Jeffrey Dahmer suffered a mental disease when he killed and mutilated young males to fulfill his sexual desires, but he is not legally insane, a prosecution psychiatrist testified Monday.

Dr. Frederick Fosdal took the stand for questioning by defense attorney Gerald Boyle as the serial killer's insanity trial entered its third week.Fosdal said Dahmer suffered a mental disease called paraphilia, which includes a wide range of sexual disorders. However, he said that despite Dahmer's mental illness, he knew right from wrong and could have controlled himself.

"This type of disorder in and by itself . . . in my interpretation does not usually cause the degree of impairment" to make the individual unable to obey the law, Fosdal said.

Dahmer, a 31-year-old former chocolate factory worker, has pleaded guilty but insane to 15 slayings in Wisconsin.

To be found insane and sent to a mental institution instead of prison, he must convince the jury he suffered from a mental disease that made him unable to tell right from wrong or unable to prevent himself from doing wrong.

Fosdal testified earlier that Dahmer suffered from necrophilia "before, during and after" his slayings.

"It explains his behavior but didn't cause him to break the law," Fosdal said Saturday. "He was able to refrain and had some control as to when he followed through on his sexual desires."