There is little research into the idea of women being attracted to sex offenders, such as Johnson, but Stephens suggests it plays into their sexual fantasies - and will be particularly common in people with sadomasochistic tendencies.
"It's almost like there's a risk and it's attractive to them," he explains. "They might find it exciting. It's like they're saying 'I want to be a victim'. It's not that they necessarily want the man to do that to them - it's more like a fascination."
He suggests that this is why the letters being sent to Johnson are so sexual in nature: "It's all part and parcel of the fascination and trying to attract someone. They might not be able to feel attracted to people outside. Some even see themselves as helping these people."
Of course, there's the possibility that, in Johnson's case, these women were always fans of the footballer and are drawn to him because of his status as a celebrity. But Dr Elie Godsi, clinical psychologist and author of Violence and Society: Making Sense of Madness and Badness, says it's likely that anyone who's attracted to Johnson has problems of their own.
"They don't know him. So what they're bringing to the table isn't to do with him but their own issues. I'd suggest some have abuse in their own history. In order to project this fantasy figure, you have to ignore the realities of the abuse he has committed. Some people who have been abused are already in denial; that's part of their mechanism to normalise it."
There are other examples of women falling for sex offenders. The former Lost Prophets singer, Ian Watkins, has hundreds of devotees even though he was jailed for child abuse for 35 years.
There are Facebook pages called 'Ian Watkins is our crush' and 'Ian Watkins is a sex god'. One fan wrote that Watkins also receives fan mail: "I am in touch with female fans who have written to him sending pictures, telling him they'd wait for him when he is released. Others don't believe he is really guilty and still want to marry him. I know people who have sent money and shampoo and deodorant so Ian realises his fans are thinking about him."
But usually, that's as far as it goes. Stephens explains that when the convict is released, the fans tend to disappear.
"It's fear," he says. "They don't want the real thing. It's fine when they're locked away. They're protected. They often don't even visit them. But when they're out, they don't want to take the risk."
The unreality of it all is what appeals to these women. "They can't meet them or spend time with them or share a bedroom with them," adds Dr Godsi. "It's safe.
"When you're writing to someone from a distance you don't have to deal with reality. There's no real boundaries. It's completely artificial."