I feel so sorry for (spoilers)
Bob.
He is an example of how some children who have survived abduction, abuse and rape in their childhood, grow up to be damaged adults if they are not 'treated' (therapy).
They did not believe him probably (which is bad enough) and the experience messed up his head up. At the end he rather committed suicide to end his own misery.
That just made me really, really sad. This film made me think about the recent news item about the large child pornography network being shut down. It had 45.000 members who were actively 'producing' pornography. Those poor kids..
Back to Prisoners: When I tuned the fi was probably half an hour away so I did not catch much of Alex's background story.
Was he mentally 'disabled' or just also severely traumatized?