Suda EI 323 Adler (tr. David Whitehead):
It is prophesied into Trophonios' [cave]. The proverb is applied to gloomy and unlaughing people. For those descending into Trophonios are said to spend the entire time unlaughing.
εἰς Τροφωνίου μεμάντευται. ἐπὶ τῶν σκυθρωπῶν καὶ ἀγελάστων ἡ παροιμία τάττεται. οἱ
γὰρ καταβαίνοντες εἰς Τροφώνιον λέγονται τὸν ἑξῆς χρόνον ἀγέλαστοι εἶναι.
The effect is reversible. See Pausanias 9.39.13 (tr. W.H.S. Jones):
Afterwards, however, he will recover all his faculties, and the power to laugh will return to him.
ὕστερον μέντοι τά τε ἄλλα οὐδέν τι φρονήσει μεῖον ἢ πρότερον καὶ γέλως ἐπάνεισίν οἱ.
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