'The red flags our child was a psychopath.'

From a young age Samantha had a mean streak. She would push her siblings, pinch them, trip them over, even tear up their pocket money.

Psychologists suggested delayed empathy, impulsiveness, reactive attachment disorder, and –
hurtfully – even poor parenting.

But then, at the age of six, Samantha placed her hands around her two-year-old sister’s throat and squeezed.

The little US girl’s story has been told by Barbara Bradley Hagerty of The Atlantic, and offers rare insight into what lead to her eventual, disturbing diagnosis: conduct disorder with callous and unemotional traits, a key indicator of a budding psychopath.

Having adopted her at the age of two, Samantha’s parents observed her petty cruelty escalate; her behaviour become premeditated, calculated, vengeful.

Once, after being scolded by her mother at the age of five, she washed the woman’s contact lenses down the drain.

“She knew exactly what she was doing,” her adoptive mother told Bradley Hagerty. “There was an ability to wait until an opportune moment to exact her revenge on someone.”

It became darker still when she choked her little sister, then four months later did the same to her two-month-old brother. Disturbingly it wasn't only them she'd plotted against.

"I want to kill all of you," she reportedly told her mother.

This was no offhand comment. After the first choking incident Samantha showed her mother a book containing drawings of weapons - a knife, a bow and arrow, poisonous chemicals - and demonstrated on her stuffed animals how she would use them.

“I wanted the whole world to myself,” the now 11-year-old told Bradley Hagerty from the treatment facility where she lives. “So I made a whole entire book about how to hurt people.”

Psychiatrists tend to avoid a psychopathy diagnosis for children, using instead the term given to Samantha - callous and unemotional traits, which are associated with increased risk of adult psychopathy.

Last year, researchers from the University of New South Wales found such traits - lack of guilt and limited empathy - in 10 per cent of the 200 three to six-year-olds they tested.

However, as Bradley Hagerty notes, the vast majority (four out of five, by some estimates) will not grow up to be psychopaths. Researchers are still working to determine why this is; why some will grow up to be normal, well-adjusted adults while others don't.

With Samantha in a state-of-the art institution, her mother feels there's hope.

“The hard part is, it’s never going to go away," she said. "It’s high-stakes parenting. If it fails, it’s going to fail big.”

For more, you can read Barbara Bradley Hagerty's brilliant piece here.