The jailing this week of Roderick and Mark Newall for their respective parts in the bludgeoning to death of their parents and the disposal of their bodies has led to massive media speculation about motive in this, perhaps the ultimate taboo crime. Was it simply for money, or the product of an affectionless family life? So far the explanations offered have fallen short - the boys were not poor, and they do not appear ever to have been physically or sexually abused by their parents.

An indication of the complexity of motivation in parent killing comes from the only academic study comparing adolescents who murder their parents with those who murder strangers or more distant relatives. It was published in the American Journal of Psychiatry in the 1970s and found parent-murderers suffered much less from poor impulse control and past violent behaviour, compared to stranger-murderers and relative-murderers. The same study also speculated that the murder of one parent by a child may have been the result of a child responding to unconscious wishes or commands from the other parent.

Psychiatrists call the murder of one's parents parricide, and Freudian psychoanalysis has long contended that resolution of the Oedipus complex - the unconscious impulse to kill one's parent of the same sex - dominates even normal psychological development.

Even those sceptical of this bizarre theory might be impressed by how often parricide recurs in myth and fable. Apart from the Greek myth of Oedipus, in Babylonian mythology, Marduk slices up the body of his mother, Tiamet, from which the universe is formed, and in Egyptian mythology Horus decapitates his mother, Isis. Female-perpetrated parricide is extremely rare in both myth and reality; both matricide and patricide are almost always carried out by sons.

Patricide by sons suggests an underlying theme of conflict based on competition between father and son over masculinity and dominance. In healthy families the father can afford to let sons become different without feeling threatened, while in patricidal families a lasting crisis exists between fathers and sons.

In contrast, matricide by sons appears to be dominated by incestuous elements, and the term 'Orestes complex' (after Orestes who, under the influence of his sister, kills his mother, Clytemnestra, as revenge for the death of his father, Agamemnon) has been coined to describe a sexually immature son trapped in a dependent but hostile relationship with a possessive mother culminating in a murderous psychological crisis.

In the only national study of parricide, published in the 1970s, investigators examined the incidence of parricide in France from 1958-1967 and found 62 cases. Parent slaying was more likely to occur in the northern agricultural areas, 65 per cent were of father by son, 20 per cent mother by son and only 6 per cent father by daughter. Sixty-four per cent of perpetrators were characterised by severe psychiatric disorder in which hallucinations and delusions, often about the parents, dominated.

In England and Wales there are perhaps nine cases of matricide a year, with a similar number of patricides, while in the US a recent estimate was that some 400 homicides a year (of an annual homicide rate over 20,000) are parricides.

The much higher rate in the US is partly accounted for by the availability of guns, but one is also struck by the repeated themes of a younger generation who refuse to tolerate parental nagging.

The most recent case is that of the Menendez brothers, currently on trial in California, who on 20 August 1989 shot their wealthy parents, the prosecution claims, for their money.

Others include Wesley Underwood, a 15-year-old Texan, who shot his mother three times in the back after they argued about whether he had misplaced a knitting needle. The district attorney told the judge he was a 'spoiled, lazy young man who got fed up with his mother and got rid of her'.

On the night of 18 February 1988, 16- year-old David Brom told a friend of his plan to move away from home, but when asked what his parents would think of that, David said: 'They won't be around to oppose it.' Later that night, he used an axe to kill his mother and father. The jury concluded he was not insane at the time of his killing. In March 1989 Brian Britton, a 14-year-old New York boy, shot and killed both his parents after an argument over his school attendance.

In Britain guns are much less available, but are not uncommon in farming families - hence perhaps the case of Jeremy Bamber, currently serving five life sentences for the shooting of his adoptive parents, sister and her twin sons at their home in Essex on 7 August 1985. Again the main motivation appears to have been the inheritance - an estate of 300 acres and pounds 430,000 from his parents' wills.

The similarities between the Newall and Menendez cares are uncanny: apparently contented families, mentally healthy, well-educated sons, claims of emotional neglect and money the apparent motive, despite the fact the children were already in well-monied families.

The notion that parents are killed because they are perceived as getting in the way of their children's lives resonates in the Newall and Menendez cases, where the parents were seen by the press as thwarting their sons' financial ambitions.

What is particularly intriguing about this motivation is the fact that we also inherit or are hugely influenced by our parents' own wishes and motivations; so, for example, materialistic parents tend to have materialistic children, and yet it is this very materialism which may eventually produce a murderous conflict between parent and child, if the parent is seen as getting in the way of the child's acquisitiveness.

The Menendez and Newall cases illustrate the classic dilelmma for onlookers in parricide. Are these, as the defence claims, children killing after years of parental abuse or emotional neglect respectively, or, as the prosecution claims, spoilt rich kids who see their parents as merely creditors, to be disposed of if they get in the way of impatient ambitions?

But perhaps the attempt to take sides is the first mistake. Family life is such a complicated web of cause and effect that family therapists have long dispensed the attempt to allocate blame and instead take what they call a systems view - the family is a system where each individual makes a contribution to the eventual outcome, even if that is their own downfall.

The complexity of the situation also explains the prominence of guilt following the crime, which in the Newall case eventually produced a confession directly leading to the sons' capture. The most notorious historical mother-killer, Nero, was guilty of much cruelty, but it was only the crime of matricide that weighed on his conscience and led him to suicide.

Despite the rarity of parricide, the family makes the news repeatedly as a violence-prone institution, from child abuse to domestic violence against women. Yet despite the urgent need to study it, the modern family remains the most private of institutions, insulated from outside eyes and ears. After all the attention even the Newall family life remains largely a mystery.

Dr Raj Persaud is clinical lecturer at Maudsley Hospital, London.

(Photographs omitted)