Conscience is considered the seventh sense.

It is felt instinctively.

For the most part, it can't be taught or conditioned or rehabilitated.

The absence of conscience is a non-correctible disfigurement of character. There is no effective treatment.

In a word: Guiltlessness. More words: Remorseless, deceitful, manipulative, self-centred, callous, unscrupulous, emotionally shallow.

There was a time when the condition was described as "moral imbecility," which still fits. The modern shrinking heads bible, Diagnostic and Statistical Manual of Mental Disorders IV, refers to the clinical diagnosis of little or no conscience as "anti-social personality disorder."

More commonly, it's called psychopathy or the slightly semantic alternative, sociopathy.

M.T., the 17-year-old girl convicted of first-degree murder on Friday for orchestrating - demanding, via sexual blackmail - the death of a perceived rival, 14-year-old Stefanie Rengel, is arguably a psychopath, or sociopath, on the evidence put before the jury.

This crime was not the unintended consequence of typical teenage psychodrama with a contemporary cyber twist: girl meets boy, girl seduces boy into murder, girl and boy plot killing online, girl rewards boy with sex.

It was sinister and calculated.

But psychopathic meets neither the legal nor psychiatric threshold of insanity. As Robert Hare, a renowned expert in the field and professor emeritus of psychology at the University of British Columbia, explains in his book Without Conscience: The Disturbing World of the Psychopaths Among Us, psychopathic killers are not mad.

"Their acts result not from a deranged mind but from a cold, calculating rationality combined with a chilling inability to treat others as thinking, feeling human beings. Such morally incomprehensible behaviour, exhibited by a seemingly normal person, leaves us feeling bewildered and helpless."

They are utterly logical, to their own way of thinking, and don't see where they need to be "fixed," even if that were possible.

M.T. will be subjected to a battery of psychological tests in preparation for her sentencing hearing and, again, when a personalized custodial plan is formulated.

One of those tests could be that devised by Hare, the "Psychopathic Checklist," an inventory of symptoms now used by clinicians globally. These manifestations include lack of empathy or remorse, egocentric and grandiose behaviour and the need for excitement.

But so what? They are merely descriptive labels. Nobody really has much of a clue how psychopaths are created so the why question can't be answered.

There is some evidence suggesting psychopaths are born that way, possibly due to a neurodevelopmental fritz in the cerebral cortex.

In plain-speak - the bad seed. They emerge from the womb predisposed to amoral - unfeeling - conduct and are reinforced in that no-limits attitude as they grow up.

Most don't become killers. But they're all ruthless, which often makes them quite successful in their careers and frequently charismatic. There's no compunction about lying or seizing what they want or luring others to engage in dangerous ventures.

We all know such people.

Here's the truly frightening part: It's estimated that 4 per cent of the population is psychopathic.

The Harvard psychologist Martha Stout writes, in The Sociopath Next Door: "About one in twenty-five individuals are sociopathic, meaning, essentially, that they do not have a conscience. It is not that this group fails to grasp the difference between good and bad; it is that the distinction fails to limit their behaviour.

"Without the slightest blip of guilt or remorse, one in twenty-five people can do anything at all."

Further: "If you decide to kill, the only difficulties will be external ones. Nothing inside of you will ever protest."

From the thousands of pages of retrieved MSN chats and Facebook postings that nailed M.T. at trial, it was evident the accused believed her obsession with Stefanie - a girl she'd never met - was entirely rational. Her wish to see Stefanie dead - allegedly stabbed by M.T.'s sap of a sexually manipulated boyfriend, D.B. - never struck the teen as unconscionable.

She didn't see Stefanie as a human being. Stefanie was an imagined threat and an annoyance, someone who got under the skin of M.T. and deserved to die.

A more practised psychopath - perhaps someone older - would at least have faked remorse for the cops.

There is just about zero probability that M.T. will emerge from prison a chastened woman. If sentenced as an adult, she would still be eligible for parole after five to seven years, as determined by the judge, with the clock ticking from the time that she was taken into custody, age 15.

But if sentenced as an adult, she will at least have a criminal record that won't dissolve as it does for youths, five years after the sentence ends.

She will always be formally branded a murderess, even if, for M.T., there is neither shame nor stigma in that.