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Types of Legal System: Adversarial v. Investigatory Trial Systems

Mirroring the difference between Common Law and Civil Law systems is the difference between adversarial (also known as accusatorial) and investigatory trial systems. England and Wales, and most if not all common law countries, have an adversarial system. Civil Law countries, such as France and the Netherlands, have an inquisitorial system. But it does not follow that if you have a common law tradition you must have an adversarial trial system, or that if you have a Civil Law system that you must have an investigatory system. Indeed a number of Civil Law countries are edging towards an adversarial system and the Royal Commission on Criminal Justice (1993) (appointed to investigate the causes of miscarriages of justice in England and Wales), was encouraged to consider the potential benefits of this country developing a more investigatory trial system.

So what are the basic differences between the two systems, appreciating that there will be many differences of details even between countries running similar systems? In our adversarial system the prosecution accuses the defendant of certain specified crimes. The prosecution then has to prove the defendant guilty, of those charges, before a neutral judge or jury. The prosecution and the defence call their witnesses and examine them before the judge and any jury. The judge can ask questions but should only do so to clarify matters, not to investigate, argue or prove the case for him or herself. That is the lawyers' job. The lawyers, for prosecution and defence, will argue their best that the defendant did - or didn't do it. The assumption is that, by allowing the lawyers maximum freedom to find, present, assess and challenge the evidence, the truth is most likely to be found. Once the prosecution and defence have finished examining all the witnesses that they have called, and they have made a summary speech and argument, the judge or jury will make a decision. They must decide whether the prosecution has proved the defendant guilty, beyond reasonable doubt. If they have a doubt, and they consider that to be a reasonable doubt, they must acquit, must find the defendant 'not guilty.'

However in an investigatory system, basically, a judge is in charge of the inquiry into the allegations right from the start. These judges control the police investigation, specifying the enquiries they want undertaken. Over time they prepare a written document, a dossier, of the evidence they have collected. In due course this dossier is presented to a court , with a different judge, which will inquire into its accuracy. The court calls and questions any witnesses it wants to hear from. It investigates the sufficiency and accuracy of the dossier and reaches a verdict.


The adversarial system is the one we regularly see portrayed in films and on television. It appears natural, appropriate, exciting. Whether it is more reliable than an investigatory trial system - for both can be challenged and criticised - is one of the great questions about the law.

Notice that, within the adversarial system, the issue is whether the prosecution's charges are sufficiently proved. It is not, directly, about guilt or innocence! The court does not, as in an investigatory system, inquire into whether the defendant is guilty. It is only allowed to consider the specific charges. It does not find people innocent. It may declare someone 'not guilty,' but that simply means that, they have not been found guilty to the standard required. Defendants, who have been acquitted, may declare to the world that they have been proved innocent. But they are wrong. They may actually be innocent - we can never know - but being found not guilty, of the specified charges, is not the same as being proved innocent! Perhaps our criminal courts should inquire into innocence as well as guilt. Perhaps they should pronounce people innocent and not just that they have not been found guilty. But they do not and, under our system, cannot.

There are, of course, other types of legal system. But we are only concerned with these two.