Liu, a consultant at Castle Peak Hospital, said she interviewed the mother, now 30, on four more occasions, during which she reported suffering from a low mood as a result of stress from childcare and her first experience of managing such a large household.

But the expert found these symptoms were not pervasive or severe enough to warrant a diagnosis, because they were reactive to stress and not as generalised as one would expect of depression, which affects the person as a whole, given its nature as a biological illness involving an imbalance of neurotransmitters in the brain.

That finding was supported by the fact the mother did not require any medical attention in the three years between her arrest and trial, as well as a five-month record of more than 15,000 messages with her husband and a friend, which offered an insight into her overall mood and motivation to connect with others.

In particular, Liu noted the mother’s irritability and claim of impaired judgment was specifically directed to the deceased and her eight-year-old stepson, without any impact on her loving relationship with her husband and biological daughter, then aged seven.

“You won’t just be irritable towards two people,” the prosecution witness said. “This is not mental illness, it’s just human nature – and if it’s human nature, it’s an issue for the jury.”

The stepmother and the children’s 29-year-old father have admitted to child cruelty inflicted upon the deceased and her brother over 150 days from August 10, 2017, but denied murdering the girl, who died of septicaemia on the following January 6.

Nursery school teacher breaks down while testifying in 5-year-old’s murder trial

Prosecutors said the couple had indirectly but significantly contributed to the child’s death because their prolonged abuse and neglect had considerably weakened her immune system and ability to fight the bacterial infection that ultimately killed her.

The High Court heard both defendants had sought to plead guilty to manslaughter – a bid rejected by the prosecution – with the mother contending she was suffering from a recurrent depressive disorder at the material time.

Liu’s expert report revealed the stepmother’s account of her childhood, from being a single child having a poor relationship with her father and experiencing her parents’ divorce at about age 16, to being raped twice in her teenage years, going through an abortion with her then husband, and a divorce in 2015.

The woman then met her current husband on WeChat in February 2016, moved in with his family the following September and tied the knot two months later.

She also told the psychiatrist she had a bad relationship with her husband’s family and it was decided that they should move in with her mother, which took place on August 10, 2017.

A month later, she started to hit her stepchildren, first on their palms, then with a rattan stick on their buttocks and soles, because the boy told lies while the girl exhibited poor self-care and did not respond to her, the report said.

She added that her husband was also involved in hitting the children, claiming he was responsible for 60 to 70 per cent of the beatings and that he was more forceful as he would also throw punches.

She further admitted to the psychiatrist she had tied up the children to stop them from picking their scars, and had arranged for their absences to prevent the school from seeing the beating marks.

On Thursday, her defence counsel Caesar Lo said her childhood experiences “might have sowed the seed for future psychological illness”, and she was depressed at the material time, with symptoms such as sleeping problems, headaches, and feelings of helplessness and desperation, as reflected by her WeChat and WhatsApp messages.

“At first I was truly fond of them,” she texted her friend on December 5. “Now I find them annoying from the bottom of my heart … I don’t even want to talk to them. I’m under a lot of stress.”

A week later she said: “It’s better when the two elder ones return from school – once it’s just the two of us, I really want to die.

“Sometimes I see the injuries I couldn’t bear it,” she continued on December 11. “But once [the child] starts talking, making noises, nagging, I feel so much hate. But I don’t think I would hit [the child] again, I’ve [reached the point of] scaring myself, even when [the child] isn’t afraid.”

Later she texted the same friend: “I kind of want to strangle myself to death. The family of six can burn charcoal together. Quick and easy.”

Lawyers question credibility of boy who testified against parents in child cruelty case

The psychiatrist, however, maintained under cross-examination that these messages were not indicative of depression or adjustment disorder, as she found the mother was just expressing her emotions in a transient moment, without any actual plan to commit suicide.

Liu added the woman had specifically denied having recurrent suicidal thoughts.

The children’s 56-year-old step-grandmother also denied four counts of child cruelty.

None of the defendants or their relatives can be identified due to a gag order from Mr Justice Albert Wong Sung-hau, aimed at protecting the identities of the children.

The jury trial continues on Friday.