Nathan Matthews and Shauna Hoare
Nathan Matthews and Shauna Hoare have been found guilty of killing Becky Watts. Facebook

The gruesome murder of 16-year-old Becky Watts from Bristol shocked the nation, but many observers have also been left puzzled. How could a young, outwardly normal couple, calmly plan the kidnap and sexual assault of someone they both knew well, then make sick jokes the same day they cut up her body before tricking friends into helping them conceal the evidence?

One suggestion put forward by Professor David Wilson, a criminologist at Birmingham University who has followed the case, is that murderer Nathan Matthews and his partner Shauna Hoare, who was convicted of the 16-year-old's manslaughter, may have been influenced by a psychological condition called folie à deux - also known as "shared psychotic disorder".

Becky Watts
Matthews, 28, had admitted killing Becky and dismembering her body but denied it was murder. Facebook

Folie à deux is a shared delusional state which affects two or more people, often in close proximity to one another, such as mothers and daughters, siblings or couples. In exceptional cases three, four and sometimes even 12 people share the delusion, in which case it is called folie à deux, folie à trios, folie à quatre, folie à cinq and so on, up to folie à douze.

People who suffer the condition feed off one another, sharing their delusion and in so doing making it stronger. However, not everyone who suffers the condition goes on to kill - and whether or not Matthews and Hoare did reinforce their own sick fantasies by sharing them, there is no suggestion they did not know exactly what they were doing when they killed Becky Watts.

Matthews and Hoare's names will now be mentioned alongside those of other couples who kill, including Ian Brady and Myra Hindley - the Moors Murderers - and Fred and Rose West. Whatever sentences the two are handed on Friday (13 November), it is almost certain they will never see each other again - and never again have the chance to share their sick and deadly fantasies.