What is reverse genealogy and why do you care?
First, I wouldn't say reverse genealogy is the opposite of genealogy. I won't be answering the title question, I don't know what's the opposite of genealogy, maybe hiding your ancestry? Reverse genealogy can help you if your family tried to hide their ancestry.
Let's get back to what it is, though.
What is reverse genealogy?Reverse genealogy is searching for the living or descendants. It's doing genealogy in reverse (not "opposite"). You start in the past and move to the present. However, you still start with what you know.
Why?Why would you want to find the living? There are various reasons and they can be quite personal. I want to mention some of the personal reasons because they might be of most interest to "non-genealogists." If that's you and you've gotten to this post, I want to help you figure out what else to learn about.
Non-genealogists might want to do reverse genealogy because they can't do traditional genealogy. They might have a non-paternity event in their family history they want to learn more about. Examples can include an illegitimate birth, an adoption, or a mystery paternity event.
The last, the mystery paternity event might not technically be a non-paternity event because you know the name of both birth parents but you don't know anything about them. That's just like an adoption case where the birth parents' names are known.
Some people don't think of it that way because it's not an adoption. Someone recently asked me about this because they had an in-law who's great-grandfather died at Pearl Harbor. The child was legitimate and not put up for adoption, but they still knew nothing about her father. If that person wants to do research, she should learn about researching adoptions and illegitimacies even though her case doesn't involve either.
All three types of cases are the same from a research standpoint, a biological parent who's name is known but nothing else. Adoption cases might have additional records (if they are recent enough for adoption records to exist) and additional legal hurdles. Illegitimacy cases might involve more mis-information. My point is. use resources recommended for each type of case as they might be equally relevant. It doesn't hurt to expand your knowledge into related specialties.
Reverse genealogy for genetic genealogyAll the talk about using DNA for genealogy has gotten lots of people interested in solving 20th century problems of paternity. Genetic genealogy (using DNA testing for genealogy) is perhaps the number one reason people consider reverse genealogy, today. There's just as good a second reason I'll mention in a moment.
No matter why you're considering using DNA (if it's an adoption case or just general genealogy---any reason), reverse genealogy is important. Genetic genealogy requires your test be compared against other tests. All testing companies offer the option for you to compare your results against those (who have given permission) in their database. You have no idea what you'll get.
If possible, it's better to (also) hand pick additional people to have tested to see how their results compare. To find new test subjects, you can use reverse genealogy. This applies to any type of DNA test. The difference is in who can be tested (male or female and if they are in the appropriate line for the type of test). That's a different post and one many people have already written.
Reverse genealogy for traditional genealogyReverse genealogy is useful in a traditional genealogy problem for another reason. It identifies subjects for cluster research or the "FAN principal." In this case, it may not be necessary to research living descendants, the purpose may simply be to find descendants of someone.
The purpose is to identify descendants who will have additional and different records. When you get stuck, researching collateral relatives can provide new research opportunities. Sometimes you research collaterally in the same generation. Sometimes you can research backwards (your ancestor may be unknown but you know he or she is the niece/nephew/cousin of someone).
Researching backwards is often not an option, though. If you try to stay within the same generation, the farther removed the relative, the less helpful they become. What is often possible, and the most helpful, is to identify descendants of the known relatives in the generation you're stuck on.
The descendants will become farther and farther removed in relationship to you, but they may still be the descendant of your problem ancestor. Just because you wouldn't invite them to the family Christmas doesn't mean they aren't useful for genealogy.
This is just a little information to introduce you to the what and why of reverse genealogy. The follow-up post with more details about "how," is available here.
Have you used reverse genealogy for a particular project? Leave a comment.