TL;DR Underrepresented groups are underrepresented because they self-select into EA circles at lower rates due to divergent values. Concerted initiatives to increase diversity are strongly biased towards the values of the people running the initiative. Communities are exclusionary by necessity.

I hope as one of the rare developing world EAs, my perspective is useful here. This post is a nice thought, but like many nice thoughts relating to diversity I believe it collapses on deeper consideration of how it would be implemented. There is a reason EA mainly consists of WEIRD (Western, Educated, Industrialized, Rich, Democratic) people, and underrepresented groups are not underrepresented due to some conspiracy to exclude them. It's because EA is based on a set of assumptions derived from WEIRD patterns of thought that are not common outside certain groups. For example, statements such as "All people's lives should be valued equally regardless of their age, gender, sexuality, religion etc." or "Empirical reasoning is the best path towards truth" would receive very different levels of agreement at Germany  universities and Bangladeshi villages. Be careful what you wish for! Those who claim to seek diversity are also the most likely to be horrified when they find it.  As a result most diversity initiatives in WEIRD circles tend to only select members of underrepresented groups from an elite, liberally educated minority within those groups who are rarely representative of the values of the groups as a whole, and much more representative of the values of the diversity seekers. So I more or less agree with other commenters on this post - demographic diversity is a fairly poor proxy for intellectual diversity, especially  when it is brought in through a concerted effort to increase diversity, rather than in an organic manner that reflects the changing norms of different groups and the communities they are joining. 

Communities are exclusionary by necessity : the questions you ask to determine entry are biased by the perspectives of existing members because they have to be! There is no reason for the community to exist if it is perfectly representative of all opinions in society at large - to paraphrase Syndrome, if you have everyone's values, you have no values. Would I be happy to meet more EAs who shared my background? Of course! Would I be happy if this happened at the cost of losing the values that make EA such a powerful lens with which to view the world? Hell no! Diversity is good at the margin, but it is not an intrinsic good. There is a balancing point that is best for the overall health of the community - it's important to try to rigorously determine what that is before making calls for increasing diversity.

One concrete way to do that would be to carry out two very large surveys - one of EAs, and one of randomly sampled people. They would measure agreement with certain values that are generally considered axiomatic to the EA way of thought. We could then statistically determine whether certain values cluster with certain identities, and the degree to which those values/identities are represented within EA. Would be a lot of work and money, but I believe it's the only rigorous and safe way to run a diversity initiative!

As we cannot measure the diversity of perspectives of a person directly, our best proxy for it is demographic diversity, as our life shapes our assumptions.

This seems like quite a key premise to your argument, but you don't seem to spend much time arguing for its plausibility; indeed, it seems quite likely false. Could we not simply ask someone their views on a variety of subjects? If we know what their views are on Sports, Reincarnation, the Holy Roman Empire, Two Dogmas of Empiricism, Abortion, Pokemon, the Axiom of Choice, MIRI, and Socks with Sandles, I feel like we could have a pretty good sense of whether they add a new perspective. 

Similarly with education and career; I would in general expect more perspective diversity from a group consisting of an economist, a biologist, a nurse, a poet, a cop and a prostitute, even if they were all the same race, age and sex, than I would from a demographically diverse group of harvard-educated lawyers.

I agree with something in this direction, though not with everything as stated.
Some ways I see research affected by demographics:

• Research of all kinds has been shaped by the viewpoints of people with more status, money, etc.
• Sometimes this leads to serious slants in our understanding of the world - for example, the fact that so much psychology research has been done on WEIRD undergraduates means that our understanding of human psychology is badly skewed. Medical research that’s primarily carried out on one demographic group may not be generalizable to other populations, meaning worse health outcomes for less-studied groups.
• In some other cases, findings are less susceptible to bias coming from the demographics of the researchers or research subjects. I can imagine how some aspects of climate science are swayed by demographics: do we look at how people of different geographies, ages, and genders are affected by food and water shortages, for example? But it's hard for me to see how some other aspects, like how polar ice melts, are all that susceptible to demographic bias.

Aside from what or whom is being studied, there is also the question of who is taken seriously in the research community itself.

