I see Gwern's/Aaron's post about The Narrowing Circle as part of an important thread in EA devoted to understanding the causes of moral change.  By probing the limits of the "expanding circle" idea for counterexamples, perhaps we can understand it better.

Effective altruism is popular among moral philosophers, and EAs are often seeking to expand people's "moral circle of concern" towards currently neglected classes of beings, like nonhuman animals or potential future generations of mankind.  This is a laudable goal (and one which I share), but it's important that the movement does not get carried away with the assumption that such a cultural shift is inevitable.  The phenomenon of the expanding circle must be caused by something, and those causes are probably driven by material conditions that could change or reverse in the future.

As I see it, the strongest part of the argument for a "narrowing circle" is the "Ancestors" and "Descendants" sections.  It seems plausible to me that preindustrial "farmer" culture placed nigh-obsessive emphasis on pleasing the wishes of your ancestors and securing a promising future for your descendants.  (I suspect this is probably because, in a world where income came from farming the land rather than hunting/gathering or performing skilled industrial-age work for wages, inheritance of farmland from one generation to the next becomes crucially important.)  Much of the modern world seems to have essentially abandoned the idea that we should place much weight on the values of our ancestors, which should be concerning to longtermists since valuing the lives of ancestors seems very close to valuing the lives of unborn generations (see for instance Chesterton's quote about how "tradition is the democracy of the dead").

The idea that concern for descendants has also decreased is certainly a worry worth investigating -- perhaps a logical place to start would be by investigating  how much the recent worldwide decline in fertility rates really reflects a decreased desire for children.  A drop in respect for ancestors might also directly cause a drop in concern for descendants -- it might be logical to disregard the lives of future generations if we assume that they (just like us) will ignore the wishes of their ancestors!

Anyways, here are some other pieces that seem relevant to the thread of "investigating what drives moral change":
- AppliedDivinityStudies arguing that moral philosophy is not what actually drives moral progress.
- A lot of Slate Star Codex / Astral Codex Ten is about understanding cultural changes.  Here for instance is a dialogue about shifting moral foundations, expanding circles, and what that might tell us about how things will continue to shift in the future.

Finally, I think that investigating the "expanding circle" is doubly important because it's not just an assumption of a couple people within the nascent EA movement... it's very similar to one of the core legitimizing stories that are held up to justify mainstream liberal democracy!  I am talking about the whole civil-rights story that "the moral arc of the universe bends towards justice", that democracy is good because it has lead an expanding-circle-style transition towards increasingly recognizing the civil rights of women, minorities, etc.  I think this story is true, but I don't know exactly why and I don't think the trend is guaranteed to continue.  (Was democracy itself the cause, or merely another symptom of a larger force like the Industrial Revolution?)

For all these reasons, I think this is a good post worthy of inclusion in the decadal review.

Much discussion of Moral Circle Expansion seems hampered by lack of conceptual clarity about what the Moral Circle means.

There are a lot of distinctions that need to be drawn, but here are two positions on one dimension:

1. The moral circle merely refers to which (groups or types of) entities are viewed as possible targets of moral regard
2. The moral circle refers to the amount of actual moral concern granted to such entities

A lot more distinctions should be drawn on this dimension alone (e.g. for "actual moral concern" are we interested in abstract attitudes of concern, actual amount of effort extended, or actual treatment extended), but even these suffice for now.

On the first view, which seems somewhat closer to original uses of the term, it does seem like retrenchment of the Moral Circle should be expected to be quite rare, at least once you reach contexts like our own (in WEIRD societies) where there are extremely prevalent memes about at least potentially considering entities as possible moral targets if they might be persons in any sense (or more generally in contexts where the conditions for considering the possibility of including some group in the moral circle are as extensive and plural as they are now). It seems relatively hard for groups to fall entirely out of the moral circle in the first sense, in such cases, except in cases like those you mention where we decide that certain entities don't exist or aren't sentient.

With the more expansive second sense of Moral Circle (which seems to be what people are using), where all that is required for Moral Circle expansion/retraction is an increase or reduction in moral concern extended (as seems to be implied by examples such as more/less care being granted to the elderly and so on), it seems like the Moral Circle should be expected to be expanding and retracting near constantly on an individual or group basis. This is especially so if we understand degree of moral concern as meaning the actual extend to which needs are weighted and help extended (in which case this will, almost necessarily, be pervaded by tradeoffs in a near zero sum fashion) which is why further distinction being drawn within this category is so important.

