What caused EA movement growth to slow down?

This seems likely to me. I can think of several instances of cases where an EA organization had the chance to market certain content widely, but chose not to take it, because they preferred to focus on other work.

As one example, the EA Newsletter grew its subscriber list dramatically by advertising on Facebook, but no longer does so. I've surveyed subscribers to find out which actions the newsletter may have prompted (donating, career change. etc.), and almost all subscribers who took significant action found the Newsletter "organically" through prior involvement in EA (rather than having it advertised to them). This indicates that not trying to optimize for "number of subscribers" could make sense for the newsletter.

Yeah, that looks interesting. Thanks for letting me know. I'll check it out.

Knowing about the history of various social movements, one thing about environmentalism and feminism is they were much more growth-oriented than EA is. I.e., massive growth was a necessary instrument for those movement's to achieve their historical goals. Some of the goals of the EA community will benefit from a strong, growth-oriented social movement more than others. For a lot of the EA community's goals, high-fidelity growth could be beneficial, but it doesn't seem like one of the most crucial factors in goal achievement.

So, there is a lot less enthusiasm to growth in EA than there historically has been in environmentalism and feminism. That isn't to say the quality or quantity of resources dedicated to community-building in EA is way worse than it could or should be. I think how EA has handled movement-building has been decent or adequate so far. I know a lot of other movements and communities that are a lot less organized and coordinated. Relative to other movements, environmentalism and feminism were two of both of the most growth-oriented, and most successful at growth, among all social movements of the 20th century. Since the achievement of feminism's and environmentalism's goals have historically required them to be much bigger than EA has needed to be so far, and for each member of these movements to be so involved, they have pursued these goals much more than EA has ever needed to so far. These movements have also been around for much longer, and because they are so much bigger, they have had opportunities in even the last several years to grow that EA never has.

Another thing to keep in mind with the model of an average annual growth rate of 100% I introduced for thinking about movement growth in the OP is not actually representative of real data in EA. For periods of exponential growth of various movements, I would think modelling the movement's membership size doubling every year would have a rough fitness for average growth of many different movements. However, it is very imprecise and often inaccurate for the quantitative reality of how different movements grow, and it excludes crucial info about the quality of growth. So, for example, in the history of EA, I wouldn't be surprised if the growth rate in EA from one year to the next was for some years much higher than 100%, and for some years somewhat below 100%. It just turns out the average growth rate of EA over the years 2009 to 2015 or 2016 can easily be modelled at 100%.

I know in the history of feminism, feminism has indeed come in waves.

• 1st-wave feminism: started in the mid-19th century, but achieved its most critical growth and success in the English-speaking world between the 1890s and 1920s.
• 2nd-wave feminism: hugely successful in the 1960s-70s. Fractures within feminism led to the movement's collapse by the early 1980s.
• 3rd-wave feminism/4th-wave feminism: 3rd-wave feminism began in the 1990s, as the academic/intellectual turn feminism began taking with the 2nd-wave had developed over many years to the point it had solutions to the problems that had led to the 2nd wave's collapse. Intersectional feminism and sex-positive feminism began finding more success within the feminist movement among a new generation who didn't share the beliefs of second-wave radical feminists. While 3rd-wave feminism was characterized more by the evolution of feminism, as opposed to its rapid growth and superlative public success of the 2nd-wave, this laid the groundwork for the 4th-wave feminism of the present, with the return to political activism, consciousness-raising, and movement-building, and has seen huge growth.

Between the 1920s and 1960s; and between 1980 and 1990, the size of feminism, its rate of success, and its amount of influence, didn't merely decline but collapsed. So, it doesn't appear feminism has had average growth rates of 100% for the last century. Rather, between waves of feminism, the organized/activist feminist movement would have gone through a long period of negative growth, perhaps with an annual growth rate below -100%. Conversely, during a critical period of a few early years with each successive wave of feminism, the average annual growth rate appears as though it could have easily been well over 1000%. So, a generic 'doubling-every-year' model doesn't fit the historical peaks and valleys of feminism.

Environmentalism as a modern social movement has also existed in some form since the late-19th century. It doesn't fit a model of doubling in size or support every year until about the 1960s, at which point the rate of growth takes off to bring environmentalism to the level of support we see today. Looking at the history of environmentalism the growth rate of support has probably been in the triple-digits during some critical periods of growth, and definitely in the double-digits for most years since 1960.

