Epistemic Status: An hour of googling combined with some highly dubious statistics and very rough notes. Take with at least 500 grams of salt.

I don't like wearing bike helmets, and have a bunch of friends who were horrified at me not wearing helmets when I occasionally ride a bicycle. On their request, here is a rough cost-benefit analysis of riding a bike without a helmet to my office and back (a total of ~20 minutes):

I decided to break down the question into the following three subquestions:

  • How great are the benefits from exercise when riding a bike?
  • What is the baseline risk of riding a bicycle?
  • How elevated is the risk by not wearing a bicycle helmet?

How great are the benefits from exercise when riding a bike?


According to the first episode of More of Less (haven't gotten to the second one yet), the benefits of cycling outweigh the risks by about 5-10 times.


Micromorts are a unit of risk measuring a one-in-a-million probability of death. You can say that a mode of transport results in so many miles per micromort and the result is a number you can really use. For cycling, this is 10-20 miles, for driving it is about 230 miles.
Microlifes are a millionth of an (average remaining) lifespan = 30mins of life, so activities which extend your life or decrease it in a chronic rather than acute way can be measured.
In looking at the risk associated with cycling to work, people often quote the mort factor You also have to include the life factor. First 20mins of excercise = +2 microlifes . Sedentary behaviour = -1 microlife


First 20 minutes of moderate exercise: 2 microlifes
Subsequent 40 minutes of moderate exercise: 1 microlife

However, this is more complicated by the other costs of wearing a helmet, as well as a preference of mine to not die at a particularly young age. All-cause mortality is really low in my age-range, so bicycling might be a significantly higher proportional increase for that age range, and with shorter GCR timelines I might not care super much about my long-term health, which also reduces the value of exercise (though then there are also immediate cognitive benefits of exercise beyond all-cause mortality that start taking effect immediately, which seems maybe like a sufficient counterbalancing consideration).

Concretely, if I cycle for 20 minutes every day and that is my only exercise, I gain about 2 microlives. If I have some other source of exercise, I gain more around 0.5 microlives. On average I think it's more something around 1 microlive for the 20 minutes of cycling. So let's go with that number for now.

What is the baseline risk of riding a bicycle?

Wikipedia for one micromort: Travelling 10 miles (16 km) (or 20 miles (32 km)) by bicycle (accident)

Average bicycle speed (Wikipedia): 15.5 km/h

Micromorts per hour of cycling (baseline): ~1

That results in about a gain of a third of a micromort over 20 minutes of cycling.

How elevated is the risk by not wearing a bicycle helmet?


Results: Estimates of helmet effectiveness were similar from odds ratios (ORs) using hospital controls or from relative risks (RRs) using helmet use estimates (Seattle: OR = 0.339, RR = 0.444; Victoria: OR = 0.500, RR = 0.353). Additionally, the odds ratios using hospital controls were similar when controls were taken from a larger cohort for head injury of any severity (Seattle: OR = 0.250, alt OR = 0.257; NSW: OR = 0.446, alt OR = 0.411) and for serious head injury (Seattle: OR = 0.135, alt OR = 0.139; NSW: OR = 0.335, alt OR = 0.308). Although relevant exposure data were unavailable for The Netherlands, the odds ratio for helmet effectiveness of those using racing, mountain, or hybrid bikes was similar to other estimates (OR = 0.371).

Ok, so if we assume that all micromorts come from head injuries, then this suggest a reduction of around 2/3 in risk.


Results: A total of 43 studies met inclusion criteria and 40 studies were included in the meta-analysis with data from over 64 000 injured cyclists. For cyclists involved in a crash or fall, helmet use was associated with odds reductions for head (OR = 0.49, 95% confidence interval (CI): 0.42–0.57), serious head (OR = 0.31, 95% CI: 0.25–0.37), face (OR = 0.67, 95% CI: 0.56–0.81) and fatal head injury (OR = 0.35, 95% CI: 0.14–0.88). No clear evidence of an association between helmet use and neck injury was found (OR = 0.96, 95% CI: 0.74–1.25). There was no evidence of time trends or publication bias.

