A German who calls himself Clark Rockefeller has a delusional disorder and a narcissistic personality disorder that impair his judgment, a psychologist hired by the defense in his kidnapping trial told a judge Wednesday.

Forensic psychologist Catherine Howe's testimony offered a glimpse into what the insanity defense will look like at the trial, which begins with opening statements Thursday.

The suspect's real name is Christian Karl Gerhartsreiter. He is accused of snatching his 7-year-old daughter, Reigh, in Boston in July after losing custody of the girl to his ex-wife, who lives in London. Authorities say he shoved a social worker to the ground during a supervised visit, then hustled his daughter into a waiting car and fled. They were both found in Baltimore six days later, the girl unharmed.

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• Partial List of Witnesses at the 'Clark Rockefeller' Trial

Prosecutors say Gerhartsreiter has used multiple aliases — including the family name of famous oil baron John D. Rockefeller — to ingratiate himself into wealthy circles in New York, Los Angeles and Boston since he came to the United States in the 1970s.

Howe, the defense psychologist, said she spent 16 hours interviewing him after his arrest and diagnosed him the two mental illnesses. Together, the illnesses mean he has delusions of inflated worth and power, a sense of entitlement and a lack of empathy, Howe said.

"His judgment is impaired by this belief system," she said.

The psychologist's testimony came during a hearing on a motion by defense lawyers, who are trying to keep prosecutors from using statements Gerhartsreiter made to police and in an interview he gave to The Boston Globe after his arrest. The jury was not present for the arguments.

Gerhartsreiter's lawyers argued that his statements to police and his interview with the Globe were not have been made voluntarily, because of the mental illnesses. Howe said that his disorders made it impossible for him to turn down a request to be interviewed and to stop himself from talking to police.

"This is a gentleman who does not stop talking," she said.

Assistant District Attorney David Deakin rebutted her diagnoses by reviewing all the truthful statements Gerhartsreiter made to police and to the Globe, including details about his daughter, his wife's job, his divorce agreement and the fact that his wife was the sole wage earner in their family.

"Not a delusion, right?" Deakin repeated after Howe confirmed that he made truthful statements during the interviews.

Judge Frank Gaziano did not immediately rule on whether the jury will be allowed to hear the statements and a tape of the Globe interview. He said he will hear testimony from a defense expert Thursday before issuing his decision.

Earlier Wednesday, the judge ruled that prosecutors cannot play a call made by the social worker to police. But the judge said the social worker, Howard Yaffe, will be allowed to testify about a statement he made to a private investigator who had been hired to observe the father-daughter visit.

Also, prosecutors agreed Wednesday that they would not tell jurors that Gerhartsreiter is a "person of interest" in the 1985 disappearance of a California couple. He was living in a guest house on their property, under the alias Chris Chichester, when they disappeared.