So what drives a father to kill his children?
STEVE Clark thought, in his own words, that he'd "seen it all" – but yesterday was to bring the detective chief inspector a most shocking and unwelcome surprise.
A crime that has rocked a continent left the Melbourne officer struggling to put into words the almost unimaginable horror that confronted him after a local man, Arthur Freeman, pulled over on a highway bridge, took his four-year-old daughter, Darcey, out of the car and dropped her over the side to her death.
The tragedy shocked Australia, leading national news bulletins and prompting an outpouring of sympathy.
But however shocking the murder, it is only the latest in a catalogue of distressing episodes in which fathers have taken the lives of their children.
Ashok Kalyanjee, 46, was jailed for at least 21 years last week for stabbing his two sons to death before trying to set their bodies alight. Police found the blood-soaked corpses of the two boys in the car with their father slumped unconscious in the front seat of the vehicle, which was parked in a lay-by in the Campsie Fells near Lennoxtown, East Dunbartonshire.
A tape recording by Kalyanjee, recorded in Punjabi, contained the chilling message: "These children are mine and they go with me. This death is near."
The same day, Robert Thomson murdered his disabled daughter and young son, before turning the knife on himself.
A court heard the 50-year-old, from Buckhaven, Fife, was unable to give any explanation for repeatedly stabbing Michelle, 25, who had learning difficulties, and his son, Ryan, seven. At the time he and his wife, June, were going through a divorce.
Speaking after he was sentenced to a minimum of 17 years in October last year, Mrs Thomson condemned the "pure evil and badness" of her husband.
It is difficult to speculate about what was going on in the minds of these two men as they planned these most disturbing and despicable of crimes.
But there are similar features in these cases which experts say can help to explain why such filicides take place.
In Thomson's case, background reports suggested that he wanted to get back at his wife, whom he feared would leave him with nothing after their divorce.
Kalyanjee was also estranged from the mother of his children, although he was given access to them.
In both cases, questions were raised over the men's mental state – but neither was insane.
Dr Vince Egan, a senior lecturer in forensic psychology at Leicester University, divides the killing of children by their fathers into broadly two categories.
On the one hand, there are those fathers who do terrible things as a result of a lack of rational thought, linked perhaps to extreme mental illness.
On the other, there are "callous acts" where fathers will harm, and in some cases kill, their own children as a way to "get at" their former partner.
It's a phenomenon that is likely to continue as increasing numbers of families experience painful break-ups.
While mental illness can be treated, there is no cure for callousness.
"It's about seeking vengeance against their wives – an extension of custody battles, in a crude sense," Dr Egan told The Scotsman.
"These men are thinking, 'How do I get back at somebody if I cannot otherwise upset them, because they care so little about me.' It's a last resort."
He says the almost unimaginable evil of the crime can be understood when one considers just how much can be riding on the success of a relationship.
"People behave badly in marriages and then realise what it actually means – they find they've lost an awful lot in life," he says. "Losing the house and losing money is one thing, even losing love, but losing the children can be tremendously painful. No amount of time can heal these wounds."
It is this degree of pain – and loss – that can turn into anger towards their former partner, and themselves.
Dr Egan says these extreme emotions, suffered over a period of time and triggering, depression and a sense of desperation, can – in rare cases – lead parents to inflict such violence on their children.
"They may not have any disagreements with their children – but they may just want to get at the partner, to penalise them by not letting them have access to the children," he says.
Prof Egan reckons there is little doubt that fathers are more likely than mothers to kill their own.
He gives two reasons for this. One is that men are more likely to lose custody of their children. The other is the distinct biological bond between mother and child.
"Something would have to be very wrong indeed for a mother to kill her own children," he says.
Studies have shown children can be more at risk where the man involved is not their biological father.
Martin Daly and Margo Wilson, two Canadian academics, analysing data from England and North America, found that the risk of being killed by a step-parent was some 50 to 100 times higher than by a genetic parent.
Stepfathers seem to kill more brutally, suggesting greater aggression and hostility. One of Daly and Wilson's surveys found that 80 per cent of under-fives killed by their stepfathers were beaten to death, twice as many as the under-fives killed by their genetic fathers.
Heather Coady, child policy officer at Scottish Woman's Aid, says it is impossible to separate the phenomenon of fathers killing their children from the far more widespread issue of domestic abuse.
Killing your own child can simply be an extreme way of exercising power and control over former partners, she says.
But Dr Ian Stephen, a consultant forensic psychologist who has helped police with numerous murder investigations, identifies an altogether different causal factor that helps to explain some of these cases.
He believes men's role as breadwinner can – when it all goes horribly wrong – place an intolerable strain on some.
Such a backdrop of crushing failure can be seen in the case of the millionaire Christopher Foster, who killed his wife and teenage daughter before ending his own life. Mr Foster's company faced legal action and owed about 800,000 in tax.
It is in such scenarios where mental illness can be seen to play a significant part.
The Daly and Wilson study found that genetic parents were more likely to kill themselves after killing their children – suggesting depression as a factor.
"What can come across as a simply callous act can often conceal severe depression," says Dr Stephen.
ARTHUR Freeman is the latest in a long line of fathers who have killed their children. Others include:
ASHOCK KALYANJEE: Jailed for a minimum of 21 years after slitting the throats of Paul Ross, six, and Jay, two, and trying to set their bodies alight, in East Dunbartonshire last year.
ROBERT THOMSON: From Buckhaven, Fife. Murdered his disabled daughter in May last year.
JOHN HOGAN: Pushed Liam, six, and Mia, two, off a fourth-floor hotel balcony in Crete in 2006. Mia survived.
MARCUS WESSON: Sentenced to death in California in 2005 after being convicted of killing nine of his children, many fathered through incest.
PERRY SAMUEL: Sentenced to 30 years after smothering Caitlin, five, and Aiden, three, in north Wales in 2006, because he thought his former fiance, Sarah Graham, was having an affair.
JAMES HOWSON: Killed his 16-month-old daughter, Amy, at their home in Doncaster by snapping her spine "in two" over his knee. He was sentenced to at least 22 years in prison last month.
MOHAMMED RIAZ: Murdered his wife and four daughters in a house fire at the family home in Accrington, Lancashire, in November 2006.
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