"My daughter is 16 months old and for about six of those months, she's been scratching others--me, my husband, my mother, babies and children at the gym daycare, her playmates and her cousins. My initial reaction was to take her hand immediately after each episode and sternly tell her, "No scratching!" Eventually, that morphed into, "No scratching. Scratching hurts!" And, after consulting our pediatrician at her 15-month appointment my correction became, "No scratching. It hurts! Show (name) how you can touch softly instead," while I took her hand and stroked the offended party gently (giving her a positive way to replace the negative behavior). I know that she understands that scratching is a bad thing. I know that she does it either to a) get a reaction because she's curious or b) to be defiant and show that she is angry.
We've been battling this problem for what feels like forever and her pediatrician said it could be many more months before she stops. He said this is something that we should continue to correct but that she'll eventually outgrow it. I'm not ready to accept that this could go on for months yet. I don't like that she hurts me and her father and especially other babies and children. She's a happy, darling, wonderful, smart girl and I want her to be well-liked. But, her little playmates and her cousins are starting to stay away from her for fear of getting scratched. The caregivers at the gym daycare are running out of patience with her too. I need your help, and that of your readers, to find a way to get her to end this behavior. Do you have any suggestions?"
I have to say I'm with your pediatrician on this one. My older son went through what seemed like a million months of hitting at that age, and my younger son is going through months and months of biting right now (he's 15 months). Even when they understood our anger and frustration, they didn't stop because they couldn't really.
Just because they understand that it's wrong (not that they really understand the concept of "wrong"--they just know that we don't want them to do it) doesn't mean that they can stop themselves from doing it. They don't have the power to choose at this age--they just act. Have you read Harvey Karp's thing about toddlers being like little cavepeople? He's definitely onto something with that analogy. They're all id, and it takes months and months and months of constant, grueling, soul-sucking alertness and monitoring by parents to instill even a teeny bit of super-ego in them. (OK, so maybe it takes years.)
When my older one was going through an endless phase of trying to hit our cat, a mom of older kids told me that in her experience the kids didn't seem to be able to stop it until around 21 months or so. So I had to be there almost constantly to pull his hand away from her for a long, long time. The 21 month mark was pretty accurate for him. Any other readers want to add data points?
One concept I found particularly helpful during this phase was the idea that you shouldn't tell a child that young not to do something, sit around and watch the kid do it, then punish the kid. Instead, you'll help your child to learn self-control by watching carefully and intervening before the child can do that thing. Kids this age often respond better to physical intervention than verbal commands. So when you see her raising her hand or doing whatever she usually does right before she scratches, put your hands on hers and help her make the "soft" motion while pulling her away from the kid she's about to scratch. If you do it before she scratches (or even mid-scratch) then you're reinforcing the good behavior instead of letting the bad behavior happen and then punishing her for it (which is kind of a cruel set-up when you think about it this way).
This stage is truly awful. You feel like you're raising an out-of-control demon seed, that all the other parents are judging you, and that your child will never have friends. But it's such a common stage that kids go through that I have to think that it's just a normal part of development. I can't think of a way that would absolutely stop it (barring painful physical punishment, but that would only create a different set of problems) at this age. Once she's closer to 2 years old, if she's still scratching, you could try giving her a special doll to scratch (similar to the "toby" I gave my older one to bite when he was going through a biting phase at 2 years). You could even try a scratching doll right now, although she might not connect with it yet. It's also probably too early to start painting her fingernails (it might work with an older scratcher to paint her nails and tell her they're special and can't scratch once they're painted, or something like that).
I think you're just going to have to keep helicoptering around her in social situations to prevent her from scratching when you can. Obviously you're keeping her nails cut as short as possible already, so the scratches probably aren't really injuring anyone. It probably will go on for a few more months, but eventually she will stop doing it. She'll have friends and other kids will want to be around her and you'll know you've been doing a great job raising her. Just try to hang in there a few months longer.