Okay, this isn't completely correct but it's the closest I've come to figuring
out the chords for the song. Just try it, trust me it's extremely close the key 
is just a little different. What I mean is the key in the verse is a tad higher, 
and the chorus is a bit lower than the original, but that isn't much of a 
problem when playing the song. This will hold you over until someone 
(maybe me :)) is able to figure out the true chords Tegan and Sara play.

Capo 2

G2: play a Bm on the 7th fret
D2: play an F#m on the 7th fret

Intro: D A F#m G

Em (022000@1)                                    GBuild (320003@1)a wall of books between us in our bed
Em (022000@1)                                          GRepeat, (320003@1)repeat the words that I know we both said
Em (022000@1)                                   GRelax (320003@1)into the need we get so comfortable
Em (022000@1)                                    GRemember (320003@1)when I was so strange and likable

D (xx0232@1)       A (x02220@1)            G (320003@1)        DA F#mG
I Just want back in your head
D (xx0232@1)       A (x02220@1)            G (320003@1)        DA F#mG
I just want back in your head 
D (xx0232@1)             A (x02220@1)         G (320003@1)     DA F#mG
I'm not unfaithful but I'll stray
G2                    D2When (x43141@2)I get a little scared
                      G2
When I get a little scared
                  D2When (x43141@2)I get a little

Em (022000@1)                                      GWhen (320003@1)I jerk away from holding hands with you
Em (022000@1)                                         GI (320003@1)know these habits hurt important parts of you
Em (022000@1)                                  GRemember (320003@1)when how sweet and unexplainable
Em (022000@1)                           GNothing (320003@1)like this person unlovable

D (xx0232@1)       A (x02220@1)            G (320003@1)        DA F#mG
I Just want back in your head
D (xx0232@1)       A (x02220@1)            G (320003@1)        DA F#mG
I just want back in your head 
D (xx0232@1)             A (x02220@1)         G (320003@1)     DA F#mG
I'm not unfaithful but I'll stray
G2                    D2When (x43141@2)I get a little scared
                      G2
When I get a little scared
                      D2When (x43141@2)I get a little scared
                           
When I get a little
G2      D (xx0232@1) GRun (320003@1)run run
D2Run (x43141@2)G2      D (xx0232@1) GRun (320003@1)run run
D2Run (x43141@2)
                                    DA F#mG
I Just want back in your head
D (xx0232@1)       A (x02220@1)            G (320003@1)        DA F#mG
I just want back in your head 
D (xx0232@1)             A (x02220@1)         G (320003@1)       
I'm not unfaithful but I'll stray
D (xx0232@1)             A (x02220@1)          G (320003@1)       
I'm not unfaithful but I'll stray
D (xx0232@1)             A (x02220@1)          G (320003@1)       
I'm not unfaithful but I'll stray
D (xx0232@1)             A (x02220@1)     F#m (244222@1)  G (320003@1)       
I'm not unfaithful but I'll stray

There you have it! I know, the key change is a bummer 
but these chords are phonetically the most accurate I've 
found, and I've been searching around for a while. Rate 
this if you think it's not complete rubbish!