Sometimes on the podcast we will mention "The Theory" and not expand further, and inevitably, someone on Twitter asks for clarification. I guess I should probably write it down somewhere. We talked about this in the pilot episode towards the end, but I'll give a brief summary here. I originally came up with this after binge-watching the first season and a half of the show over a few days on Netflix.

DISCLAIMER: I don't actually think that this is going to be the secret twist ending of the show. I just think it's fun to watch the show this way, though if you are an Emily or Hanna fan, you may disagree.

The Theory: Emily and Hanna are not real. Emily is Spencer's imagined best friend/alter ego, and Hanna is Aria's. Tulpas, if you will. Aria and Spencer are the only two real people of the four liars, and they are both profoundly insane. Spencer is the killer, and Aria is A.

Now, when we start bringing Ali and Mona into this in the later seasons, it gets pretty dodgy, but it's especially fun to watch the first few seasons this way, noting things like seating arrangements of the liars, which characters drive the plot, how they interact with each other, etc.

Evidence for:

Aria and Spencer are both clearly nuts
Spencer and Emily both seem to live near Ali and Toby
Hanna's parents are divorced due to her dad's infidelity, Aria's parents separate early in the show due to Byron's infidelity
Spencer and Emily are both good students, tall, and athletic
Aria and Hanna both love fashion
Emily was friends with Toby first until her prime self Spencer stole him away
Sparia often team up for major plot developments. E.g. they investigate the Lost Woods Resort while Hanna and Emily wait in the motel room; they meet with Jonah the cell phone guy, the go to Noel Kahn's cabin with CeCe, etc.

And feel free to handwave in your own reasons from here.

-Benjamin