Why do we laugh or smile in private when we're watching something funny? Why do we say "Ow!" when we're hurt, or scream when we're startled? Why does our body p

Why do we laugh or smile in private when we're watching something funny? Why do we say "Ow!" when we're hurt, or scream when we're startled? Why does our body posture change based on our mood, even when no one's around to watch? Adding in an extra check that says "if (someone is watching) { ... do this signaling thing ... }" more costly than just doing it.