Big mistakes = Doing something that is actively harmful or useless by their own lights and values, i.e. doesn't help them achieve their life goals. (Not: Doing something that isn't in line with your values and goals.)

A lot of people think that others in the EA-ish community are trying to do something impactful but end up doing something harmful or useless. Sometimes they also work on something that they are just not very good at or make other big mistakes. A lot of people never end up telling the other person that they think they are making big mistakes. Sometimes people also just have one particular argument for why the other might do harmful or useless work but not be sure whether it's a bad overall. This also often goes unsaid.

I think that's understandable and also bad or at least very costly.

Epistemic status: Speculation/rant.  I know of another person who might post something in this topic that is much more rigorous and has actual background research.

Upsides of telling others you think they are making big mistakes, wasting their time, or doing harm:

• It's good on a community level because people get information that's useful to decide how to achieve their goals (among them, having impact,) so people end up working on less suboptimal things and the community has better impact overall.
• It's good on a community level because it's pushes towards good intellectual conversations and progress.
• I and probably others find it stressful because I can't rely on others telling me if they think I'm doing a bad job, so I have to try to read between the lines. (I find it much less stressful now but when I was more insecure about my competence, I found it really stressful. I think one of my main concerns was others thinking and saying I'm "meh" or "fine" (with an unenthusiastic tone) but only behind my back.)
  • Note that anxiety works differently for different people though and some people might find the opposite is true for them. See reasons against telling people that you think they are wasting their time or worse.
• I and probably others find it pretty upsetting that I can't rely on others being honest with me. It's valuable information and I would like people to act in a way that helps me achieve my stated goals (in this case, doing good), especially if their motivation for not being honest with me is protecting my wellbeing.

That said, I often don't do a great job at this myself and think telling others you think their efforts would be better spent elsewhere also has significant costs, both personal and on a community level.

Downsides of telling others you think they are making big mistakes, wasting their time, or doing harm:

• Hearing that somebody thinks you're doing harmful or useless work can be extremely discouraging and can lead people to over-update, especially if they are insecure anyway. (Possibly because people do it so rarely, so the signal can be interpreted as stronger than it's intended.)
  • At the same time, we often have noisy information about how good another person's work is, especially how good a fit they are or could be. 
• Criticising someone's work could lead to an awkward relationship to them. They might also get angry at you or start talking badly about you. This is especially costly if you have a friendly and or a professional relationship.
• An increase in people telling each other what they think about each other's work could create or amplify a culture in which everyone constantly feels like they have to orient themselves towards impact all the time and justify their decisions. This could lead to feelings of guilt, shame, judgement, higher risk-aversion, and an over-emphasis on doing things that are mainstream approved.
  • That said, I think for some people (e.g. hopefully me, unclear) the opposite might be true because stressing about what people are secretly thinking induces more anxiety than knowing they think I'm wasting my time. I would guess that this is true for the type of insecure person who already expects the worst anyway or feels like an imposter, but that's just speculation. (Perhaps people in the comments could speak up.)
• While I think people currently don't tell each other enough that they think they are wasting their time, everyone doing it a bit more could result in a situation where many people get way too much feedback to be worth the time to engage with all of it. This is bad if it also makes it harder to filter for the important feedback.
• Often the people with useful information about other people's work are also in particularly difficult situations when it comes to sharing their honest opinion about their work (either for reputational or social reasons or because they have a lot of power over you, making it easy for you to over-update on their opinion): Mentors, funders, employees, collaborators, hiring managers. I think the mentors one is particularly sad because a lot of short-term junior research fellowships have the stated purpose of helping mentees test their fit for research.
• Given the above and probably more, it can be a great drain on your time to tell others you think they are wasting their time and to do it well. I sometimes would like to shoot someone a quick text but know that I realistically would end up investing a lot of time into crafting a careful message and that I would (maybe rightfully) feel obliged to engage if they reply. Especially if you don't know someone very well, aren't sure of your assessment, or are not sure the person could find something more useful to do (other than doing nothing if you think they are doing harm), it often simply doesn't seem worth the time.

So, where does this leave us?