This piece examines the accuracy of Peter Singer’s expanding moral circles theory by reasoning and examples. Since the validity of this arguably fundamental thesis can have wide implications, I recommend this post.

Neat post! Feedback:

• One population neglected in a lot of conversation on moral circle expansion, and in Gwern's consideration, is how the treatment of children after infancy has changed over time. I'm only knowledgeable of the history of the last couple hundred years as it relates to legal treatment, such as child labour laws. The study of how the treatment of children has changed will be complicated by the changing definition of 'children' over time; over time adulthood in societies has been treated to begin as early as the onset of puberty up to twenty years of age. That stated, people older than two or three and lower than the historical lower-bound for age of adulthood seem to have stably been regarded as 'children' throughout history.
• Another kind of potential moral patient neglected in this conversation are abstract entities, such as concern for the overall health of a tribe, local community, or society; and, more recently in history, cultures and nations, the environment, and biodiversity. One thing all these entities have in common is there appears to be a common moral intuition one can evaluate their overall moral well-being that is greater than the sum of the well-being of their individual members (such as humans or other animals). This differs from how EA typically approaches similar entities, such as more often conflating their moral well-being with the aggregate well-being of their individual members. I'm guessing there are ways moral psychology regarding these entities differs significantly from how people think morally about individual moral patients. I don't know enough about what those differences might be to comment on them, but to understand them better seems crucial to thinking about this topic.

One argument for why people don't proportionally care about future generations is because they're such a distant concern. A pattern I notice with the moral shifts you describe is most people have become more distant from the relevant populations over time, such as prisoners and animals. We're also more "distant" from our ancestors and deities in the sense we may care about them much less in large part because we're exposed to memes promoting caring about them in our everyday lives much less frequently.

Summary: One theory for the pattern of moral shifts over the last few hundred years from a perspective of social science and history is the apparent moral shifts have followed a transition from more traditionalist and religious worldviews to more liberal ones, as largely driven by economic and political changes produced by industrialization and modernization. While this narrative model has limitations, it definitely seems significant enough to change how EA thinks about moral circle expansion.

One possibility is the pattern of moral shifts observed in the last couple hundred years in the Western world, and other parts of the world to a lesser extent, is driven by modernization. With the modernization, industrialization, urbanization, and rationalization (i.e., integration of society with advanced science and technology) of societies, popular consideration of different populations of moral patients has shifted along common lines. The upshot for this is EA should consider the possibility moral shifts are driven more by the influence of a changing material and technological environment, and less to do with whole societies intentionally shifting the exercise of their moral agency.

Modernization has given rise to the modern nation-state and greater political centralization, giving the rise to various forms of liberal political ideologies. While liberalism started with the Enlightenment, its popular spread followed the Scientific and Industrial Revolutions, and greater urbanization. The increased contact between different groups, such as differing ethnic groups and the sexes in the workplace, accentuated societal prejudices by making apparent how superficial and arbitrary the material deprivation between different groups of people was, at a historically unprecedented growth in global material wealth. This has a lot of power to explain civil rights and more moral consideration being extended to ethnic, religious, and sexual minorities, and women and children.

At the same time, the decline of more agrarian and religious society alienated more people from traditional communities and religion. This is consistent with the analysis why moral consideration of elders, ancestors, deities, and other groups traditional local communities and religion gave people more moral exposure to.

On one hand, a single convenient narrative explaining how apparent moral progress across societies is actually a natural political and social progression driven almost exclusively by technological and economic changes seems too convenient in the absence of overwhelming evidence. It definitely seems to me intuitively unlikely apparent moral circle expansions would necessarily have happened in the course of history. On the other hand, the idea that the moral circle expansion is an apt evidence-based theory for explaining historical moral progress could be recognized by EA as a largely confused notion, and we could spend less time trying to frame moral shifts through a flawed lens. From there, we could view the theory of moral circle expansion as more of a prospective model for thinking about how various societies' moral circles may likely expand in the present and near future.