As a political and social movement, public or institutional support of socialism of any kind around the world, outside of pre-existing communist countries like China, Cuba, or Vietnam, collapsed to virtually nothing after the fall of the Soviet Union. However, since then, support for socialism has regrown to the point:

• Socialist parties have been elected to government in several countries in Latin America.
• Socialist parties have occasionally formed government or part of a coalition government, and generally found continuing success (perhaps until the last several years) in Europe.
• Support for social democracy/democratic socialism, and support social democrat/democratic socialist politicians like candidate for Democratic Party nomination for the 2020 U.S. Presidential election Bernie Sanders, is as high as it has ever been in the United States.

So, similar to feminism, support for socialism as a political movement rapidly collapsed, i.e., extremely high negative growth rate. Also like feminism, the growth rate of support for socialism has steadily increased in various countries around the world, and could be experiencing growth rates of over 100% in countries like the United States in the last few years.

For me, one thing that is different about my perspective is I think the things that would need to change about EA that would make it tend to grow more are things that would make it more effective. I think I'm more willing to believe than other community members that EA as is isn't nearly as effective as it could be, and this is part of the reason why EA seems saturated. It's only saturated in its current form.

For example, I think it is possible to make the EA community both much more effective and much bigger while retaining its fidelity for each of its current goals, and I also think it would make things better and not worse for each cause. I think there is a fear that introducing new causes into EA is going to fracture the attention, and so whatever specific cause people are focused on, which they consider the most important problem in the world, will receive proportionally less attention from new people introduced into EA. However, I haven't seen evidence this is the default growth model EA actually follows. It also doesn't take into account the following:

• Focusing on new, different causes will introduce new people into EA who are already put off of EA by its lack of focus on causes they consider fit for EA, like climate change. So, it only grows EA, and attention to EA as a whole, to introduce new causes, as opposed to steering away resources from cause areas with a pre-existing presence in EA.
• EA as a whole has not previously employed growth strategies for the movement's overall goals.
• Sub-communities in EA can capitalize on growth in EA for themselves, and have also not previously employed optimal growth strategies for their goals.

In general, I wouldn't be surprised if there are a lot of deficiencies in how we have thought and still think about movement growth, and community-building and consciousness-raising for various goals in EA.

Yeah, this is an issue with defining EA. "Self-identification" is the only clear criterion for "being part of EA". There is nothing useful as a metric about it. EA is not the kind of movement where it makes sense to ask "how many effective altruists are there?" It's just how I framed my thinking about growth rates and bottlenecks, since that is the question I see lots of people asking people all the time. It is a familiar framing.

If someone has a specific question about change in the amount of resources moved through EA in a particular way, it is a lot easier to pinpoint a bundle of things to measure, and that usually provides an answer, like with what you're doing.

Yeah, there were a couple other organizations that might have been able at one point to take advantage of naturally occurring opportunities for growth of the EA movement, or what we might call EA's constituent sub-communities. For various reasons, they also couldn't or at least didn't end up prioritizing the growth of EA.

I think it would be beneficial, and perhaps necessary for some goals, to attract more and more different kinds of people than are currently in the EA community to join. One could also specify just the 'existential risk reduction' community, or 'effective animal advocacy', but for whichever community, it's hard to imagine whichever group in EA as it exists now is more optimal than any other way they could be configured in the future. So to become better at achieving our goals, perhaps EA doesn't need to grow as much as we need to adapt or change. That seems like it will eventually entail attracting some more new people to effective altruism as a movement/community.

I think one reason other effective altruists are less optimistic about prospects of growing the EA movement than I am is because they think for the amount of resources that have previously been invested in movement growth, the success has been mixed, and it's unclear what among what has been tried worked best. I agree, and that is why I have been trying to figure out ways to grow EA as a movement with greater fidelity than we have a community have been able to in the past. I hope to publish some of those ideas in the near future.

A lot of those ideas are still more theoretical than practical. So I don't think they can be quickly applied to grow EA anyway. Ultimately, I agree it's not a bad thing that EA's movement growth has slowed, since it's unclear right now how the community would take advantage of such continued rapid growth.