On 15 September 2017, a jury in Wisconsin decided that 15-year-old Anissa Weier was mentally ill when three years ago, along with friend Morgan Geyser, she stabbed 12-year-old Payton Leutner 19 times in a forest in Waukesha. When the case broke in May 2014, it attracted widespread attention for the brutality of the attack by two 12-year-old girls and the reasons they gave for why they tried to kill their friend.

Now known worldwide as the Slenderman case, both girls told police they had tried to kill Payton Leutner to please the fictional internet folklore character ‘Slender Man’. A character that both girls appeared to believe was not only real but by killing their friend they would be welcomed into his world and taken to live with him in his mansion in Wisconsin’s Nicolet National Forest. They also told police they believed that if they didn’t carry out the attack, Slenderman would harm them and their families.

Morgan Geyser and Anissa Weier in court in 2015

Morgan Geyser and Anissa Weier in court in 2015

Both girls underwent psychiatric assessments while the courts decided how best to proceed with their case. Under a great deal of debate, it was decided in March 2015 that the two girls would be charged with attempted first-degree murder and tried as adults, despite their young ages.

This week’s verdict for Anissa Weier comes after a guilty plea to attempted second-degree murder due to mental illness, and it means Weier will now be committed to a mental institution and not sent to prison. Within her mental capacity trial which started on 11 September, psychologist Melissa Westendorf, who assessed Weier, told the jury that she suffered from shared delusional disorder. This disorder is the reason why she shared Morgan Geyser’s delusional belief that the character of Slenderman was real and that by killing their school friend they would be accepted into his world.

CBS News reported how Melissa Westendorf explained to the court that shared delusional disorder in children is rare, most often seen in between spouses, siblings or a parent and a child. Highlighting the website the two girls were visiting which told stories of the Slenderman character, she told the court, “If adults have trouble distinguishing fake news, 12-year-olds will, because their brains can’t yet discern or analyse as well.”


The Journal of the American Academy of Psychiatry and the Law reports in a paper on Shared Psychotic Disorder and Criminal Responsibility, that the criteria for a diagnosis of shared delusional or psychotic disorder includes:

  • an individual developing a delusion when in a close relationship with another who has already established this delusion,
  • the delusion being similar to that established delusion and,
  • the incidence of the delusion cannot be explained by another psychotic disorder such as schizophrenia or a mood disorder with psychotic features.

A decision by a jury that an individual was indeed suffering from shared delusional disorder at the time a crime was committed is essentially equal to an acceptance of temporary insanity.

The idea of a criminal act being carried out under a shared delusion between two people also arose in the murder case of 16-year-old Becky Watts, in Bristol, south-west England in 2015, who was killed by her stepbrother and his girlfriend. Their shared distorted sexual interest in school girls and kidnapping led to criminologist Professor David Wilson suggesting the pair were operating under shared psychotic disorder, where the dominant partner of Nathan Matthews held control over his girlfriend, Shauna Hoare, transferring his fantasies and delusions onto her whereby she too believed these delusions.

In the Slenderman case, Morgan Geyser was diagnosed with early-onset schizophrenia in 2014 during psychiatric assessments for her competency to stand trial. She has pleaded not guilty to attempted intentional homicide by reason of mental disease. Anissa Weier, no doubt due to this testimony on shared delusional disorder, has avoided a possible lengthy prison term and she will now spend at least three years in a psychiatric unit receiving treatment before she will have the opportunity to be released. The trial of her friend Morgan Geyser is due to commence in October 2017.