Much like every rock, twig, or stick you see is really made up of atoms at the most basic level, the harm delivered by the various forms of child abuse are in actuality comprised of negative social messages at the most basic level. When you look underneath the superficial shell of particular actions, what you find is messages. It's generally not any particular action that causes a child lasting harm, but the messages delivered by that action which create the deepest and most enduring wounds. In virtually every case, at the most fundamental level, child abuse is about messages.
For example, let's take the topic of physical abuse. We all know physical abuse to be a bad thing that is capable of producing a serious psychological injury. But let us ask you this: why should hitting a child be any worse than the experience a child endures when they fall down and scrape a knee - something that also delivers quite intense physical pain and is an occurrence that takes place frequently throughout childhood? Or, what about youngsters in other cultures who go through painful rights of passage and endure things that would be considered horrific abuse in the United States? Why do they meet such pain with pride and look back upon it positively?
The answer to this conundrum is that the harm from child abuse is about messages, not actions. When a caretaker hits, slaps, or otherwise shows physical aggression towards a child, it delivers a lot more than physical pain. Such an action delivers some powerful negative messages: I don't like you; you're bad; you're unwanted; you're worthy of being mistreated; I don't care for you; and so on. After all, people don't mistreat the things they love and cherish. When is the last time you saw someone intentionally smash a treasured and valuable piece of crystal? It's a general rule for all of life: lovable things are cherished and treated well, while bad or undesirable things are mistreated. So even though such messages are never spoken and may not be intended, they are the messages that the child can't help but to receive and internalize.
During any act of physical aggression, a caretaker is (at the time) intentionally trying to inflict injury and pain onto the child. They might regret it later, but it still sends this message. So while the physical pain often subsides, the ideas that are delivered by such an action persist. This is why physical abuse is more harmful than the other bumps and bruises that come with childhood. Whereas the other pains are caused by no one in particular and are met with comfort afterwards, the lingering message from physical abuse is one of pain. As Rusk & Rusk (1988, p. 19) remark, “physically abused children are convinced they are somehow bad. What else can they believe, having received punishment when what they craved was understanding and support? Abused children always feel defective.” Especially if such abuse occurs often enough, these destructive messages become part of the child's identity, not to mention an aspect of their relationship with the important adults in their life.
We're not trying to say that it's impossible for physical experiences to matter. If a person chops off a child's arm, does them brain damage, severely neglects them in a way that causes biological or cognitive detriments, or rapes them violently in such a manner that the act causes permanent tissue damage to their genitals, then it's obvious that physical experiences can leave a lasting effect all their own. If someone keeps a child perpetually entrapped in an abusive environment, either physically or psychologically, this also will have an impact all its own.
Thankfully, such cases are in the minority. Ultimately, if a child suffers lasting harm from the majority of child abuse cases, it's because of the messages that they internalize from those actions. As noted by Albert Ellis & Aaron Harper (1961, p. 65), “it is difficult for humans to feel severely hurt by anything but physical assault or extensive deprivation unless they have traumatizing ideas about what happens to them.” Unfortunately for all of us, these traumatizing ideas are way too easy to come by.