• When there are unnecessary slants in what’s seen as professional or credible, we systematically devalue the contributions of groups who don’t fit that mold. Example: someone with an accent perceived as lower-status may be taken less seriously. Or a workplace may treat straight hairstyles as professional but textured hair as less so.
• Structural problems. Examples: a lack of all-gender or single-person bathrooms in a workplace or conference venue disproportionately affects trans people who have reason to fear harassment in bathrooms. A bad parental leave policy disproportionately affects staff who give birth.
• Reducing people to their demographic, even if it feels positive. Example: after my first child was born, I went to an EA talk with my baby. Two male friends of mine were there. One talked excitedly in front of the group about the baby and my new status as a parent. The other didn’t mention the baby and only talked to me about EA-related topics, including asking me about an area and pointing out that I knew more than he did about this topic. (He did come over at the end of the gathering to visit with the baby.) While both friends were completely well-intentioned, the first made me fear “Oh no, am I just going to be a mommy now? Is that how they’ll see me?” The second was intentionally trying to avoid that, and he made me feel I was still valued for intellectual contributions.

When you have slants like these that affect who gets published, who gets funded, who gets hired, who can focus better on their work, etc, you lose good contributions. And that’s a loss to the field as well as to those individuals. (And all of this is aside from any knowing/intentional discrimination.)

I think these are problems we should be on guard against. I’m always happy to see people in EA thinking about how to avoid them in workplaces, funding, local groups, etc.

I do agree with other commenters that different backgrounds and demographics will not by themselves necessarily be good for epistemics or truth-seeking.

Thanks, this was an interesting write-up. I have one, well, let’s call it a concern or maybe caveat. You write:

Objectivity as it is understood here is a continuum between being more or less objective. Objectivity increases with the diversity of perspectives, as more perspectives in a given discussion lead to more assumptions being challenged and, thus, to an answer that is more likely to be true.

I think this relies on all perspective-havers having some shared norms that enable them to find truth collectively. Philosophy, for example, which while not a science benefits enormously from diverse viewpoints, has norms of logic, reasoning & charity that are essential to finding truth. More generally, my impression is that groups & teams function better when they have some shared values, goals & norms. So that’s the caveat that I would add – that there still need to be shared norms, at least truth-seeking norms.

It seems like my post created more of a buzz than I anticipated. Many people seem to get the message from it: “we should only care about demographic diversity and nothing else”. I’m sorry that my wording was apparently so vague, as this is not really what I meant. 

To create a fruitful discussion you not only need diversity, but also at least some value alignment and some knowledge about the topic that is being discussed. Given that some value alignment and some knowledge about the topic are present, diversity of perspectives is a powerful way to make sure that the knowledge and insight gained from such a discussion increases. It allows the participants in the discussion to detect each other’s blind spots and challenge their assumptions. And this is where demographic diversity comes into play. I think that you cannot easily (or even at all) measure your subconscious assumptions and biases, but I think that those assumptions and biases originate from the experiences that you have in your life. Those experiences are strongly shaped by demographic markers like age, gender, race, etc.. Therefore, we should make sure to have enough people from different subgroups to not miss out on perspectives that would challenge erroneous assumptions in our thinking. 

Another post from the EA Forum which might have done a better job at highlighting a similar idea is “EA Diversity: Unpacking Pandora's Box”, as it unpacks the different facets of diversity explicitly. Unfortunately, I only came across it after I had published my post. 

"As we cannot measure the diversity of perspectives of a person directly, our best proxy for it is demographic diversity"

Demographic diversity is a useful proxy and may add something additional even if we did have diversity of general philosophy. However, we can measure diversity of perspectives directly, ie. by running surveys like Heterodox Academy has.

"The answer here is that objectivity is not something that a single person has, but that objectivity is a social achievement of a diverse community"

Feminism offers some valuable lens, but I feel it often leads to a hyperfocus on the underprivileged. Suppose we're discussing raising taxes on the rich, it might be useful to have a rich guy in a room. They might share some useful perspectives like, "It won't change the behaviour of my friends one bit. Most of us won't even notice. Our accountants handled our taxes, so we have no idea how much we're paying" or "If the California tax law passes, I'm headed to Texas". They might lie, but that's true of everyone. They might be biased, but the poor are likely to be biased as well.

I'm not claiming this is equally important as representing the perspectives of the poor, just that we shouldn't be hyperfocused.