Wish I had a great answer for you. I too have tried everything. I tried the bad tasting poilsh, painting nails daily, doing a reward chart daily with stickers and a bigger reward (like manicure) when the chart is filled. I've tried taking something away and giving it back as a reward when nails are not bitten. I've tried taking picutres of them when they are biting their nails to see how gross it looks, and even taking pictures of their nails bitten and a nice pair of nails to look at as incentive. I tried the education route about germs. I even found pictures of petrie dishes that show the growth of bacteria from fingers of nail biters vs non-nail biters. It shows a ton of e-coli growth which is in feces. Even this did not deter for very long. I tried figuring out if there was anything at school they were nervous about, but there was nothing unusual. Some kids just worry about little things like, what if the teacher calls on me and I cant' answer, who will play with me at recess, what if I am the last picked for a team in gym, etc. I spoke with my pedi, she said that it is a very common habit for kids and more than 50% stop in their teen years, some continue throughout their life. Honestly, I bit my nails until I was in my 20's and wanted nice nails to be in my sister's wedding. Even then, I had to work on it on a daily basis. I have found if I work with my girls daily, they do better, but if I forget, so do they. If you find a good solution that works, I would love to hear it and try it with my girls. Good Luck!