Smiling Is a Submission Signal

Smiling and laughing are universally considered to be signals
that show a person is happy. We cry at birth, begin smiling at
five weeks and laughing starts between the fourth and fifth
months. Babies quickly learn that crying gets our attention -
and that smiling keeps us there. Recent research with our
closest primate cousins, the chimpanzees, has shown that
smiling serves an even deeper, more primitive purpose.
To show they're aggressive, apes bare their lower fangs,
warning that they can bite. Humans do exactly the same thing
when they become aggressive by dropping or thrusting
forward the lower lip because its main function is as a sheath
to conceal the lower teeth. Chimpanzees have two types of
smiles: one is an appeasement face, where one chimp shows
submission to a dominant other. In this chimp smile - known
as a 'fear face' - the lower jaw opens to expose the teeth and
the corners of the mouth are pulled back and down, and this
resembles the human smile.
The other is a 'play face' where the teeth are exposed, the
corners of the mouth and the eyes are drawn upwards and
vocal sounds are made, similar to that of human laughing. In
both cases, these smiles are used as submission gestures. The
first communicates 'I am not a threat because, as you can see,
I'm fearful of you' and the other says 'I am not a threat
because, as you can see, I'm just like a playful child'. This is the
same face pulled by a chimpanzee that is anxious or fearful
that it may be attacked or injured by others. The zygomatics
pull the corners of the mouth back horizontally or downwards
and the orbicularis eye muscles don't move. And it's the same
nervous smile used by a person who steps onto a busy road and
almost gets killed by a bus. Because it's a fear reaction, they
smile and say, 'Gee...I almost got killed!'
In humans, smiling serves much the same purpose as with
other primates. It tells another person you are non-threatening
and asks them to accept you on a personal level. Lack of
smiling explains why many dominant individuals, such as
Vladimir Putin, James Cagney, Clint Eastwood, Margaret
Thatcher and Charles Bronson, always seem to look grumpy
or aggressive and are rarely seen smiling - they simply don't
want to appear in any way submissive.
And research in courtrooms shows that an apology offered
with a smile incurs a lesser penalty than an apology without
one. So Grandma was right.