Walter asks Peter about the brain scan, which is what I guess Peter is looking after when he's not busy making mad faces at everyone, and he says there's nothing unusual with the brain scan, but something has to be wrong, because how else does Olivia have memories from the other Olivia that are stronger than her own?
Walter's theory is empathy: during the Cortexiphan trials, he and Belly noticed how well Olivia, even as a child, was able to intuit what people around her wanted. "I suspect that Olivia may be empathizing with your strong desire to be reunited with your Olivia, and she meets that need by, in effect, becoming your Olivia," he says. Peter thinks that's bullshit, because no matter how much he wants to be with his Olivia, he can't make "this one" have memories of things she didn't experience. Walter admits to being perplexed by that as well, but he has a couple of ideas. One is some form of thought transference, and then he can't remember the other one he was going to suggest, much to Peter's irritation. "I lost the thought. Too many drugs of my own," he says. Then he snaps back to business, and asks Olivia if he can take a few strands of her hair to test, and she says yes. Walter plucks them and then gives them to a still-pissed Peter so he can prepare a slide.
Lincoln Lee comes in to see how Olivia's doing. "We don't know yet," growls Peter, because his sunny disposition is such a big help in figuring out the problem here. Olivia tells Lincoln that she feels fine, and he's got another piece of good news: transit authorities found a wad of bloody paper towels at the Roosevelt Street station bathroom. The DNA analysis suggests they're looking for a Caucasian male, but they haven't had any hits from the database yet. Walter has a look at the report, and says he's seen one like it before: Sean, their misdiagnosed schizophrenic. They put the DNA reports side by side so Walter can show them the chromosomal spur that both Sean and the suspect have on chromosome 7, the point being that a spur like that is so rare that it can only mean Sean and the killer are brothers. Actually, half-brothers, because they share just one parent, but Walter says it lends credence to his mental telepathy theory, since there are many documented cases of siblings communicating non-verbally. Usually it's identical twins, but not always.
That doesn't explain, says Lee, how Sean can hear the thoughts of all three of the killers, but it's Olivia who takes Walter's theory to the next logical step and suggests that they're all related. "According to his medical records, Sean is an only child," points out Lincoln, who seems to be getting annoyed with being the only sane one in the place. Olivia saddles up to go talk to Sean's mother, but not before the menfolk -- Lincoln and Walter, anyway -- start to decide for her whether she should be allowed to go anywhere, but Olivia's not having that. "You said there's nothing wrong with me physically. So if something shows up in the tests, let me know. Otherwise, we've got a case," she says.