Joshua explained (spoilers!!)
SPOILERS
SPOILERS
SPOILERS
OK here is my interpretation of the movie after watching and browsing through reviews and other posts:
1: Joshua is a sociopath. My theory is his sociopathic tendencies may have been exacerbated about from attachment disorder at his birth with his mother Abby. We are given clues about this when Joshua asks his parents how he was as a baby and receives some stilted and smoothed over responses that indicate he was a very difficult baby indeed. Also, when he watches the video of himself at 8 days old and sees how his mother is falling apart and is being driven nuts by his constant crying, it would seem that Joshua was a very difficult child to bond to and this probably continued through his childhood. His relationship with his parents seems to be distant and "polite", with Abby barely acknowledging him at all. My guess is his parents also knew something was
"different" about Joshua all along as well which completed the circle of no bonding or affection between them. So why did Joshua not start "acting out" until Lilly was born? See 2
2) Joshua probably had no frame of reference to judge the relationship between himself and his parents. Being a sociopath with no "real" emotions he may have accepted the situation as it brought him no disadvantages. Remember, sociopaths strive for a semblance of normality to hide their true natures. And this worked.. until Lilly.. Right away we see the difference between his parents' behavior towards Lilly and the way they act towards him, in the scene where they visit Abby and Lilly in the hospital. Joshua is left standing in the doorway while his parents and baby cuddle on the bed and coo at each other. The parents don't even invite big brother to meet his new little sister.. odd behavior indeed for most families but indicative of the relationship they and Joshua had.. Joshua on the outside always. Then when they get home with the baby, Joshua again sees the differences: Abby is happy and content with the new baby, and his parents and Lilly form the seemingly happy family..without him.. He is again on the outside, playing a (quite lovely) song on the piano all but ignored by all except the nice uncle who comes to play with him...until Abby tells him to stop. Then they proceed to sing Twinkle Star to the baby.. and Joshua expresses his disgust (resentment?) by throwing up.. and then later on playing a dirge-like version of Twinkle Star at his recital. He is unable to explain the difference in attitudes though, until he asks his parents about how he was like as a baby, and gets the stilted answers.. and sees the video tape.. so now he knows.. in his mind his parents don't like him because he was a screaming, difficult baby and now he has to live with an in-his-face demonstration of Joshua vs Lilly in his parents eyes. So I think at this point Joshua's plan to recreate Lilly baby into Joshua baby begins, see 3
3) Joshua begins a campaign of antagonizing Lilly to make her scream like he did, to drive away his mom from Lilly. We don't know exactly how he did this but we do see the abrupt change in Lilly and then Abby (when she is in the doctor's office, she says "Not again" and right away needs to hand the baby to the doc). I think at this point, this is the sole point of his plan. Make his parents not love Lilly like they don't love him (when he says exactly that on the recorded video of him in Lilly's room). He then tries to "provoke" response or love? in parents by a few things: the giving away of ALL his toys, he says he asked Abby who told him it was OK (most mothers/parents would think something was totally wrong with their 9 year old child wanting to get rid of every single one of their toys, but we are left with the impression Abby barely questioned this action of his). Dad makes a token attempt dissuade Joshua from doing this but after a brief conversation throws in the towel and leaves the room.. and this characterizes most of Dad's interactions with Joshua: he makes token efforts at the Dad and Son thing but never spends too much time or really tries delving too deeply with Joshua.. and Abby who ignores him all together. The other scene where he tells Abby he's going out to take his toys to Goodwill and she just lets him out to walk the streets of NYC by himself.. something the good uncle questioned but she blew off because "Joshua's not a normal 9 year old". Another scene, when Abby is deep in depression (but pre-medication) is when her and Dad are in the bedroom and she's trying to pump milk.... Joshua stands in the doorway (again in the doorway but not in the room with them) and leaves saying "I love you Mommy and Daddy" and they just stare in silence.. not a word of "I love you" back. I think at this point Joshua starts to see his plan to make his parents not love Lilly and love him instead backfires and he moves on to 4)
4) Joshua decides Mom is the problem.. after all she is colder and Dad at least tries.. So he starts tampering with Mom's meds.. keeps provoking Lilly into crying fits.. with the intent of sending Mom away.. culminating with the bizarre glass cutting incident and finally with the hide and seek game. I think Josh hopes now he and Dad can be together so he begins on part 5)
5) Get rid of Lilly.. I think he calculated the scene at the museum to get an opportunity to get rid of Lilly in a way that can appear accidental.. pushing her down the stairs. He almost does it but gets interrupted by grandma and Dad (who is now on to Joshua by watching the video). The point where Dad races up the stairs and grabs the baby stroller changes Josh's plans: he knows from that silent glance between them and the way Dad grabs the stroller that Dad knows about him and his plan to stay with Dad is over. He also knows if he gets rid of Dad, Grandma is an unacceptable alternative so he pushes her when Dad turns his back.. maybe also in a power play with Dad to let him know he knows.. And from that point, see 6)
6) Now that Dad's not a viable option, grandma's out of the picture, Joshua begins his campaign to end up with his uncle.. and they way he does that is set up the ruse that Joshua's been physically abused and then to provoke his Dad into the beating at the park.
7) Finally with uncle.. there's still Lilly.. some posters have stated their idea that Joshua wanted to save him AND Lilly but I disagree.. Lilly was never in his picture and I think she's still in danger. Notice his stony silence when uncle exclaims how cute his baby sister is.. and in the song he sings, he explicitly says "you and I together", not "us 3 together". And I think if there is a Joshua 2 : ) he will try to devise ways to get rid of Lilly.. and if uncle makes it known that he now suspects, if not knows.. he will be gotten rid of as well..