Sept. 11 conspirator Zacarias Moussaoui suffered a traumatic childhood that transformed him from a boy with a sense of humor who made friends easily to a man who spurned his family and embraced radical Islam, a defense witness testified Monday.

Jan Vogelsang, a clinical social worker, said at Moussaoui's death-penalty trial that the 37-year-old Frenchman was in and out of orphanages the first six years of his life. As a teenager, she said, he was rejected as a "dirty Arab" by the family of his longtime girlfriend, with whom he lived briefly and won dance contests.

Moussaoui was dismissive of the social worker's analysis. He shouted, "It's a lot of American B.S.," as he left the courtroom for the lunch recess.

The jury also heard videotaped testimony taken in France in December from Moussaoui's sister Jamilla, who described her younger brother as "a pretty little baby, always smiling ... he was the little sweetheart of the family." She also described the abusive atmosphere caused by their father, Omar, who repeatedly beat Jamilla and the siblings' mother.

Moussaoui was in jail in Minnesota during the Sept. 11, 2001, attacks. The jury has decided that lies he told federal agents a month earlier kept authorities from identifying and stopping some of the hijackers, making him responsible for at least one death that day and qualifying him for the death penalty.

Now jurors are deciding whether Moussaoui deserves execution or life in prison.

In her videotaped testimony, Moussaoui's sister said their father would intervene in their lives even after the parents had divorced and that "each time he reappeared in our lives, it was to traumatize us."

"He left us completely destitute," she testified. "He was a man who never should have had children."

At the outset of her testimony, Vogelsang said she did not intend to make excuses for Moussaoui's actions as a terrorist but wanted to explain how he had reached that point. The clinical social worker never interviewed Moussaoui herself, relying instead on interviews with 50 family members, friends and others, like teachers, who knew him.

Vogelsang said that Moussaoui's mother, Aicha el-Wafi, was beaten throughout her pregnancies -- including six before she gave birth to Moussaoui. Moussaoui, born in a French town near the Spanish border and of Moroccan descent, went to an orphanage four months after his birth, when his mother was placed in a convalescent home, she said.

She added that Moussaoui's family only nominally practiced Islam and celebrated Christian holidays because his mother wanted her children to integrate into French culture.

As a boy, Moussaoui made friends and displayed a sense of humor, Vogelsang said, despite the beatings his father inflicted on his mother and sister. When Moussaoui was 6 and his mother had divorced Omar, an uncle moved into the home and beat Moussaoui and other family members.

Mental illness was rampant in the family, according to the testimony. Omar is diagnosed with bipolar disorder and is institutionalized, Vogelsang said. Moussaoui's two sisters have been diagnosed with schizophrenia and psychosis with schizophrenic features, respectively.

In 1992, Moussaoui moved to London in hopes of becoming an international businessman. He struggled to acclimate himself and learn English. He obtained a master's degree from South Bank University. It was during Moussaoui's time in London from 1993 to 1995 that his family noticed a transformation.

"He started shaving his head and wearing a beard," Vogelsang said. "He was fussing at his sisters for how they dressed," calling one sister a "whore" for dressing in Western clothing.

Moussaoui took the witness stand in his own defense Thursday for a second time, and again did more harm to himself than good as he reveled in the death and destruction of Sept. 11 and mocked the testimony of victims and their families.

One day after Moussaoui's testimony, the judge vacated an earlier order compelling would-be shoe bomber Richard Reid to testify in his defense. The jury may instead hear a written statement summarizing some of what the al-Qaida comrade would have said on the witness stand.

Reid is serving a life sentence in the federal government's maximum-security prison in Florence, Colo., after a failed try to blow up an American Airlines flight in 2001.

Moussaoui, who calls Reid his "buddy" from their days together in al-Qaida training camps in Afghanistan, testified last month that he and Reid were going to hijack a fifth plane on Sept. 11, 2001, and fly it into the White House.