The question I asked my son Rowan's pediatrician when he was around 2 years old was "I know this sounds stupid, but when he bangs his head on stuff hard enough to actually injure his forehead ... can he give himself shaken baby syndrome or anything?"
Okay, it still sounds stupid, but Rowan was a tantrum-thrower -- a big one. He was a full-blown throwing things, screaming, hitting his head on the wall or floor, kicking the floor TANTRUM thrower. I thank the stars that Aurora is a pouter and crier instead.
He would literally make his forehead bleed from hitting it hard enough, even on carpet.
As painful as this was to watch, I learned that it was totally normal. And he learned too that some surfaces hurt more than others, though sometimes in a total meltdown, he'd forget that cement was especially painful and he'd end up with small marks on his forehead. Poor guy.
Starting around 18-24 months and sometimes lasting until 3, head banging is a normal behavior for stress relief. And even though painful to watch, the rhythmic motion CAN be soothing. It's often an outlet for frustration in the small crowd, especially those who don't have the words or signs to help express their feelings better. Sometimes it's even done for attention, especially when the parent constantly rushes in to stop it -- totally understandable, seeing how painful it can look (or be!), but try not to.
As Rowan's pediatrician put it, if he does it hard enough to hurt, he'll stop. And it was true, though like I said, sometimes it did leave marks and was painful for me to watch. Occasionally he'd forget that the cement was extra-hard.
Sometimes it can be one piece of many OTHER signs that points to a developmental disorder, but there would be other signs, too, so don't freak out about head banging alone. It can also be a kiddo's way of dealing with or distracting from some pain like teething or an ear infection, so keep an eye on it, but in most cases, it's just a painful part of toddlerhood. Teaching about emotions and helping learn words and signs for different things can often help alleviate toddler frustration.
Is or was your toddler a head banger?