A middle-aged ex-con who once starved a man to death and two of her accomplices are arrested, charged with keeping four mentally disabled prisoners chained up in the putrid sub-basement of a rowhouse in the city's Tacony section. The next day, investigators say there may have been more victims: children and teenagers.

Police suspect the accused were doing it for the money, collecting the captives' Social Security checks.

But why deprive them of food? Why make them lie in their own filth? Why leave them in the dark? What makes people so cruel?

As time-tested a deadly sin as avarice may be, greed alone does not explain such depravity.

"You can't get any lower than how they preyed on these people," Philadelphia police spokesman Lt. Ray Evers said.

Oh, but you can.

Two forensic psychiatrists who have made careers of plumbing the depths to which criminals can go say that as horrific as this case may seem, many have been far worse.

And worse can be measured.

According to the scales of grotesque human behavior the two have analyzed, the alleged villains - Linda Ann Weston, Thomas Gregory, and Eddie Wright - appear to be only middling sociopaths.

They've got nothing on Ted Bundy, Jeffrey Dahmer, John Wayne Gacy, and Mike DeBardeleben.

"The [Philadelphia] situation was not a pure exercise in sadism," said Michael H. Stone, professor of clinical psychiatry at Columbia University and author of the 2009 book The Anatomy of Evil.

Stone has identified 22 levels of heinousness, beginning with impulsive crimes of passion like murdering a cheating spouse, to the unthinkable - plotting to rape children and bury them alive.

Starvation, he said, "is a cheap way to kill somebody. Eventually they die, and when they do, they're lighter than when you started out, so they're easier to bury." It is also a cleaner method, "as opposed to shooting them."

Although starving is painful, Stone said, it's less so than having cigarettes burned into your neck or being whipped.

The brains of psychopaths who can do these things have been studied with MRIs and often show abnormalities, Stone said.

"The connections in the frontal lobe are not adequate. From birth, some of these people are predisposed to do this stuff, even if they have not been abused or neglected."

Ted Bundy, for instance, was a serial rapist, kidnapper, and necrophiliac responsible for at least 30 murders. "No one ever laid a hand on him," Stone said. "He was simply a very cold and callous person."

At the other extreme, DeBardeleben, a sexual sadist and counterfeiter, was brutally abused as a child.

"There is no greater power over another person than that of inflicting pain on her," DeBardeleben wrote in his private journal. "To force her to undergo suffering without her being able to defend herself. The pleasure in the constant domination over another person is the very essence of the sadistic drive."

Michael Welner is less inclined to attribute a criminal's penchant for atrocities to mental illness or a dysfunctional home.

Welner, an associate professor of psychiatry at New York University School of Medicine, has developed a "depravity scale" for prosecutors and defense attorneys to use when trying cases of extreme malice.

"We all have a different moral compass," Welner said. Having once interviewed Philadelphia's Marie Noe, who smothered eight of her babies, Welner said, he believes she had no psychiatric illness but rather a limited capacity to appreciate small children as people.

"The inability to appreciate the humanity of the other is a limitation," Welner said. "But to many criminals, it is seen as an advantage. A conscience is an obstacle."

People are endowed with varying degrees of empathy and a sense of right and wrong "by accident of birth or the way you are raised," he said. "But none of us are raised in a cage."

Millions survive abusive childhoods and cope with mental deficiencies without becoming sociopaths. From infancy to the point at which people make homicidal choices, he said, they are exposed to countless positive influences.

"None of this," he said, "happens in the absence of choice."