RIVERSIDE, Calif. - A psychologist testifying in the murder trial of a California boy who killed his neo-Nazi father last year told a court on Monday that the young defendant suffered mental issues from a "long history" of physical, emotional and likely sexual abuse.

Robert Geffner was called to the witness stand by defense attorneys, who concede that Joseph Hall, now 12, shot his father at point blank range in May 2011 but argue that he should not be held criminally responsible.

"It's clear that violence is the appropriate way in his world," Geffner said. "A repeated theme in conversations with him was killing. Another part of his focus was guns."

The case in Riverside County, east of Los Angeles, has drawn attention for Jeffrey Hall's neo-Nazi associations and the rarity of a parent being slain by a child as young as Joseph.

Kathleen Heide, a criminologist who specializes in juvenile offenders, has said that 8,000 murder victims over the past 32 years were slain by their offspring, but only 16 of those were committed by defendants age 10 or younger. ?

Since Hall is a juvenile, the purpose of the trial, now in its second week in Riverside County Superior Court, is not to determine his guilt or innocence but whether certain allegations about his motives are true. If he's found responsible for the crime, he could be sent to a juvenile facility until the age of 23.

The outcome of the case, which is being heard without a jury, hinges in large part on the boy's understanding of right and wrong at the time. He may testify as early as this week.

Geffner, a psychologist and president of the Family Violence and Sexual Assault Institute in San Diego, told the court that Hall suffered a "long history of abuse - physical, emotional and likely sexual" that led to Child Protective Services being summoned to his home 23 times by the age of 10.

Geffner said that such abuse, which may have included being whipped or forced to eat from the floor, can create "significant neurological and physiological problems" as well as confusing the difference between right and wrong in the child's mind.

"Children experience what's called learned helplessness, that there's nothing that can be done. They suffer internal feelings of hopelessness, helplessness," Geffner said. "There's an unwritten message that there doesn't seem to be any consequences to these types of behaviors. It teaches children this is acceptable behavior."

In a videotaped police interview played in court last week, Hall was seen to say that he was physically abused at home and committed the shooting because he "wanted everything to stop."

Defense lawyers have said the boy was conditioned by his father's violent, racist behavior, and killed the 32-year-old man to put a halt to the physical abuse inflicted on him.

Prosecutors say the boy, who lived in a house with four siblings, committed the slaying because his father was threatening at the time to divorce his stepmother, Krista McCary. Prosecutors said he was close to McCary and considered her his true mother.