Some sleepers have intelligent faces even in sleep, while other faces, even intelligent ones, become very stupid in sleep and therefore ridiculous. I don't know what makes that happen; I only want to say that a laughing man, like a sleeping one, most often knows nothing about his face. A great many people don't know how to laugh at all. However, there's nothing to know here: it's a gift, and it can't be fabricated. It can only be fabricated by re-educating oneself, developing oneself for the better, and overcoming the bad instincts of one's character; then the laughter of such a person might quite possibly change for the better. A man can give himself away completely by his laughter, so that you suddenly learn all of his innermost secrets. Even indisputably intelligent laughter is sometimes repulsive. Laughter calls first of all for sincerity, and where does one find sincerity? Laughter calls for lack of spite, but people most often laugh spitefully. Sincere and unspiteful laughter is mirth. A man's mirth is a feature that gives away the whole man, from head to foot. Someone's character won't be cracked for a long time, then the man bursts out laughing somehow quite sincerely, and his whole character suddenly opens up as if on the flat of your hand. Only a man of the loftiest and happiest development knows how to be mirthful infectiously, that is, irresistibly and goodheartedly. I'm not speaking of his mental development, but of his character, of the whole man. And so, if you want to discern a man and know his soul, you must look, not at how he keeps silent, or how he speaks, or how he weeps, or even how he is stirred by the noblest ideas, but you had better look at him when he laughs. If a man has a good laugh, it means he's a good man. Note at the same time all the nuances: for instance, a man's laughter must in no case seem stupid to you, however merry and simplehearted it may be. The moment you notice the slightest trace of stupidity in someone's laughter, it undoubtedly means that the man is of limited intelligence, though he may do nothing but pour out ideas. Or if his laughter isn't stupid, but the man himself, when he laughs, for some reason suddenly seems ridiculous to you, even just slightly—know, then, that the man has no real sense of dignity, not fully in any case. Or finally, if his laughter is infectious, but for some reason still seems banal to you, know, then, that the man's nature is on the banal side as well, and all the noble and lofty that you noticed in him before is either deliberately affected or unconsciously borrowed, and later on the man is certain to change for the worse, to take up what's 'useful' and throw his noble ideas away without regret, as the errors and infatuations of youth.
Fyodor Dostoyevsky, The Adolescent
laughter