Agree - it IS a curious question. I also agree that it depends on what you mean by ADR. I'm a trained mediator, attorney, and participant in hundreds of mediations as a participant. But what does "mediation" mean exactly. In my experience, it was one thing - a very clear procedure between people who were representing business interests and negotiating on the sidelines. The training was far more oriented toward the goal of what I will call "personal" win-win results.
So the first was about money, and win-win took into account the expenses of litigating in the United States - expensive - as well as the potential results of litigation which is a risky business, as well as expensive. Other factors to consider in this type of mediation is that the parties to the mediation, as businesses with past experience, may well want or need to continue doing business together AND be concerned about their reputation in the business or commercial world. Each mediator engaged to assist the parties was experienced and understood that the parties sought a financial settlement or similar agreement (example, how much will an insurance company pay for a house that as destroyed by fire ? . . . with two different sides to the dispute)
In this scenario, I usually refer to this type of ADR as "business mediation". The people in the mediation were all experienced negotiators and we came with plans regarding what we would, and would not, agree to in the end.
The trainings I took had a real focus on the human side of conflicts. Family members in conflict, landlord and tenants, that kind of thing. The model for mediation here involved two educated and experienced mediators. As in the business model, each party was spoken to separately either in a room with chairs, in another room or in the hallway. There was -0- pressure applied by the mediators in this type of mediation, and the results could well be that a teenager agrees to go back to school, a husband says he is sorry, or the neighboring family agrees to fix the fence that fell down after their dogs jumped numerous times on the fence to follow a cat over the top of it.
In addition to mediation, there are contracts that many people sign which have language that, by signing, compels them to resolve any and all disputes between the parties using "Arbitration". Arbitration is more like a mini-trial headed by a paid, trained and experienced arbitrator who will decide who "wins". It is more formal than mediation in general but it is not a full-blown case brought before the usual civil courts by attorneys representing one of the parties and calling upon the other to answer.
These alternatives are not right for all civil situations, although, as an example, over the last 30 years, more and more U.S. State courts have required "mediation" of divorce cases before being permitted to file a formal case that would trigger litigation. You can imagine that not all marital disputes can be resolved by mediation - they do not generally submit to Arbitration. These end up in court. As an aside, in all types of mediation, it is not necessary to resolve the entire dispute. Sometimes it is possible to reach agreement on part of the dispute - having other issues still at play for later resolution.
Criminal law is really something entirely different and I have problems with the use of ADR in criminal cases unless it is limited to certain low-risk situations and perhaps arbitration versus mediation.
Why do I feel that way? From time to time, place to place and crime to crime there can be huge differences in how convicts are treated under criminal laws, including in terms of punishment. In addition, using it as a standard raises too many questions although, again, there can be situations where part or even all of an alleged offense can be resolved by simply talking or by mediation.
Some examples of the issues raised by your question: Who will assess the defending party's ability to understand right from wrong; who will assess whether a person has the legal capacity to make formal agreements as often are reached through mediation. Civil law in the U.S. limits who can enter into contracts lawfully, based on age and other factors. If a party lacks this legal capacity - what binds them to uphold the promises they made? Not the law. Who has the intelligence, awareness, mental health to both obtain information, to evaluate it, to communicate productively, and what if one person has some, but not all of these attributes? When would mediation take place in the timeline of a criminal investigation or prosecution?
A related question is: How do we know when a person accused of a crime is actually guilty of the crime? What if they have been judged guilty, but have an appeal that will be decided in the future? What would the effect be of mediating and reaching an agreement - if the appeal results in a reversal of a guilty judgment? How would the pressures of mediators or those on the other side of the mediation effect an alleged perpetrator's willingness to agree? We know that many confessions result in statements of guilt - even when the person admitting guilt was not, in fact, guilty. Native intelligence and time, mental age, experience, literacy, fear, health, and intimidation and social pressures - to name a few - can all affect a criminal defendant participating in ADR.
That said, trial procedures differ throughout countries that have the will and tradition of using them and providing representation for those accused. The process I'm familiar with is the adversarial process with a heavy emphasis on the use of evidence and many rules that are applied to ensure that evidence if believable and without bias. Under these circumstances, while I believe that "doing justice", which is the job of prosecutors, does not always require jailing a convicted criminal - other times, it does: whether the reason is to take a violent person away from his or her victims, to deter future violence by the convict and others, to remove a person from an environment that encourages criminal behavior and, while they are incarcerated - teach them new social skills, build confidence through learning self and facilities care, help them prepare for a different life in a different place and so on, or whether the so-called "punishment" has the purpose of retribution - an eye for an eye, a tooth for a tooth.
Are there situations that are more likely to invite ADR as the best means for resolving a social problem? I believe there are - but I would not promote the idea where one's liberty/confinement is at stake, and similarly, I would likely limit them to situations where a truly capable group of educated and experienced professionals ensure that the participant is protected in terms of what they say and any subsequent use of it, that they are capable of making knowledgeable decisions have their own advisor available during the mediation, and that the person participating has the literal ability to benefit from the procedure whether the allegedly injured person or the person accused.
These are my thoughts for today for what they are worth. 8^) I am sure there are others with equally interesting thoughts on this subject!
Prior to the invasion of administration of criminal Justice in Africa by Western model, ADR had been part and parcel of offense disposal and it has continued to be relevant to as a match towards reviving restorative justice globally.