Q&A: Is it normal for a 9-month-old to slap herself?

As long as this behavior is occuring in a child whose other social skills are developing normally then, yes, this is most likely normal behavior. This means that she makes good eye contact, loves to be held and cuddled, is interested in games played with other people, seeks out the company of others and perhaps has an early word or two. In tis context, slapping or hair pulling that only happens once a day can be part of exploratory behavior that you will see more of as she enters toddlerhood. Containers get dumped over, objects get stuck into noses, walls get scribbled on, all just to see what will happen and the effects. If the effect isn't good, they learn what not to do again and move onto something else. Your daughter will soon learn form the pain not to continue slapping herself, as long as others don't inadvertently reinforce that behavior by paying it lots of attention. If frustration is part of why she is hitting herself, you should see it building beforehand, and can hopefully intervene beforehand by helping her find a way to get what see wants. If not, and the slapping seems more of a random event, try not paying it any attention and it should soon extinguish itself.