As he attempted to explain why he took his two children and jumped off the balcony of his hotel room in Crete on 15 August . . . marital discord, business failures, the suicides of two of his brothers and his father's death from multiple sclerosis . . . his actions of that night remained beyond the comprehension of many.

Hogan's six-year-old son, Liam, died instantly from massive brain injuries while his two-year-old daughter, Mia, survived only because she was cushioned by her father. Hogan himself never meant to survive. On Friday, he attempted to commit suicide in his prison cell.

The past few years have seen several high-profile cases of parents murdering their children before taking their own lives. It may be a phenomenon that has been around since the beginning of historical records, but there is no sign of it dying out. So what is it that drives a parent to kill their children as well as themselves?

Speaking in general terms on the phenomenon, known as filicide, Dr Damian Mohan, consultant forensic psychiatrist at the Central Mental Hospital, said it can be broken into six categories. These are altruistic filicide, where the mother wishes to commit suicide and wants to take her children with her; acutely psychotic filicide; unwanted child; fatal maltreatment, where child neglect is involved; spouse revenge and paternal filicide. The latter tends to be more violent than maternal filicide and is often followed by the father taking his own life.

"Although altruistic suicide, where the parent takes their own life as well as that of their child may attract more media attention, it should be borne in mind that this type of filicide is relatively rare, " said Mohan. "In Ireland, maltreatment cases [where the child is neglected through the inability of the parent to carry out basic minimum care tasks] are much more common."

"Most murders by parents are in the context of mental illness, depression, drug abuse or schizophrenia, " said Professor Patricia Casey, consultant psychiatrist at the Mater Hospital, Dublin. "Or they may be an impulsive act of revenge arising from something like a marital dispute.

This has been recorded and written about in psychiatric literature for years. It's not just a phenomenon of the millennium; sadly, it's a tragic feature of life."

The most recent and perhaps most shocking of all cases in recent years was that of 40-year-old Mary Keegan from Firhouse, Co Dublin, who stabbed her two sons to death and then took her own life while her husband was overseas on a business trip.

Friends and neighbours of the Keegans expressed their utter shock that the seemingly happy family had come to such a tragic end and everyone initially believed they had been victims of an intruder. When it emerged that Mary had stabbed her sons, Glen (10) and Andrew (6) in the back, stomach, face and neck, no one had any answers as to how it could have happened.

"In a case where a parent is suffering from severe depression, they may believe that the child is suffering as well, or has an incurable illness, " said Casey.

"They may even believe the child is infiltrated with evil spirits. When those thoughts occur, they can often believe that taking their child's life is an act of love.

"If drug abuse is involved, they tend to be so intoxicated that they may not realise they are killing a human being.

Whereas when there are marital problems, it's often a question of reaching for the nearest and most vulnerable object. Unfortunately, that is often the children."

This may have been relevant in the case of Wexford woman Sharon Grace (29) who drowned her two daughters, Michaela (4) and Abbi (3) before drowning herself last year. She had recently separated from her husband and the couple were trying to work out access to the children.

A similar case was that of Mary Collins who, in 2002, pushed her two children into the sea in Mayo. Collins and her three year-old daughter survived, but her 11-monthold son suffered severe brain damage and died. In court, Collins pleaded insanity and claimed she had suffered from domestic abuse at the hands of her husband.

In 1999, Christopher Crowley took his six-year-old daughter Deirdre from her mother's home and went on the run.

When gardai closed in on him in Clonmel, Co Tipperary, two years later, he shot the little girl before taking his own life.

"You can understand why people may have no sympathy for a child-killer, but it should be remembered that it generally occurs in the context of major psychological problems, " said Casey. "It is not something that is carried out in cold blood, except in very, very rare instances."