Lead author of the study and senior lecturer at UNSW, Eva Kimonis, said the diagnostic tool would allow children at risk of psychopathy to get earlier treatment.

"Until now we didn't really have a way to identify those traits in very young children," she said. "This is really the first study which uses tools adapted for very young children and the sooner those children are identified, the earlier they can be helped."

The children were assessed using a combination of tests adapted for their age and interviews with their parents and teachers.

The tests measured their ability to recognise changing and static facial expressions as well as their reactions to distressing and neutral images, such as a crying child or a book.

"Even very young children with these traits show that difficulty in recognising emotions in others and they are also not engaged by other people's emotions," Dr Kimonis said.