In "The Reeve's Tale," a dishonest miller cheats all his customers by shorting them on corn, "padding" their sacks with a less expensive substance like bran. When two students attempt to outsmart him only to be hoodwinked just as soundly as everyone else, they punish him by having sex with his wife and daughter right under his nose, while he's sleeping. They accomplish this partially by deception: using the "cradle trick," a common motif in medieval fabliau (bawdy stories) John convinces Symkyn's wife that his bed is actually hers, causing her to fall willingly into his arms.
In "The Reeve's Tale," no one is completely honest. Even Symkyn's family seems to be in on his stealing, with his wife baking the cake made of the students' stolen corn and his daughter possessing suspiciously accurate knowledge of its whereabouts. What we get in "The Reeve's Tale," then, is an extremely pessimistic view of humanity. At the end of the tale, the narrator declares that "a gylour shal hymself bigyled be" (a deceiver will himself be deceived), suggesting that the moral we ought to take away from this story is one about honesty. However, the events of the tale suggest a different moral: everyone's out for himself, and in this world, it's cheat or be cheated.
"The Reeve's Tale" comments upon the moral that "a gylour shal hymself bigyled be" (a deceiver will himself be deceived) by showing the way in which deception spawns further deception.
The "cradle trick" John plays on Symkyn's wife symbolizes the way in which adultery hijacks not just a man's wife, but also his bloodline.