Little ones are just as expressive as older children and adults--often more so. However, they lack the vocabulary to express themselves appropriately, so they have to use alternatives. One is to say goofy things like "I hate you." Another is hitting, biting, kicking, screaming, and tantruming. Yet another is hitting themselves. It expresses frustration, anger, sadness, and an inability to control oneself.
In a way, a child who hits himself is not only expressing frustration but begging Mom or Dad: "Please, help me get under control!" Parents have a few options when Junior starts whacking himself.
1. Interpret. Say something like, "Wow, it looks like you are really mad right now!" This helps the child learn to connect feelings to words. In time, he or she can learn how to use those words instead of hitting. The only disadvantage with this option is that it may inadvertently reinforce the pattern by giving the child your attention when being self-abusive.
2. Punish. Spanking, yelling, and other negative stimuli might stop Junior in the moment. But I don't recommend it because it will never teach a new way of dealing with the frustration that led to the behavior in the first place. Junior needs you to be in control of the situation by teaching new strategies for handling emotions.
3. Ignore. Eventually, your toddler will learn that hitting himself won't get your attention or whatever it is he wanted. This is extremely difficult to do, especially in public and especially since your child might injure himself.
4. Time Out. This is a good option, because it removes the child from the reinforcing aspects of the situation--namely, your attention. However, it might take a while for Junior to settle down once he is out of control and some toddlers are not ready to do Time Out until well into their third year. So this is better for 4 and 5-year-olds.
4. Redirect. This is the best option. Yes, it gives the child attention, but that's OK here, since safety is paramount and it provides an excellent teaching opportunity. When your child starts hitting, shift the focus to something completely different, like a chore, a song, a physical activity ("Hey, let's both twirl around until we fall down and want to throw up!"), wrestling, repeated hard high-fives, etc. Be creative. Just do me a favor: don't redirect to TV or video games. You'll have an Electric Addict in no time.
Give that a try when your toddler starts going postal on himself/herself! Remember, don't freak out; normal toddlers do crazy things.