on (one's) (own) head
Being one's sole responsibility to bear. I won't rat you out, but if you get caught cheating it's on your own head! She's the one who cheated, so in my book, their marital troubles are on her head.
on someone's head
Fig. [for something negative] belonging only to one person or group. All the blame fell on their heads. I don't think that all the criticism should be on my head.