Casteel, 44, shot at cars along the I-96 corridor in October 2012. He was convicted of several felony charges in Livingston County Circuit Court last week.

Throughout his trials, Casteel’s mental illness remained the subject of his testimony.

As part of a court order, Casteel underwent an independent psychiatric evaluation and discovered he suffers from delusional disorder.

Casteel testified during his trial in Livingston County that he believed people in the cars he shot were part of a large government conspiracy against him. He said he believed his phones were tapped into and that he was being monitored by government helicopters.

He later gave a detailed account of his family history with mental illness, including a history of paranoia and delusional thoughts on his mother’s side. Judge David Reader later told the jury to disregard his comments about family ties to mental disorder.

To a person with delusional disorder, there is often no difference between imagination and reality, MSU psychiatry department chair Jed Magen said. He said a delusional disorder diagnosis is in the same family as someone with schizophrenia or bipolar disorder, but it’s a mutually exclusive diagnosis.

“People acting on these delusions don’t only believe that these delusions are true, but they see evidence everywhere to support them,” Magen said.

Criminal justice professor Karen Holt said insanity cases only achieve acquittal about 10 percent of the time.

“You have to be able to prove that the person didn’t know what they are doing is wrong,” she said.

In 2012, state lawmakers did away with the diminished capacity plea, meaning defendants can no longer have their charges lessened based on mental illness.

If a defendant wants their mental condition examined and used in the trial, they have to plead insanity or no contest.

The issue also has come up regarding where Casteel will spend his jail time.

Holt said the procedure will depend on the judge and their decisions, but there are two options: a concurrent service time that would have him serve time for all of his trials in one jail, or consecutive, which would have him serve his sentences in individual jails.

Holt said the judge isn’t supposed to take prior convictions into account, but admits that it is easier said than done.

“It’s hard to be fair and partial completely with a trail this big,” she said.