An alternative explanation for act-omission distinction, from Joshua Greene’s Moral Tribes(emphasis added):
Forget, for a moment, about morality. Why would an animal’s brain distinguish between things that it actively causes to happen and things that it merely allows to happen? Right now, as you read this book, you are actively causing your eyes to move across the page, actively causing the pages to turn, and so on. That’s what you’re doing. But think of all the things that you are not doing. You are not teaching a poodle to dance, not writing a fan letter to Rod Stewart, not juggling flaming torches, and not installing a hot tub in your basement. And that’s just the beginning. At any given moment, there are infinitely many things that you are not doing, and it would be impossible for your brain to represent all of them, or even a significant fraction of them. (Sound familiar?) What this means is that an agent’s brain must, in some sense, privilege actions over omissions. We have to represent actions in order to perform them, in order to make sure they go as planned, and to understand the actions of others. But we simply can’t keep track of all the things that we and others don’t do. This doesn’t mean that we can’t think about omissions, but it does mean that our brains have to represent actions and omissions in fundamentally different ways, such that representations of actions are more basic and accessible. [...]
...representing a specific goal-directed action, such as choosing a blue mug, is a fairly basic cognitive ability, an ability that six-month-old infants have. But representing an omission, a failure to do some specific thing, is, for humans, a less basic and more sophisticated ability. Note that this is not because representing an omission necessarily requires substantially more complex information processing. If there are only two possibilities— choosing A and not choosing A— then representing what is not done is not much harder than representing what is done. If you were programming a computer to monitor and predict someone’s two-alternative mug selections, you could program the computer to represent “didn’t choose the blue mug” almost as easily as “chose the blue mug.” (All you’d need is a little “not” operator to turn the latter representation into the former.) Nevertheless, it appears that humans find it much easier to represent what one does rather than what one doesn’t do. And that makes sense, given that in real life, it’s more important to keep track of the relatively few things that people do, compared with the millions of things that people could do but don’t.
The fact that babies represent doings more easily than non doings makes a prediction about adults: When human adults distinguish between harmful actions and omissions ( non doings) in their moral judgments, it’s the result of automatic [intuitions], not the [conscious application of a formal principle making them different]. Cushman and I tested this prediction in a brain-imaging study in which people evaluated both active and passive harmful actions. As predicted, we found that ignoring the action/ omission distinction— treating passive harm as morally equivalent to active harm— requires more [effortful dorsolateral prefrontal cortex] activity than abiding by the action/ omission distinction.** This makes sense, given that representations of omissions are inherently abstract. An action, unlike an omission, can be represented in a basic sensory way. It’s easy, for example, to draw a picture of someone running. But how do you draw a picture of someone not running? You can draw a picture of someone standing still, but this will convey something like “person” or “woman” or “standing” rather than “not running.” The conventional way to represent what something is not is to use an abstract symbol, such as a circle with a slash through it, conjoined with a conventional image. But a conventional image can’t do the job by itself. You need an abstract symbol.
Actions, in addition to having natural sensory representations, also have natural motor representations. Reading words like “lick,” “pick,” or “kick” automatically increases activation in the subregions of the motor cortex that control, respectively, the tongue, fingers, and feet. But there is no part of the brain that ramps up when people think about actions that do not involve the tongue (etc.) because there is no part of the brain specifically devoted to performing actions that do not involve the tongue.
As we saw earlier, our emotions, and ultimately our moral judgments, seem to be sensitive to the sensory and motor properties of actions, to things like pushing. (And to visual imagery of pushing; see pages 46– 48.) Omissions, unlike actions, have no distinctive sensory and motor properties, and must therefore lack at least one kind of emotional trigger. Moreover, this basic sensory/ motor distinction between actions and omissions may carry over into the realm of more physically amorphous behaviors, depending on how they are conceptualized. For example, the idea of “firing” someone (active) feels worse than “letting someone go” (passive). This parallels the results of a study by Neeru Paharia, Karim Kassam, Max Bazerman, and myself, showing that jacking up the price of cancer drugs feels less bad if it’s done indirectly through another agent, even if the physical action itself is no more indirect.
The hypothesis, then, is that harmful omissions don’t push our emotional moral buttons in the same way that harmful actions do. We represent actions in a basic motor and sensory way, but omissions are represented more abstractly. Moreover, this difference in how we represent actions and omissions has nothing to do with morality; it has to do simply with the more general cognitive constraints placed on our brains— brains that couldn’t possibly keep track of all the actions we fail to perform and that originally evolved as sensory and motor devices, not as abstract thinking devices. Once again, it seems that a hallowed moral distinction may simply be a cognitive by-product. (But, as I’ll explain shortly, there is room for some utilitarian accommodation of the action/ omission distinction.)
An alternative explanation for act-omission distinction, from Joshua Greene’s Moral Tribes (emphasis added):