Why do some people fall in love with their close relatives?

Stories of love against all odds are the basis for some of the greatest works of art and literature. But tales of infatuation between close relatives - be they siblings or parent and children - are generally met with churning stomachs rather than misty eyes. That was certainly the reaction when mother and son Kim West, 51, and Ben Ford, 32, both from Michigan, revealed their story earlier this month.

West put her son up for adoption a week after she gave birth to him. Decades later, in 2013, Ford did as many adopted children do and made contact with his biological mother. The pair then formed a highly unusual bond. In an interview, West admitted that she started to have sexual dreams about her son, while Ford scoured the internet for reassurance that he was not alone in having sexual feelings towards his mother.

The pair later met at a hotel, and with their inhibitions loosened by alcohol, shared their first kiss and started a sexual relationship that West described as “mind-blowing.” Three days after the meeting, Ford left his wife for his mother.

Since revealing their story to the press, the couple have gone into hiding over fears that they could each be hit with a 15-year prison sentence and forced to sign the sex offenders’ register. But perhaps they could claim some mitigation at an evolutionary level. As Dr Tamsin Saxton, a senior lecturer in Psychology at Northumbria University, explains, attraction is all about the “impression of someone that you construct, which can depend to some extent on their value to you as a potential partner”.

Now hold that thought, and consider this: although you might find falling for a relative incomprehensible, you’re probably not too surprised to learn that evidence suggests heterosexual men are more likely to find a partner who resembles their mother attractive, and women a partner who resembles their father. This is known as positive imprinting, and a 2008 study by Icelandic scientists suggests that it occurs because a couple with a distant familial relationship may have a better chance of producing a large number of healthy children.

Meanwhile, negative imprinting, or the Westermarck Effect, describes the process whereby individuals reject any sexual attraction that they feel for those with whom they have lived closely in their infancy, including parents and siblings. So could the combination of positive imprinting and a lack of the negative version account for the behaviour of Ben Ford (if not his mother)?

“Some people who haven’t had the right experience in childhood can struggle to separate a sense of physical comfort or emotional comfort from sexual touch or attraction,” suggests psychologist Judith Wenban-Smith. However, she says, they still can exercise free will: “It’s not inevitable, it’s not a disease or an illness. It’s the way people interpret and react to the feelings they have when they’re reunited with a blood relative, from whom they have been separated and about whom they would have fantasised.”

Unfortunately, the psychological implications of such a relationship built on powerful emotions misfiring can be doubly devastating. Couples will not only face rejection from society, but also destroy any chance of sharing a familial love.