Some ideas and recommendations based on speculation and intuition

• My hypothesis is that we should tell each other more often when we think others are wasting their time on the macro-level but not the micro-level: I.e., I'm against unsolicitedly telling people that you think their spending their week on something you think doesn't help them achieve their goals. But I'm favour of sometimes unsolicitedly telling people that, by their own lights, maybe they want to prioritise a different cause or work a different job, or give up skilling up in the area they are skilling up in.
• I suspect that some approaches to sharing these kind of thoughts will typically go down better than others:
  • Get into the mindset of being an ally that genuinely wants to help the other person achieve their goals. Take them seriously in their stated goal to do good. (Some people might actually prefer not to hear your thoughts but I think it's good practice to start out by taking what people say about their goals at ~face value.) Remind yourself that you are being kind to them because you are helping them with their life goals instead of cruelly letting them act in ways they would potentially feel terrible about if they had your information.
  • Get into the mindset of figuring things out together. Stay curious about why they are doing what they are doing. Even if you are quite convinced that you are right, stay open to at least the possibility of being convinced otherwise. Getting them to agree with you is not the end all be all.
  • Instead of starting with "I think your work is net harmful", maybe start out with a consideration for why it might be net harmful and explore that together. (I'm not telling you to lie about or obfuscate your overall opinion or how strong it is - I'm just guessing it's sometimes useful to not start with it.)
  • Keep in mind that they might already have thought about all the things you want to bring up. If it's easier, maybe start with questions.
  • I think there are ways to vibe-check the other person and carefully ask whether they are receptive to this kind of feedback. But I think it requires skill and I don't have great ideas for what to recommend or what to do myself. Starting the meta-conversation about whether and how others want to hear your thoughts will very often make it a bit awkward to then decide not to talk about your object-level thoughts.
• If you start telling people more about the mistakes you think they are making, you might want to start praising people (Not necessarily the same people!) more, too, to balance out the overall negative-to-positive feedback ratio. Otherwise, we move from a community where people probably mostly don't get enough feedback to one where you only ever get negative feedback.
• Maybe it's enough to only focus on cases where the benefit-to-cost ratio seems highest. I tentatively think that's the case for friends and mentors. The benefit-to-cost ratio is probably highest when:
  • The person sharing their thoughts is willing and able to engage in some detail. I.e. they are willing to say more than "I think you work on something bad" or "I think you're bad at this kind of work" and then disengage (although something minimally more informative than the latter could be informative from a mentor-type that has many comparison points and feels fairly confident.)
  • The person sharing their thoughts has thought at least somewhat about the other person's work and why they think it's a waste of time. This can include having thought about one specific argument for why the other might be wasting their time even if the person sharing isn't sure about the overall sign.
  • The person sharing has some idea about whether the other person wants to hear those kind of thoughts or how to best deliver these thoughts to the other person.
  • Somebody is explicitly soliciting feedback.
• Maybe we can set up more spaces and events where people specifically opt into getting this kind of feedback
  • My understanding is that Doom Circling is meant to do this although I've never done it. It also sounds quite intense and part of a specific culture that might not be for everyone.
  • Potentially less intense versions of this branded as something like "Discussing/red-teaming once cause prioritisation/career plan" could perhaps work. Even if I don't know how exactly this would differ from doom circling, I expect this kind of branding to attract a less brutally honest and more polite crowd. This seems great from the point of view of matching feedback givers/receivers who are interested in similar levels of efficiency vs. niceness.

Finally, you don't have to do any of this. Sharing your honest thoughts can cost you a lot and you should spend your resources on whatever you want to spend them on.

Be brutally honest with me & ask me for advice

You can give me anonymous feedback. I really care about what I do with my life - If you think I could be doing better, please tell me. I'm also happy to receive short, low quality, low information feedback, although the opposite is preferred of course. If you wanna be extra fantastic and give me non-anonymous feedback, so I can engage with you, I will be over the moon and very grateful - but don't let the perfect be the enemy of the good.

Also, if you think you have important thoughts to share with someone but you don't know how, feel free to dm me. I wrote this post in one go, so maybe I'll have more useful tips for how to actually go about sharing your honest thoughts in individual cases :) I might take a while to respond or decide not to engage in detail though.