San Francisco ? Every day of his six-month murder trial, Scott Peterson marched into the courtroom with his head held high. He smiled at his family, took his seat and paid close attention, often whispering to his lawyers or taking notes.

Given a chance to defend himself in the murders of his pregnant wife Laci and their unborn son, the slick, handsome salesman with the megawatt smile had nothing to say.

His demeanor seemed to infuriate jurors and many trial watchers, who came to see Peterson as a manipulative, pathological liar with a grandiose sense of self and an inability to empathize.

Experts say this absence of emotion is the hallmark of a psychopath.

“They don’t have the internal psychological structure to feel and relate to other people,” forensic psychologist Reid Meloy said. “Sometimes they can imitate it, so they can fool other people, but there will come a point when they can’t maintain it.”

The times Peterson did display emotion were rare. He winced and put his head down when prosecutors showed autopsy photos of his wife and their fetus. He wiped tears from his eyes as his mother pleaded with jurors to spare his life. He wept softly when his sister-in-law recounted the first time she met his slain wife.

But passionate, angry and accusatory outbursts from Laci’s family members when he was sentenced to death Wednesday didn’t appear to faze him. Meloy said that fits with the inability of psychopaths to form truly intimate bonds with others.

Such an absence of heartfelt emotion “gives the psychopath the ability at times to kill without remorse and to kill for reasons filled with banality,” he explained. “Others’ emotions of grief and rage and fury are like water off a duck’s back.”

That apparent lack of emotion raised investigators’ suspicions in the first place, police and prosecutors said.

“His major concerns weren’t Laci at the beginning of this case,” explained Modesto Detective Al Brocchini. “He is very calm, cool, nonchalant, polite, arrogant. He thinks he’s smarter than everybody.”