Ok, so this also suggest something around the 2/3 number, so I think that’s a pretty decent bet.

For arguments sake, let's just assume that all of the bad consequences of accidents are covered by measuring head-injuries, resulting in a total risk increase by a factor of 3.

Conclusion (very sketchy)

Ok, so if you add this up super naively in terms of micromorts, then we get a (1/3) * 3 = 1 micromort loss at a 1 microlife gain for my average office commute. If we commit a statistical atrocity and just add those up naively, we end up with a net gain of 0, suggesting that cycling without a helmet is roughly equally risky as doing some random sedentary activity.

There are obviously a lot of complicating factors to this, but I am not sure in which direction they point. I care more about short-term injury risk than I care about long-term gain from exercise, but I also care about the cognitive benefits of exercise and want to maximize my peak-potential more than my average potential.

I also know that you can’t just add micromorts to microlifes together, though it seems hard to figure out what the correct thing to do is, and I only set aside around an hour of time for this.

Overall, my conclusion is that if I have the choice between riding my bike without a helmet, and staying at home, I should probably be mostly indifferent between the two. Since the alternative is usually paying for an Uber to my office, or walking, or riding with a helmet which I find quite actively annoying, I think I will continue riding without a helmet for now, in the absence of me noticing some new considerations or evidence (or discovering some way to be less annoyed by helmets).

Or maybe someone feels motivated and does a better analysis than I did here and corrects me. My overall estimate on the importance of this isn't super high, so I probably won't do much more analysis of it.

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I don’t like wearing bike helmets

If I understand your conclusions correctly, by not wearing a bike helmet you'd incur 260 * 2/3 * 30 = 5200 extra micromorts over 30 years of working 260 days each. This equals $260,000 if valuing a micromort at $50, which seems like a lot (although with the caveat this should perhaps be time discounted), and may justify trying to train yourself out of not liking to wear bike helmets.

ETA: Actually multiplying by 30 years doesn't make sense because it's unlikely you'll be biking for all 30 years of your work life, and because of time discounting. Perhaps 10 years would be more reasonable, which would yield $86,666 as the potential value of finding some way to get rid of your dislike of wearing bike helmets.

It feels like the per-experience costs are more relevant than the lifetime costs, since *also* you have to aggregate the lifetime annoyance. "Is it worth wearing a helmet this time to avoid 2/3rds of a micromort?"

It could be the case that the "get used to it" costs are a single investment, or there are other solutions that might not be worth it for someone who can tolerate a normal helmet but are worth it for habryka.

In an old post I argued that for acausal coordination reasons it seems as if you should further multiply this value by the number of people in the reference class of those making the decision the same way (discounted by how little you care about strangers vs. yourself). This makes decisions about things that only affect you personally depend on the relative sizes of their reference classes and on total population (greater population shifts focus of the decisions further away from yourself). Your decision inflicts the micromorts not just on yourself, but on all the people in the reference class, for the proportionally greater total number of micromorts that given this consideration turn into actual morts very easily.

The idea doesn't seem to have taken root, people talk about this argument mostly in the context of voting, where it's comforting for the argument to hold, even though it seems to apply in general, where it demands monstrous responsibility for every tiny little thing. It's very suspicious, but I don't know how to resolve the confusion. Maybe it's just psychologically unrealistic to follow through in almost all cases where the argument applies, despite its normative correctness.

It feels to me like people in our community aren't being skeptical enough or pushing back enough on the idea of acausal coordination for humans. I'm kind of confused about this because it seems like a weirder idea and has less good arguments for it than for example the importance of AI risk which does get substantial skepticism and push back.

In an old post I argued that for acausal coordination reasons it seems as if you should further multiply this value by the number of people in the reference class of those making the decision the same way (discounted by how little you care about strangers vs. yourself).

But if "the same way" includes not only the same kind of explicit cost/benefit analysis but also "further multiply this value by the number of people in the reference class of those making the decision the same way", the number of people in this reference class must be tiny because nobody is doing this for deciding whether to wear bike helmets.

