Placing zero value on the ability to look, dress, and act like a non-nerd. I seriously overestimated the effort and underestimated the benefits.

Not precommitting to be on my own before making a major life decision.

I once bought something in an New York shop through high-pressure sales. I looked at it and said something about how I would like to have it but I couldn't nearly afford it, and he asked me how much I would pay for it. Foolishly, I named a price; he looked insulted and said that it was far too low. I tried to explain that that was what I meant, that I couldn't afford it at any reasonable price, but he skilfully turned it into haggling, and I walked out with the thing and considerably poorer. I then resolved never to buy anything expensive without leaving the shop first, so I could just walk off if I changed my mind.

Many years later, I met up with my girlfriend's girlfriend for dinner and drinks so we could discuss whether it would work for her to move in with us. There were a lot of warning signs that it wouldn't, to say the least. I pressed her on things that were worrying me, and got wholly unsatisfactory answers. But we very often had good and enjoyable conversations, and this was one of those times. So at the end she sort of said "OK, that's all great, shall we announce online that I'm moving in?" ... (read more)

Rationalising my feelings of social anxiety by adopting the view that I should not be socialising with my "intellectual inferiors" anyway: one must choose to be social or intelligent. When I entered university and met people I considered intellectual peers I was hardly able to socialise with them either because my social skills were so inadequate. Post-university it still took me a few years working at a low-paid job with a forced social element (working at a shop in a shopping mall) to kickstart my social development.

I once thought I'd skip learning to cook, thinking I'd specialize in something else and trade money for someone else's cooking. I've since learned to cook moderately well, and there are benefits:

• Much better control of your nutrition
• It impresses people; I can create dinner dates and dinner parties
• It's tradable; you can swap your cooking for people doing other chores or buying food
• Cooking is an activity with enjoyable depth, satisfying geeky tendencies to learn
• Cooking is easy to learn, giving you success spirals
• You can influence the nutrition of those close to you

Starting university with chemistry rather than computer science. Started on chemistry because I'd really enjoyed it at school, because my teacher was fantastic; realised I didn't actually like it at all. If I'd started on comp sci (and this was in 1985), I'd have done well, I'd have had much more money earlier, and my ridiculously intensive personal hobbies wouldn't have been affected.

tl;dr unexpectedly hazardous: good teachers.

1 I would say "Marrying the wrong person," but phrased that way, it's not really useful as a warning.* So let's go with "Marrying for the wrong reasons (e.g. because then low self-esteem and limited prior success with the opposite sex led me to settle far too quickly despite what should have been obvious warning signs). The emotional and financial consequences were catastrophic -- a lost decade of life, basically, And due having to children, another decade of not completely being able to escape their malign influence even after divorce.

*The more specific form "Not realizing I was marrying a borderline may be somewhat more useful. I know I've seen other Lesswrongers talk about similar issues, though I think they escaped before marriage and children.

2 Not actually doing any networking in college/grad school despite attending very prestigious institutions (due to introversion and cluelessness about the usefulness of this)

3 Not doing anything to deal with social anxiety much earlier, thus missing out on so many different opportunities.

In fairness to me, I'm older than the median LWer, and the Internet was a lot less helpful for finding the answers to life problems then than it is now.

Not seeing medical professionals as soon as medical problems arose. I now live with (likely) permanent chronic pain which may well have been preventable were the causes addressed at an earlier date.

The mental model that states that since all past problems have been inconsequential, all future problems must therefore be inconsequential is a problem here. Holding that mental state (my past problems have evaporated without issue, therefore my future problems will do likewise) is problematic because most people have not experienced enough major problems to draw on a meaningful sample size here.

Mistake 1: Trying to convince others that I know more than I really do.

Mistake 2: Thinking that I actually know more than I do.

Not stepping back and thinking strategically about where to go and why before beginning a PhD program.

Because of a deteriorating relationship and problems in my current lab, I felt completely overwhelmed by the prospect of choosing a PhD. I was being told by my superiors that I had loads of potential and therefore had to get into the most prestigious institute possible. I therefore copped out of the difficult task of doing a PhD and essentially allowed my supervisor (who I knew was incompetent) to choose one for me. I never asked myself if I really wanted to do the research I was being pushed towards, if labwork was a rational choice for someone with my skillsets, or, more importantly, if I wanted to do a PhD at all.

Imagine my embarrassment when I start reading lesswrong and discover that stories like this about grad school are a stock example of irrational behavior...

• Not taking the wisdom of nature seriously and, in particular, failing to appreciate the possibility that a considered intervention (e.g. polyphasic sleep) might do more harm than good.

• Failing to do all sorts of things that I would have enjoyed merely because they involved some trivial inconvenience.

Majored in philosophy. I don't think I learned much worthwhile and it's makes it harder to get a job.

Not taking risks of audio noise exposure seriously. Hearing loss is gradual, and tinnitus often starts as a temporary thing, so it's very easy to accumulate major damage before you realize it's a problem.

-Taking the non-thinking NPC college route (going to whatever school was close enough to commute to, majoring in classes you thought were easy or letting your family influence your decision too much)

-Getting married extremely young because you think (probably correctly) that you can't support yourself, and then staying married much longer than you should have for the same reasoning (probably incorrect, by then).

