This is the majority opinion. Tate's ability to interact with solid objects is also seen in Moira, and we know she's a ghost. In "Home Invasion", he was able to teleport throughout the house and walk through walls (though he is the only ghost we've seen to do this). Additionally, the ghosts of the students he killed were said to have been in their thirties, had they remained alive. Tate, on the other hand, remains a teen. This has been confirmed in "Piggy Piggy" by Constance's medium friend. She refers to Tate as one who "doesn't even know he's dead" and urges Violet that she must help him "move on."
By definition, a living human who can see ghosts. Given the frequency with which this occurs with random house visitors, this ability seems to not be needed to interact and see the spirits within the house.
Constance said she had a perfect son and that he was taken from her by "other things." This leads to the the speculation that Tate is Constance's son and may have been corrupted by Infantata as a child. Interestingly, Tate appears to be the only ghost in the house who interacts or has an established relationship with Infantata.
In the attic/ 1970s section of "You're Going To Die In Here" on the FX website, the following story, which supports this theory, is written on the wall:
"Alex was small and didn't know how to talk yet. But he had friends. Every night when his mom would put him in bed, he would play with a small monster with a really dark face. Alex wasn't afraid of the monster because the monster told Alex all about dying and that it wasn't that bad. When Alex told his mom about the monster, she sent him away to camp to get rid of his bad ideas. Alex hated the camp, but it was OK because the dark monster came with him. After a week of trying to be a better baby, Alex got to go home. He promised his mom he wouldn't talk to the monster again and Alex really tried. But the monster knew so many things. He taught Alex things that most babies don't know and Alex liked that. So they kept playing. And then the dark monster invited other friends to play. A black cat and a rhinoceros. They were great animals and Alex loved them. But the dark monster told Alex that he had to kill them or else he would tell his mom about their playtime. So Alex asked the dark monster for a knife and he cut up the animals and put them in a drawer. No one ever found the animals even though they looked and looked for twenty-four hours. It was hopeless. But Alex knew where they were. And the dark monster never told Alex's mom about what he did. Ever."
This story, if it is connected with Tate, would also explain why during therapy he expressed the idea that death is freedom for his loved ones and how Tate's fantasy about killing people he likes may allude to his desire to make peace for the spirits in the house to escape the cycle of murders.
This theory is additionally supported by Tate's constant need to "be good." He states to Dr. Harmon several times during their meetings that he wants to be good and that he doesn't want to have "these bad thoughts anymore."
Tate murdered his other sibling, a sister, as a child
Related to the above theory, as described and seen in the Attic (see here for further detail). He could also have found his murdered sister.
Tate seems to be connected, acquainted, or knowing of all the entities, ghosts, and other beings that live in the house. Violet's initial attraction to the house, despite its nature, could also be evident in her interest in Tate. Tate's feelings for Violet could be due to her (presumed) connection to the house. Additionally, Tate often proclaims his "innocence" in all of the murders in the house since the Harmons moved in, yet he has been involved in all but one. Tate may very well be a personification of all the malevolent energy (for lack of a better word) in the house.
It was revealed in episode eight that Tate is indeed the Rubber Man.
Tate has a twin
In the first episode of American Horror Story, Tate sees a bloody version of himself standing behind Ben during their fist session. Upon closer look, you can see that this is not Tate. The character is actually dressed identical to him, but has shorter hair. We could say that the shots are from the SWAT team, but Tate was shot in the chest. This may or may not be his twin. In addition to this, other evidence could back up this theory, such as:
In a promotional poster for the show, you can see twins boys sitting up in the left corner of a room in the Murder House. They appear to not be the twins (Troy and Bryan) who were murdered in the 70s. They look younger and are dressed differently, whereas Troy and Bryan are always seen wearing the same striped shirts and jeans. Also, one of the boys appears to have shorter hair, which is similar to the boy Tate saw in the corner of his room. (NOTE: Another possibility is that the actors portraying the twins were changed after the promotional pictures were taken.) (NOTE: When they show Tate as a little kid, he appears to be wearing the same clothes as one of the twins in the promo poster).
Some scenes, such as ones seen in "Piggy Piggy", take place in 1994 and Tate appears to have scars on his face. Yet other times, the scars are absent. In addition, it is presumed that Tate would have evidence of being shot like Nora and Moira, who were both killed by gunshots. In the sneak preview of "Rubber Man", Tate is seen topless without any scars or evidence of his death. (NOTE: There could be a significant reason for this, or it could be a goof by production.)
When Violet's reading online about the High school shooting, it says there are two shooters. Tate's twin could be one of them.
In the scene where Chad and Patrick are trying to re-do the twins' nursery, they have two cribs. When Constance walks in, she asks, "Where did you find all this old stuff?" and seems mesmerized like they were hers. The three children we know of, though, are too far apart in age to need cribs at the same time.
We have met three of Constance's four children and the twin could be her fourth child. Ryan Murphy has stated that it is possible that we will never get to see Constance's 4th child.
Ben and Tate are siblings.
Ben could be Tate's brother, and Constance's fourth child. He may have been institutionalized in his teen or childhood years for doing something bad. It's also possible that Troy and Bryan are young Tate and Ben (note the same first letters of the names). Also, Troy and Bryan are always seen wearing striped shirts and jeans and images of someone who looks like Tate wearing a striped shirt and jeans appears during one of Tate's therapy session with Ben.
disposition2=proven|option2=Tate has an alter ego.
In the pilot episode, Tate describes his "fantasies" to Ben, which include him strolling through a busy highschool hallway dressed in civil war attire and skull makeup with his hair slicked back. In actuality, as it is shown in "Piggy Piggy", Tate wore the jacket to the school shooting but kept his hair and face the way it usually is. It could be presumed that Tate's fantasy of his school shooting could be his imagining of how he sees himself/ or an effect from the crystal meth he is seen snorting that morning. Another part that shows this is later in the pilot when Tate writes "Taint" on Violet's chalkboard in her room, in no relation to anything currently happening in the plot. It is likely, him being a psychopath, that Tate created an alter ego for himself to cope with the violent deeds he seems to remorse and grieve about over time.
|disposition4 = proven