Suppose two people did "further multiply this value by the number of people in the reference class of those making the decision the same way", but their decision making processes are slightly different, e.g., they use different heuristics to do things like finding sources for the numbers that go into the cost/benefit analysis, I don't know how to figure out whether they are still in the same reference class, or how to generalize beyond "same reference class" when the agents are humans as opposed to AIs (and even with the latter we don't have a complete mathematical theory).

people talk about this argument mostly in the context of voting

I'm skeptical about this too. I'm not actually aware of a good argument for acausal coordination in the context of voting. A search on LW yields only this short comment from Eliezer.

Your decision inflicts the micromorts not just on yourself, but on all the people in the reference class, for the proportionally greater total number of micromorts that given this consideration turn into actual morts very easily.

But your decision also causes the corresponding benefits to accrue to all the people in the reference class? So the decision you make should be the same, it just becomes more consequentially important.

The voting case is different because the benefits are superlinear in the number of people you affect (at least up to a point) -- a million people voting the same way as you probably have more than a million times more chance at swinging the election.

ETA: Never mind, misunderstood habryka's reply, I'm basically saying the same thing. Though I still think that the case for applying the argument to voting is much stronger than the case for applying it in other decisions where benefits are linear.

The absolute size of a reference class only gives the problem statement for an individual decision some altruistic/paternalistic tilt, which can fail to change it. Greater relative size of a reference class increases the decision's relative importance compared to other decisions, which on the margin should pull some effort away from the other decisions.

That the effective multiplier due to acausal coordination is smaller for non-voting decisions doesn't inform the question of whether the argument applies to non-voting decisions. The argument may be ignored in the decision algorithm only if the reference class is always small or about the same size for different decisions.

Yeah, I agree with all of that. (I didn't realize the point about the relative sizes of reference classes until I read your reply to habryka more carefully.)

Perhaps another way to make the point about the argument for voting being stronger is that it affects your decisionmaking even if you are not altruistic. Here by stronger I mean that the argument is "more robust" or "less suspicious".

Sure, for voting the effect on decision making is greater. I'm just suspicious of this whole idea of acausal impact, and moderate observations about effect size don't help with that confusion. I don't think it can apply to voting without applying to other things, so the quantitative distinction doesn't point in a particular direction on correctness of the overall idea.

Sure, but doesn't that apply mostly uniformly to all decisions, making tradeoffs mostly the same, just having everything have a larger magnitude? I don't see why this is the kind of decision that should be more influenced by that line of reasoning than e.g. my heuristics around choosing what to do with my career, how I choose romantic partners, whether to get exercise, etc.

The magnitude depends on the sizes of reference classes, which differ dramatically. So some personal decisions are suddenly much more important than others simply because more people make them, and so you should allocate more resources on deciding those things in particular correctly. Exercise regimen seems like a high acausal impact decision. Another difference is that the goal that the personal decisions pursue shifts from what you want to happen to yourself, to what you want to happen to other people, and this effect increases with population. (Edited the grandparent to express these points more clearly.)

Hmm, I think that's being too shallow. My decisions aren't made in isolation, they are the result of applying general principles and broad heuristics. The biggest impact should come other people adopting the same heuristics and principles, not just making the same object level decisions.

That influences sizes of reference classes, but at some point the sizes cash out in morally relevant object level decisions.

But if you spend more time thinking about exercise, that time cost is multiplied greatly. I think this kind of countereffect cancels out every practical argument of this type.

New information argues for a change on the margin, so the new equilibrium is different, though it may not be far away. The arguments are not "cancelled out", but they do only have bounded impact. Compare with charity evaluation in effective altruism: if we take the impact of certain decisions as sufficiently significant, it calls for their organized study, so that the decisions are no longer made based on first impressions. On the other hand, if there is already enough infrastructure for making good decisions of that type, then significant changes are unnecessary.

In the case of acausal impact, large reference classes imply that at least that many people are already affected, so if organized evaluation of such decisions is feasible to set up, it's probably already in place without any need for the acausal impact argument. So actual changes are probably in how you pay attention to info that's already available, not in creating infrastructure for generating better info. On the other hand, a source of info about sizes of reference classes may be useful.

