frown and grimace

[keannu]

What is the difference between frown and grimace? I saw their images on google, but there seems to be little difference. Are they only synonyms?

st56)...Investigators have found that unpopular children are more likely to frown and grimace when they lose at a game and less likely to contain their smiling when they win at a game than pupular children...

 

[Grumpy]

I would say that it's a matter of degree. A frown is mostly confined to the eyebrows and forehead, whereas a grimace also includes the rest of the facial features.