Moussaoui, 37, is scheduled to be sentenced next month in a US federal court when a jury must decide if he should be executed or imprisoned for life without parole.
His lawyers have submitted to the court a list of expert witnesses prepared to testify about Moussaoui's upbringing in France by his Moroccan mother, his alienation from society and his mental state.
A neuroscientist and author, Dr Nancy Andreasen, will tell the court that Moussaoui "suffers from a major thought disorder, most likely schizophrenia," the defense lawyers wrote, court papers revealed.
Dr Andreasen, chair of psychiatry at the University of Iowa's college of medicine, based her analysis on Moussaoui's rambling writings, his appearances in court and the findings of another psychiatrist who examined Moussaoui, the lawyers said.
Another witness, clinical social worker Jan Vogelsang, will testify that Moussaoui had a traumatic childhood, spending five years in orphanages after his mother separated from his "violent alcoholic" father.
"While Zacarias' mother struggled to support her four children in a strange country where she had to learn a new language, Zacarias and his brother basically raised themselves," the lawyers said.
The witness will also recount how Moussaoui suffered from racism in France due to his Moroccan heritage and that his condition deteriorated further after moving to Britain. He eventually was attracted to "radical mosques which flourished in London in the 1990's," they said.
The social worker will also point to an alleged history of mental illness in Moussaoui's family.
US law recognizes mental illness as a possible mitigating factor in a death penalty case, but the defense strategy may prove difficult given Moussaoui's often defiant attitude during court proceedings. He has also refused a psychiatric examination.
In his trial, Moussaoui showed contempt for the United States and held forth about his belief in Islamic militancy.
Moussaoui is the only person who has been brought to trial in a US civilian court in connection with the September 11 attacks on New York and Washington.
He signed a statement of fact in his guilty plea that identifies him as the "20th hijacker" in the September 11 attacks in 2001.
The sentencing hearing starts with jury selection on February 6.