I also expect prioritizing animals over global health in EA to correlate with being more engaged in online EA discussion, in part because I would guess:

1. The animal advocacy community intersects so much with EA, whereas global health has a relatively larger non-intersecting space with EA, including the effective parts of each. Effective animal advocacy supporters engage relatively more with other EAs and people close to EA than do EA global health supporters.
   1. Basically all cost-effectiveness/cost-benefit discussion in animal advocacy is close to the EA community, whereas substantial cost-effectiveness/cost-benefit discussion in global health happens independently of EA. Effective animal advocacy also accounts for a larger share of the relevant research being done in its space. So relevant animal advocacy researchers are closer to EA than are relevant global health researchers to EA, on average.
   2. Global health seems to have less direct use/need for EA and EA-adjacent labour than animal advocacy does, normalizing by the relevant EA subcommunity sizes. The pool of people EA orgs can hire or engage for global health has a larger share of people further from EA than does the pool EA orgs can hire or engage for animals.
2. EA volunteer opportunities drive further EA engagement, and there are more of those for animal advocacy per effective animal advocacy supporter than there are for global health per effective global health supporter.
3. Within global health, it's hard to predictably beat GiveWell charities, and cruxes in global health are mostly pretty technical and kind of boring/tedious for most people, like checking studies and calculations in sheets. Many cruxes in animal advocacy are more accessible or interesting, like consciousness, moral weights, indirect effects, backfire risks. That drives more engagement among animal advocates.
4. Animal advocates are more activisty.
5. The Hive Slack channel pulls in non-EA animal advocates and directs them to EA in general and the EA Forum in particular (as well as EAA orgs, EA events, and so on). And I'd guess the share of EAA resources spent on community building is larger than the share of EA global health resources spent on community building. So participants are disproportionately likely to come from the animal advocacy community.

That's true. 

However, even given these incentives, I would have expected more votes/interactions from people favouring global health - given that it is a very established field that feels instinctively good and well-known by most. Animal welfare was supposed to be the underdog here. 

Moreover, there were fewer arguments favouring global health, and they felt much less convincing (personal opinion, but this felt reflected in votes). 

So, although there is probably a bias to factor in, I still think that most people on the forum genuinely think animal welfare is the better choice for an additional $100m. 

Another data point might be how well organized the animal-advocacy folks seemed to be on the Manifund EA Community Choice project (although I believe Manifund did some outreach as well). I assumed some of the same reasons were in play there.

More generally, discussions on this topic have a flavor of GH being the known quantity and AW being the option with much more uncertainty. Or stated differently, the crux for most participants was going to be predominately how good the marginal dollar for AW is, more so than how good that dollar would be in GH. That's an easier topic for AW people to write on. Also, people are typically more motivated to write in favor of their cause area than in an attempt to deflate the effectiveness of a different cause area.

Quickly throwing in a related dynamic. I suspect animal welfare folks have more free time to post online.

Career advancement in animal welfare is much more generalist than global health & development. This means there's not as many career goals to 'grind' towards, leaving more free time for public engagement. Alternative proteins feel like a space where one can specialize, but that's all I can think of. I'd love to know of other examples.

In contrast, global health & development has many distinct specialities that you have to focus on if you want to grow your career. It's not uncommon for someone's career to be built on incredibly narrow topic like, say, the implications of decentralization for regulating groundwater pollution. There are even 'playbooks' for breaking into the space, and they rarely align with writing EA Forum posts, or really any public writing.