Yeah, I do think this document updated me on that a bit. I do think right now my priority in this space should be to get a reliable exercise regime at all, and the subtle sense of guilt I have been feeling from the helmet thing has been preventing some progress on making that happen by biking more. I think after I have a biking routine, I should figure out how to do the helmet thing correctly.

There seems to be some evidence that a norm of helmet-wearing discourages people from cycling. When people don't wear helmets, they are taking on some personal risk, but by challenging the norm it could mean that more people take up cycling. This is likely to make cycling safer (fewer cars on the roads, drivers are more used to cyclists, safety in numbers at intersections etc), and so the need to wear a helmet is reduced.

I'd prefer to live in a world where more people cycle and helmets aren't (as) needed (like in European cities such as Berlin and Amsterdam), so I tend to feel grateful towards people who don't wear helmets.

I also ride my bike in Berkeley without a helmet.

Some other considerations which influence me:

  • helmet use seems to make crashes more likely (by making bikers and/or drivers less cautious), so it's misleading to use data about harm that's conditioned on there being a reported accident.

  • I'm fairly careful to avoid roads with heavy traffic, or with cars driving more than about 30 mph. I expect that fatality rates vary a lot by these road factors.

  • I use a cheap bike that doesn't go as fast as the average bike.

Alas, I don't have good evidence about how to quantify these considerations.

It seems like it would be most effective to wear something that doesn't look like a helmet but protect like one.

Interesting. The ability to fold it enough to fit in a backpack should reduce the hassle of storing it at my destination, which has been part of why I've been reluctant to use one.

or discovering some way to be less annoyed by helmets

Are you aware of this? It's a helmet-airbag that only deploys in the event of a crash.

I think it’s important for you to make more prominent the fact that you have an unusually strong preference against wearing a helmet, because for someone like me for whom wearing a helmet is basically costless I think the evidence you found pretty clearly indicates that I should wear a helmet, since I can get a 3x reduction in mortality risk at no cost. Other than that, fine, I concede :p

Also I know you don’t want to put more time into this, but I found this post to be pretty hard to follow overall (though the conclusion was clear). I could maybe edit if you care.

I think taking risks is okay if it's fun, but biking on car roads isn't very fun, so why? And I'm not sure about the exercise benefits: breathing heavily while next to cars is probably more harm than help, and having aerobic as your only exercise is overrated anyway. So I guess in your place I'd choose the most pleasant way to commute, without regard for risk or exercise or whatever. If it's still biking, then fine!

There is a bike route on my way to work that basically never has cars on it, and is specifically designated to be a "bike boulevard", which makes the whole thing reasonably fun.

Unrelated: Why is aerobic as the only exercise overrated?

Compare photos of sprinters (anaerobic) to marathon runners (aerobic) and ask yourself who you'd rather look like.


Observed-expected survival was highest for high jumpers (7.1 years for women, 3.7 years for men) and marathon runners (4.7 years for men) and lowest for sprinters (−1.6 years for women and −0.9 years for men).

I don't really care how I look, and more about how good I can think and how healthy I will be overall. And most studies I've found seem to suggest that aerobic cardio is the best for improving broad health outcomes.

Exercise can also make you stronger, more flexible, improve your posture, give you muscle mass that burns fat just by existing, teach your mind that you can overcome difficulties, etc. Biking to work every morning has almost none of these benefits. Why not spend the same 30min/day on something that gives you all the benefits?

I would be interested in suggestions, but I've so far found most forms muscle-building exercise exceptionally boring or generally aversive. Going to a gym would cost me at least an hour every time I do it, and probably more if you factor in stress and subscription costs and other things that I expect to come with it.

I think if you try weightlifting twice a week for a month and pay a lot of attention to form, there's a >20% chance that by the end of the month you'll find the fun in it and keep going. Though I agree that going to a gym can be a hassle. Luckily we have a gym at the office, so I just go every day.

I assume you have already chosen your route to interact with the fewest number of cars? https://www.berkeleyside.com/2019/02/08/map-injury-crashes-with-pedestrians-or-cyclists-in-berkeley-in-january

Yeah, I mostly just drive down Milvia, which is pretty save.