example, Berl Kutchinsky, a professor of criminology at the University of Copenhagen, measured sex crimes in Denmark, Sweden and Germany as they legalised porn in the late 60s and early 70s. He found no correlation between a rise in crime and decriminalisation – and in fact, some types of sex crime fell during this period, including rape and child molestation.