Self-diagnosis is the process of identifying a medical condition in oneself. You look at signs and symptoms that you have and try to match them to a medical condition. This process is prone to error under the best circumstances and potentially dangerous under worse ones.
In the autism community, there are some number of adults who have self-diagnosed themselves with a form autism. They may refer to it using other phrasing, such as ASAN's "self-identif[ied] with the Autistic community", but it is really the same thing. These people have looked at themselves and decided that, based on their behaviors, they have some form of autism.
The people will typically say that the self-diagnosis was a wonderful thing for them or how it changed their life. And while, at first glance, it might seem like there is nothing wrong with this practice, it has been my experience that self-diagnosis is almost never a good thing for anyone involved.
In my opinion, if a person thinks that there is something wrong with them, they should not try and guess what it is themselves. They should instead go talk with an unbiased professional who will be able be able to help them. This is true for strictly physical problems and is doubly true for profound mental problems such as autism.
There are many reasons for this as a general statement, but here are some specific ones for autism.
1. Everybody has strengths and weaknesses. Everybody has things that they are good at and other things that they struggle with - this is "normal" and expected. So just because you have problems or struggle in some areas does not mean that you have a mental disorder. Mental disorders are defined by their extreme behaviors - behaviors that are so far out of the realm of what is typically seen that they cause problems.
Autism is (currently) defined by profound impairments in three areas - communication, socialization, and repetitive/restricted interests and activities - that must appear in early childhood and that must act together to impair every day life.
For example, the ability to focus on a subject and become extremely knowledgeable is well within what is considered "normal". You don't have autism (or a "trait" of autism) simply because you find a subject very interesting and can spend a lot of time learning about it. It is only when the ability to focus turns into a constant fixation or obsession that you go from what is "normal" to something that isn't.
So, to be perfectly blunt, just because you are don't relate well to others and have some strong interests does not mean you have autism. Along the same lines, just because a person is a "geek" or in some other technical profession doesn't mean that they are automatically on the spectrum somewhere.
It means that you have areas that you need to work on and improve in, just like everybody else.
2. It is hard enough for a "typical" person to objectively and accurately analyze and understand their own behaviors. If you do actually have a mental disorder then that means that your ability to understand and objectively analyze your own behaviors will likely be impaired. This is true not just in autism but most of the other major mental disorders such as ADD, schizophrenia, depression, and addiction, to name just a few.
To put that another way, if you are relying on your own opinions and observations about your behaviors and you do actually have a disorder then you will most likely be getting it wrong.
3. There are very few "objective" measures of autism and all of the reliable ones require a trained professional. But there are some rather crappy ones that are promoted online by various that will likely give you the wrong idea.
Probably the most well know and most abused of these is the Autism Quotient. The AQ is a meant to be used as a screening test in an appropriate setting with appropriate supports in place. It is not meant as a test to see if you have autism. Even if you get an extremely high score that will only raise your chances of having autism to like 1 in 10. And the AQ cannot tell the difference between autism and other conditions such as schizophrenia.
I cannot count the number of times I have run across people who have decided that they have autism based on their own observations of their behaviors and their score on the AQ. Some of these people might well have a form of autism but the numbers suggest that most would not.
4. Even if you do have some profound problems in the three core areas of autism, that does not mean that you have autism. There are several other conditions, such as ADD, schizophrenia and OCD, that overlap with autism and it can be difficult to tell them apart. You have to consider the complete picture of all of the behaviors that a person has, when those behaviors appeared, as well as what behaviors are missing to be able to tell them apart.
This is why an objective, unbiased professional is critical to the process. They should know not only what the different conditions look like and what the signs of each are but also how to tell them apart. They will know the little things like an AQ isn't a diagnosis and that a high score could mean schizophrenia or autism.
Accurately determining what (if any) condition you have is absolutely critical for the next point.
5. You have to make sure that any treatments are appropriate for your condition. If you taken it upon yourself to decide that you have a condition then you may use treatments that are completely inappropriate for your actual condition. More importantly, if you have decided on the wrong condition, you could miss out on treatments that could really make a difference in your life.
If you have decided that you have a form of autism but also decided that you don't need any form of treatment, then I would ask why you think you have a disorder in the first place. The point of a medical label is to identify a condition so that you know what you are dealing with and how to treat it. A medical label is not an identity.
(And no, "the rest of the world needs to just accept who I am" is not a treatment. Not that acceptance itself is bad or unneeded, but it will rarely be enough to give you back the ability to function in the world. More on this in a bit.)
6. You have to ask yourself what a self-diagnosis of autism will actually do for you. While at first glance it might seem like the label can help you make sense of your life, it really can't. The autism label might provide a plausible sounding rationale for your behaviors but the label alone doesn't necessarily help you deal with or improve those behaviors. For that you need an objective person who will be able to consider all aspects of the behaviors and help you arrive at an appropriate strategy.
And if you are just looking to use the label of autism as an ongoing justification for your actions and are refusing to change yourself in any way (i.e. you are in the "autism is just a difference camp"), then I have two little words for you - grow up. You are supposed to change and adapt your behaviors in response to your environment, even when you would rather not and it is uncomfortable to do so. Autism is not an excuse for behaving badly.
Just to be perfectly clear here, if you are disabled by a mental disorder then you are entitled to the same rights that everybody else is. You are not less of a person simply because you have a disability. You have the right to expect that others will still treat you as an equal and you have the right to expect that accommodations will be made to help you function better.
But if you have the ability to change and improve yourself but choose not to because you think that you are defined by a medical label then you really do have a problem. And the problem isn't that you have autism.
Which leads to the last and very unfortunate problem.
7. There are certainly many people in the world who do have autism, who struggle because of it, and who are able to speak about it publicly. These people provide an invaluable service because they enable the rest of us to better understand what it is like to have autism. I applaud their courage to speak out about their condition and am extremely grateful for the insight they provide.
But then there are a number of people who have self-diagnosed with autism and then rush to tell the word "what it is like" to have autism. These people often spread inaccurate information about what autism is and what effects it can have on a person. These people cause direct and lasting harm to people who actually do struggle with autism by giving a false impression of what autism is and by drawing attention away from people who are actually disabled by their autism and truly do need the help.
These people often encourage other people to self-diagnosis or "look into it" for themselves. They are often the people who use autism as a justification for their actions and claim that it is the rest of the world who have a problem and not them. These people certainly do have issues but, in my opinion, that problem is rarely autism.
So, for these reasons and quite a few others, if you think you have a serious mental disorder such as autism, please don't try and diagnose yourself. Instead go get the help you need. There is no shame is trying to get help when you need it and you have everything to lose and very little to gain from diagnosing yourself.
P.S. The practice of diagnosis shopping, i.e. going to multiple professionals until you find one that agrees with your self-diagnosis, is almost as bad as diagnosing yourself in the first place. There is certainly a time when a second (or third or fourth) opinion is certainly a good idea but there is a clear difference between trying to find a professional who can help and going to multiple professional until you find one that agrees with you.