[jquip]

Well, adults have multiple threads of parallel thought. They are definitely more engrossed in their internal world than children. For example: Consider meeting a friend and not recollecting their name, sometimes their name pops into our minds after a couple of days without us actively pondering or being burdened by the finding process. A majority of adults truly aren't in the moment so to speak, juggling multiple choices that surround their day does cause cognitive load.

Also, the need to instinctively smile is replaced with conditional reciprocation based on what they see and assess. That's learned behaviour.