Head down means judgement.
The head down shows a judgment or negative internal emotion. In fact, my son who is just nine months old, will pull his chin in and put his head down so that I can’t see his eyes when I instruct him not to touch the buttons on my desktop computer tower. He also smirks when he doesn’t want to internalize and obey me. Next, he raises his left arm (usually) to the back of his neck and either (short) grabs his ear or the hair at the side of his head, or reaches all the way and scratches the back of his neck showing restraint. I say short-grab because the real cue is the grab the back of the neck, but his arms, being a baby are short and inflexible. Scratching the back of the neck is like “holding yourself back” by the scruff of the neck. This cue cluster has been important for my wife and I as we ascertain what level of obedience we should expect based on whether or not he has actually internalized our instructions.
As with any head motion it is important to examine the adjoining clusters of body language to determine it’s true origins. For example, adding arm crossed to head down would signify more prominently that there was negativity present. Bending the head down, but looking up, can easily be confused with a lowered head yet means something totally different. The bent head means agreement, confirmation, or even shame.