Key Points:

The mafia invented it. New Zealand gangs adopted it. Now, it seems, omerta - the code of silence - is hindering police investigations of child abuse and domestic violence.

Our right to remain silent is enshrined in the Bill of Rights and derives from a centuries-old common law privilege: we cannot be expected to incriminate ourselves. It dates from a time when most people accused of crimes were uneducated, there was no legal aid and few could afford a lawyer.

In an age when police questioning can be videotaped, and prosecution rules are narrowly defined, there are calls to reconsider the long-cherished "right" of silence.

A blanket refusal to co-operate was cited in the high-profile police inquiry following the deaths of the Kahui twins last year. Police accused the close-knit family group of "hiding behind a web of lies and deception".

Lawyers point out the alleged obstruction did not prevent the police eventually laying charges [and the twins' father, Chris Kahui, was last month committed for trial after a depositions hearing].

But in another case, the death three years ago of 4-month-old Alyssa Wilson, lack of co-operation from family members clearly hampered police efforts to bring a successful prosecution. Alyssa's father, Joseph Wilson, was charged with manslaughter but two trials ended with hung juries.

Last year a third trial was aborted after one of Mr Wilson's relatives spoke to a juror. The Crown decided not to proceed with a fourth trial.

The case had its sorry conclusion at this week's inquest, which no family members attended.

For the police, the law in this area is the law.

"It's often very, very frustrating," says Detective Inspector Steve Rutherford, of Counties Manukau. "You get cases where kids so little they can't even speak are subjected to abhorrent sexual or physical abuse and all of a sudden nobody knows. These aren't usually stranger-danger situations."

It rankles police prosecutors that an accused can exercise the right to silence, but at trial come up with an alibi moulded around prosecution evidence. Defence lawyers argue equally vehemently that the police should disclose all the evidence against their client from the outset.

"Court should be about establishing truth or innocence, not people hiding behind certain barriers put there to assist them," says Rutherford. "It would be nice to see some real balance brought into it - I'm not quite sure that balance is there at the moment.

"But at the end of the day, if that's what the Government and the Law Commission and the Court of Appeal want, that's fine."

The long-running police probe into corruption allegations against MP Taito Phillip Field was also allegedly hampered by witnesses' refusal to co-operate. Is it happening more often?

Police say the code of silence has been part of society for a decade or more. "You can go back to Lillybing [Hinewaoriki Karaitiana-Matiaha, a toddler killed by shaking after a history of sexual abuse] or the Wanganui gangs," said one source.

Lawyers say even some judges favour doing away with the right to silence - although that's hearsay and is inadmissable.

But to suggest it's time to change the law is to run into sage shaking of the head, as if you wanted to remove a foundation stone of a teetering justice system.

"Forcing people to answer questions is entirely counterproductive and unlikely to achieve anything," says Warren Young, deputy head of the Law Commission. "All you are doing is encouraging people to tell lies.

"One way to force people to answer questions is to put them on the rack - that is where logically that sort of requirement ends up leading you. When people won't answer questions then you devise methods to make them and that's a highly undesirable track to go down."

Professor Mark Henaghan, dean of the Otago University law faculty, has a similar view. "The public really want blood in these situations - they often don't care who gets convicted as long as someone gets convicted," he says.

"If you look back through history that's the reason these things were brought in in the first place. People under police questioning feel very vulnerable - there's an uneven balance between the individual and authority."

Forcing people, particularly witnesses, to talk can produce unreliable statements made under duress, says Criminal Bar Association president Graeme Newell. "Witnesses may make self-serving statements which can't later be tested in cross-examination. People may make untrue statements through panic or stupidity which later influence a verdict."

Retired QC Kevin Ryan drew raspberries from learned colleagues last year when he called for the right to silence to be scrapped. He is unrepentant: "We're now living in a century where there's so much protection pre-trial police should be entitled to get an answer."

