I've been breeding for an Impish Drifloon, and at first, I was breeding my only Drifloon with an Impish Ditto. The Ditto was holding an Everstone (50% chance of passing down nature, iirc), yet it took me at least 15 Eggs to hatch an Impish Female Drifloon. Now she is in the Day Care with a Haunter who knows Hypnosis. She's holding an Everstone, and I've hatched about 20 of her eggs. So far, 3 were Impish. According to my rough calculations, if its a 50% chance of passing down the nature, then the possibility of getting 3 out of 20 is about or .0007%. I think... IDK, I didn't pay attention much in Statistics.
Is there some method to this trick that I don't know about? It's worked for me before...
Is there some method to this trick that I don't know about? It's worked for me before...