Others argue might makes right. If you get there first, you can make the claim. If you're willing to fight for that armrest, it should be yours.
Mashable agrees, the person in the middle seat on an airplane gets to use both armrests. Watch the man in the video below fight for his armrest rights!
The logic is pretty simple. The person seated in the aisle seat can lean towards the aisle and the person seated in the window seat can lean on the window. However, the person stuck in the middle has nothing, so therefore, they get to use both armrests of either side of them.