-Spending most of college trying (and failing miserably) to take a large course load before realizing that you really just can't do it, and the only way for you to actually succeed is to only take two or three classes at a time so that you can do perfectly in all of them (i.e. stop trying to fight your perfectionism if it's causing you to fail, and just give in to it).

-Waiting much too long to start on a career track.

• Failing to learn one's social norms quickly enough, and failing to make any falsifiable tests as to whether I was making mistakes. I was nearly asocial in elementary school, middle school was just weird, and then high school was this horrible mess of thinking people were being freaked out by me, or avoiding me, or not avoiding me, or literally anything. In reality, lots of people loved me and I didn't need to fear or be awkward about asking favors of people or asking to hang out with them.

• Buying big-ticket items such as computer equipment by numerical stats only. Compactness, physical construction quality, compatibility, and battery life (which is remarkably often not really rated, or degrades significantly) may be as important or more important than non-numerical quantities. For the specific example of laptop computers, this means to go for low-end Macs, business-level computers, and if you desire Linux, Lenovo Thinkpads. And the worst part is that I didn't end up spending that much less than I would have for something with much better construction quality, etc.

• Not having any friends or interests outside of STEM (during university), or even outside a very specific nerdy mindset.

An interesting subset is the bad decisions you made that you knew at the time were bad, not just in retrospect, but did it anyway.

-Thinking I should follow advice I got from older respected people. The best advice I've ever had is that "lots of advice is BAD advice, or at least bad for you." I wanted to be open to criticism, and I'm into self-improvement, so I used to take advice really seriously when it came from a remotely intelligent source. Lots of it was contradictory, lots of it didn't apply to me and my specific situation and personality, lots of it arose from the special particularities of the advisor, and lots of it was just people saying vague things that sounded wise to feel like they had something to offer me. Instead, I started taking surprising/unusual advice (which really is the only useful kind, in my opinion) seriously only when it came from someone who knew me and my situation well, I understood the reasoning behind it, and I had gotten similar second opinions from other respected sources.

(Also, asking the advice of people I respected at a young age (12-19) instead of going online to find solid, well-tested, data-supported strategies to deal with my problems and questions).

-Doing research in a field that didn't interest me early on because I felt like I should be doing res... (read more)

I don't know how to phrase it best so it fits here, but I feel it does, because it complicates my life regularly and therefore is pretty major: Not sticking to very beneficial routines that I know to be highly useful for my overall functionality in the short term and neccessary in the long term.

I know that I am happier and better-functioning when I regularly make journal entries and meditate. I am more productive at work, and I have more energy in my free time for other projects. Yet, I'm in a cycle where, when everything is going very well, I neglect both activities. Everything continues to go well for a while, then I get unbalanced, less energetic and overall less happy and productive, and only then do I remember that I already know what I can do to improve my situation.

For example, I neglected journal-writing and meditation in the past three month, which was a critical time because this left me with little energy to learn for a test (I'm currently doing a second M.Sc. to improve my chances of entering the economic sector I'm especially interested in). It won't have any unalterable consequences yet - if I fail on Friday, I can repeat the test next year, without having to study longer than intended - but it will make things more difficult and time-intensive next year, because I will have to prepare for the repeat tests as well as the tests scheduled for next semester. And I fear it's only a matter of time till this leads to bigger consequences (Seriously affecting job performance, for example).

20 years of organized religion. Let me count the ways in which this is harmful.

• 20 years of religiosity. (Oof!)
• Quitting programming after 1 yr due to boredom.
• Not studying math and science early (because: religiosity).
• Going to college.
• Not moving to the Bay Area at age 18.

I have made several serious misjudgements that are all instances of mistaking "I find this easy" for "this is easy in some general sense". Finding something easy is indeed evidence in favour of the thesis that it's easy, but it is also evidence that you are particularly good at it. I've mistakenly ignored the latter.

Not autobiographical, all but one are people I know or have known personally. Which is not to say I have avoided all of these mistakes, but none of this is telling my own story. The ones about reasons for choosing college majors are speculation about other people's motivations, but are obviously mistakes whether they are the true reasons for people's bad choices or not.

Waiting through several years of depression and suicide attempts before telling anyone or getting it treated.

Not living on campus while you attend college, and also not having a car. (Edit: ... (read more)

The numerous mentions of soured relationships make me think that "not knowing some basic relationship failure modes, or not weighting the possibility that they will happen strongly enough" is another metamistake.

Perhaps not truly life-altering, but I'm currently somewhat regretting not having taken a formal course in Computer Science to complement my physics education. The problem with being self-taught isn't so much that there are gaps, but that you don't know where the gaps are, and you don't have the language to discuss (or Google) them easily.

Turning from myself to others, I see many physics graduate students who would benefit vastly from just one introductory formal course in programming - not computer science per se, just basic programming concepts. Flow co... (read more)

Not flossing for years. Once it is a habit not flossing will seem gross to you. Knowing that your future self will feel this way you should feel this way now.

That thing I'm going to do next month that I don't even know about yet.

As alluded to in other comments, another interesting question is: given your past mistakes and your present circumstances, what do you expect your next "major life-altering mistake" will be?

Potential examples: continue to play football despite repeated concussions, getting together with yet another abusive boyfriend, relying on magical free will to get you through college until finally burning out and dropping out.