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It is never safe to simply tell someone that they are transgender when they haven't asked themselves, even when you are 100% certain that they are. You can educate them on gender dysphoria and you can show them parallels between their feelings and your feelings, but you *cannot* simply say to a person, "You are transgender".
Why? Because most of the time they won't believe you.
Internalized transphobia has indoctrinated us all to believe that it's impossible that we are trans, or that being trans is something negative and reviled. Pressures from within a person's family or from their upbringing can make it extremely hard to accept themselves.
Trying to tell someone who isn't already questioning that you think they're transgender triggers a self-defense mechanism; their subconscious actively tries to reject the statement, and there is a high probability that the suggestion will not only push them further into the closet, but can even make them hostile towards you for making it. Many transphobes show clear evidence of fighting their own struggles with gender, and there is no shortage of trans people who [have a history of being transphobic](https://curvyandtrans.tumblr.com/post/661595258598113280/interview-with-an-ex-radfem) out of self-preservation.
Even when the person accepts your declaration, the fact that you told them instead of letting them discover it themselves leaves an opening for their own self-conscious to instill doubt about their dysphoria and believe that the idea was suggestive, or that they were manipulated into believing they were trans. The only safe pathway forward for someone to learn they are trans is to realize it on their own.
Finally, the entire purpose of being trans is self-assignment and self-actualization. Telling a person that they are trans is surely as coercive an assignment as what was done when they were born. If you want to help them figure themselves out, tell them about your life, tell them how dysphoria works, send them to this site, and give them ways to see how what they experience isn't something that cis people live with.
Unless, of course, they ask you if you think they're trans... then the prime directive no longer applies.
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