Yet the privilege is not as sacrosanct as is made out. Exceptions include serious fraud cases, traffic offences, customs and fisheries - where we are compelled to answer certain questions or our silence is held against us.

In the area of domestic violence, the new Evidence Act has axed the long-held "spousal immunity" privilege, where a spouse could not be called to testify against his or her partner.

Even in England, where the common law principle was founded, the right to silence is no longer universal. Since 1994, British judges can instruct juries to draw an "adverse inference" about an accused who maintains a right to silence until after police have disclosed their evidence, but then comes up with an alibi. This effectively means a jury can take silence as an indication of guilt.

The Sensible Sentencing Trust believes New Zealand should adopt the English system - and extend it to pre-trial processes. "If they choose to use their right of silence we believe the judge should inform the jury," says trust spokesman Garth McVicar.

Under current law, prosecutors risk censure if they point out to the jury that the accused has waited until trial to open their mouth.

McVicar says it's one of the flaws of New Zealand's adversarial system of justice: "It's more about the best man may win on the day rather than establishing the truth."

He says there are merits in the inquisitorial approach used in some European countries, where judges can lead investigations and compel witnesses to co-operate.

"If the right to silence is an impediment to establishing the truth we have to find a way around it."

And there may be ways which stop short of threatening the right to avoid self-incrimination, or otherwise eroding civil liberties.

Under the Texas penal code, it is a crime to fail to report the sexual assault of a child, punishable by a US$4000 fine or up to 12 months' imprisonment. In Australia, reporting of child abuse is mandatory and most states make it a crime to conceal serious offences.

Another option would be to allow investigators to haul reluctant witnesses before a judge and seek an order compelling co-operation - at risk of contempt of court if they refuse. Witnesses fearing gang intimidation could be offered police protection.

Warren Young says the Law Commission has proposed a change to the right of silence in criminal pre-trial processes once a suspect has been charged.

"We don't think it is desirable to force offenders to give evidence in court, but where people plead not guilty they should be required to indicate what it is they are denying."

This would have the effect of establishing the issues in dispute - "do they deny that they did it or were they acting in self-defence, etc".

But New Zealand stops short of criminalising people for a failure to do things, says Young. Mandatory reporting of suspected child abuse overseas has led to dramatic over-reporting and is a huge drain on resources, he says.

Graeme Newell believes we should avoid the temptation to legislate in response to a handful of high-profile cases. Depositions hearings are an opportunity to compel reluctant witnesses to give evidence.

"If people aren't going to come forward about a crime that they've seen, I can't help but wonder whether having an offence provision is going to change that.

"The better position is if the community is able to encourage people to speak up rather than be forced to by the authorities."

Newell suspects the current concern about unco-operative witnesses is fuelled in part by media expectations and the demand for a quick result.

"Many police investigations take months to complete before witnesses come forward and tell the truth. It may be a factor of time before the suspect's community is prepared to speak about them. Responding to that delay in co-operation with a law is probably bad law."

Auckland University associate law professor Scott Optican says the mooted alternatives are "too tough".

"I'm not a believer in putting people in jail to coerce co-operation. People get the idea the Crown and the cops have no power to enforce co-operation but that's not true - there are formal legal mechanisms."

For instance, a 15-year-old who refused to testify against her father in a sexual assault case spent a traumatic 45 minutes in a court cell after she refused to testify.

Optican says the right to remain silent doesn't stop police getting statements. "People want to talk for different reasons. Some want to confess, some think they will get more lenient treatment, some are keen to shift blame."

He acknowledges that gang members and those "familiar with the criminal justice system" are less likely to talk.

Steve Rutherford believes the justice system should be about seeing guilty people identified and punished.

"If a detective sits down and talks to a person and asks common sense questions and the person has something to say about it - there's their opportunity, not coming back 12 months later and taking the witness stand and telling us things have rekindled in their mind.

"I think it's an issue for learned experts to have a look at."