David Wilson, professor of at Birmingham City University, offered comments for Britain’s Daily Mail. “It is impossible to say with certainty whether a person is born a killer or becomes a killer,” he stated, “but most criminologists believe that it is a messy combination of the two. A person can be born with a genetic make-up that means they have the potential to kill, but it is not a given that they will then go on to fulfill that potential.”

I’ve followed the Dennehy case closely, myself, to try to discover how and when the switch to her self-described killing compulsion flicked on. This question brings to mind a case from May 2007 in India when an eight-year-old boy was arrested for killing a baby. It turned out that he'd already killed two others. An eight-year-old!

The victim was the infant of a neighbor in the village of Musahari, Bhagwanpur in Begusarai. Apparently, the child was asleep in a crib at a school, unattended, while her mother did chores nearby.

Amarjeet Sada crept close to the baby’s bed, quietly grabbed her, and took her to a field where he knew that no one could hear her cries. (So, he knew it was wrong.) With a brick, he bludgeoned the baby in the face until she died. He then covered the body with grass and leaves.

When the child’s relatives frantically searched for her, they found the battered body. The obvious suspect was Amarjeet, because several people knew it wasn’t the first time a baby had died when he was around. The police took him in for questioning. He admitted to the grisly deed, apparently stating that he’d “put her to sleep.” He showed police where he’d placed her. When they asked why he had done it, Amarjeet just smiled.

A few months earlier, he’d come under suspicion for strangling his infant sister, as well as bludgeoning a one-year-old cousin while staying with his mother’s brother. Both incidents were hushed up, but neighbors were aware of Amarjeet’s nasty tendencies, generally avoiding him. Many were certain that he’d killed his two relatives.

It wasn’t difficult for investigators to learn about these incidents, but no one could quite fathom a serial killer this young.

A from Patna, Shamshad Hussain, surmised that the boy was a developing sadist. In other words, he was a born killer. Had he not been stopped, he would have looked for other vulnerable infants to kill and would have gotten more brutal. He seemed to show no remorse.

However, there was no evidence of the type of prolonged torture characteristic of sadists, so it seems more likely that Amarjeet is a thrill killer, deriving pleasure from the act of killing. Like Joanna Dennehy. But how did it develop and get past their moral inhibitions (if they ever had any)? That's the mystery.

Dennehy’s family and friends seem to think she started to change around age 14, when she began abusing substances. She dropped her life ambitions, skipped classes, and hung out with boys who drank and smoked a potent form of pot. She became rebellious and argumentative. Eventually, she developed what seemed a compulsion to kill. But this doesn't really clarify anything.

Experts disagree as to whether or environment has the greater influence on the development of violent , but has identified brain structures implicated in certain types of violence. No one has suggested that Joanna Dennehy or Amarjeet Sada suffer from brain damage or physiological disorders, but it’s likely that their unique behavior will draw such researchers’ . I hope a scientist with sophisticated tools looks more deeply into Dennehy's case.