When and attorney is getting ready to go to trial, one of the many things that has to be explained to a client is the standard or level of proof necessary to prove the case. In the U.S. there are basically three levels; a preponderance of the evidence, clear and convincing and beyond a reasonable doubt. For a simplified discussion see Burden of proof in Wikipedia. If a legal case is tried to a jury, the jury is specifically instructed by the Judge as to the standard of proof through written statements called jury instructions. For example, here is some wording from a common jury instruction about preponderance of the evidence:
"Preponderance of the evidence" means evidence that has more convincing force than that opposed to it. If the evidence is so evenly balanced that you are unable to say that the evidence on either side of an issue preponderates, your finding on that issue must be against the party who had the burden of proving it.
About a hundred years ago, some genealogists began applying these legal standards of proof to genealogical research and kinship determination. This application has become generally accepted in the professional genealogical community, but outside of the professional realm, the fact that there even is a standard is not generally known.
In my experience in hundreds (probably thousands) of law cases, I have seldom found a client who had the vaguest clue or understanding of the standard of proof. They figured that was why they hired an attorney. However, usually, the practice of law is limited to those instructed in the law. In the case of genealogy, the vast majority of the practitioners have no instruction at all in the standard of proof and so it is completely disregarded.
This lack of knowledge on the part of the non-professional genealogical community causes some hand wringing on the part of the professionals and a few classes and seminars, but the existence of a standard really has little or no impact on the majority of those researching their family lines.
The Genealogical Proof Standard (sometimes referred to as the GPS, which is somewhat confusing given the very common use of that term in direction finding, but there is a nice symbolism in the relationship of the two terms) goes way beyond the mere legal standard of proof. For a reference see The Board for Certification of Genealogists articulates the standard in detail in their BCG Genealogical Standards Manual.
The fact that someone would read a blog on this topic probably indicates that the reader is aware of a need for a proof standard in genealogy, but I have found very few of the day to day researchers at the Mesa Regional Family History Center and even most of the volunteers, who even knew that genealogy blogs exist, much less were aware of any kind of standard for research.
Notwithstanding this situation, I think it would be a great loss to the world if people were forced to hire professional genealogists to do their own genealogy, like a person is forced to hire an attorney, in most cases, to represent them in court. First of all, unlike attorneys and the law, most people don't even know professional genealogists exist. It is true that from time to time people attempt to represent themselves in court. However, in my long experience, this usually results in a disaster for everyone involved, including the court system itself. The only time self representation works in the court system is with very small claims. Unfortunately, even hiring an attorney doesn't guarantee adequate representation. One of the most popular features of our local Bar Association Journal is the list of attorneys disciplined by the Bar Association for failing to follow legal standards of one sort or another. Likewise, hiring a professional genealogist would not guarantee adequate family research.
Although there are professional genealogical organizations, unlike the law, they do not have the ability to force all of the genealogists to join and adhere to their standards. In today's world, anyone wanting to be a professional only needs to advertise and begin charging. There are no state laws requiring genealogists to be certified. Certification is very desirable but not required and unfortunately, few people know that certification exists.
Now back to proof. Without a standard, there is no way that anyone can determine if the research done on a family line is valid or not. For example, in reading a lot of surname books published by families or individuals, there is a vast difference in the validity of the information presented. I find many books without even one source citation and others where the source citations do not correspond to the information presented. Without a standard of proof, there is no way to determine whether a claimed kinship is valid or not.
As an attorney, I am appalled at the lack of standards for most of the research I see every day. Meanwhile, I will just keep writing and teaching and hoping that a few more people interested in genealogy start to adhere to some kind of standard in the presentation of their family history.