when one of my friends had a new son, when he was about a year old, we gave him a lemon slice to taste. we thought he’d be turned off by how sour it was, but it became quickly clear that he wasn’t sure just WHAT to think about it, and our guffaws only made it worse. “wait, i don’t think i like this… but they are so happy, so maybe it’s… good?”
It works with dads too, but they did the experiment for 3 minutes.
I can’t see why it would be especially different if the father has a close relationship with the baby normally and the baby is used to a lot more responsiveness from him. My wife and I have changed working patterns a few times since my daughter was born, but I’ve found that when I was the one who spent a lot of time at home with her, I was often the one she would run to if she was upset or hurt and her mother was also in the room. One of the things I’m very grateful for is that my wife has never been defensive about her role and has always treated me as an equal partner in raising the kids. It probably sounds like a non-issue to be worried about, but it meant a lot to me.
Is the baby reacting to that face or the sudden change in the persons behavior. In other words, if the mother always had that face when interacting with the child, would be become upset if the mother suddenly started making funny faces when the child was accustomed to an expressionless mother.
Also, her interaction and talking stops as well. Those things should have been continued unchanged if we want to make any sort of determination as to what upset the baby.