People lie for a variety of reasons. Sometimes those reasons are to hide scandal or the eyes of law enforcement. Sometimes those reasons are to protect individuals from knowing certain information. I’m not going to debate the ethics of lying–whether or when it is appropriate or not.
Paper documentation, since it contains statements by people, can also contain lies. Sometimes those records contain statements that, while not true, are what the informant actually believed. And sometimes those records contain information apparently dreamed up by the informant.
DNA reflects reality. Not the perception of reality that we may, through no fault of our own, believe to be the truth. DNA does not care one whit about family tradition, what it says on the birth record, and what Uncle Herman said on his death bed.
DNA does not reflect the altered reality your grandfather wanted to pass along to his descendants. It does not reflect the modified story grandma told her family to “save the family name.” DNA reflects the parental events as they actually happened, not how someone later wished they had. DNA is not about wishes.
When genealogists take a DNA test they are often hoping to solve an old family mystery or to see “for fun” what their ancient ethnic origins are. Those are good reasons. But there are other discoveries that may be thrust upon the test taker, ones that are unexpected. Discoveries involving parents that aren’t parents, siblings that aren’t siblings, or unexpected siblings make headlines and draw attention. Growing up as an only child and learning another submitter is a genetic sibling can be shocking to say the least. These discoveries can have a significant impact on other people besides the test taker.
But there are other discoveries that could be made as well.
How will you and your first (or second) cousin react if it turns out that you really are not cousins at all, but in fact share no DNA? How will your cousin react to you if you were the one who convinced them to take the test? What if your test turns up first cousins when your family structure, as you believe it to be true and as you have been repeatedly told, suggests no first cousins? Test results can impact other living individuals besides yourself. What if you and several descendants of your great-grandparents and all you match each other save one who matches no one else?
Any discovery into genealogy has the potential to shake up your living family members. Your Aunt Myrtle may argue with what is in the birth record if it does not agree with her view of reality. It’s difficult to argue with DNA.