🧠 99 Mental Models
Principles to Guide your Thinking.
99 Mental Models is a curated list of mental models based on principles from various disciplines such as mathematics, physics, economics, and philosophy.
1
First Principles Thinking
Break everything down to the basics.
DEFINITION
Deconstruct a problem into fundamental truths and build up from there.
EXAMPLE
If you're trying to reduce living costs, first list the absolute essentials (rent, food, utilities) before cutting anything else.
2
The Pareto Principle
80% of results come from 20% of inputs.
DEFINITION
Focus effort on the small part that yields the biggest impact.
EXAMPLE
At work, identify the few tasks that generate most of your productivity and prioritize them.
3
Inversion
Think backwards to avoid failure.
DEFINITION
Plan by imagining how something could go wrong, then prevent those pitfalls.
EXAMPLE
When planning a family trip, list what might ruin it (e.g., no backup plans, running out of cash) and address those issues first.
4
Occam's Razor
Simplicity usually wins.
DEFINITION
When multiple explanations exist, choose the simplest one with the fewest assumptions.
EXAMPLE
If your internet goes out, check the router or cable first instead of assuming a complex technical failure.
5
Parkinson's Law
Work expands to fill available time.
DEFINITION
Tasks consume all the time allotted, so shorten deadlines to get things done faster.
EXAMPLE
If you give yourself a day to clean your room, it’ll take a day. If you give yourself an hour, you’ll finish it in an hour.
6
Opportunity Cost
The cost of the next best choice.
DEFINITION
Every decision has a trade-off; what you give up to do something else.
EXAMPLE
Choosing to watch TV for an hour means losing time you could’ve spent exercising or studying.
7
Compound Interest
Exponential growth through reinvestment.
DEFINITION
Gains generate further gains over time, leading to faster growth.
EXAMPLE
Investing a small amount regularly in stocks can grow significantly as returns compound.
8
Circle of Competence
Operate where you’re knowledgeable.
DEFINITION
Focus on areas you truly understand and avoid those you don’t.
EXAMPLE
If you’re great at design but not coding, specialize in design to maximize results.
9
The Bridge Problem
Tackle bottlenecks first.
DEFINITION
Identify and fix the weakest link to improve overall performance.
EXAMPLE
If your online store has many visitors but few checkouts, optimize the checkout process before increasing ads.
10
Tool vs. Goal Fallacy
Don’t let tools distract you from actual goals.
DEFINITION
Tools should serve your objectives, not overshadow them.
EXAMPLE
Buying fancy kitchen gadgets won’t make you cook more unless you actually use them consistently.
11
Confirmation Bias
We seek out what we already believe.
DEFINITION
Tendency to favor information that supports existing views and ignore contrary data.
EXAMPLE
Reading only news sources that share your political views reinforces them without challenge.
12
Dunning-Kruger Effect
The less you know, the more confident you are.
DEFINITION
People with limited knowledge often overestimate their competence.
EXAMPLE
A novice programmer feeling they can build a complex app quickly until they face real challenges.
13
Law of Unintended Consequences
Actions can have unexpected outcomes.
DEFINITION
Well-intentioned changes may produce side effects we didn’t anticipate.
EXAMPLE
Offering a bonus for sales can lead employees to oversell or ignore customer needs.
14
Anchor Effect
Initial information lingers in decisions.
DEFINITION
Relying too heavily on the first piece of information offered.
EXAMPLE
Seeing a high original price on a sale item makes the discounted price seem like a steal, even if it’s not.
15
Sunk Cost Fallacy
Don’t stick with something just because you’ve invested in it.
DEFINITION
Future decisions should only consider future costs and benefits, not past investments.
EXAMPLE
Watching a movie you hate to the end just because you paid for a ticket.
16
Prisoner’s Dilemma
Individual interest vs. group benefit.
DEFINITION
Demonstrates why two rational individuals might not cooperate, even if it’s in their best interest.
EXAMPLE
Two coworkers blame each other for a mistake instead of working together to fix it.
17
Cobra Effect
Incentives can backfire.
DEFINITION
A solution that tries to fix a problem but ends up making it worse due to unintended responses.
EXAMPLE
A bounty on snakes might encourage people to breed snakes for reward money.
18
Butterfly Effect
Small changes can have large impacts.
DEFINITION
Tiny initial differences can lead to vastly different outcomes over time.
EXAMPLE
Being 5 minutes late to a meeting could alter the entire day’s schedule and subsequent events.
19
Law of Diminishing Returns
Beyond a point, more input yields fewer gains.
DEFINITION
Increasing one factor of production eventually leads to smaller improvements.
EXAMPLE
Studying 14 hours a day might be less productive than studying 7 hours effectively.
20
Lindy Effect
The longer it’s been around, the longer it’s likely to last.
DEFINITION
Life expectancy of non-perishable things increases with each day of survival.
EXAMPLE
A classic book still read after 50 years will likely remain popular longer than a recent trend.
21
Boiling Frog Syndrome
Gradual changes go unnoticed.
DEFINITION
If a change is gradual, people might not notice until it’s too late.
EXAMPLE
Spending slightly more each month without noticing it’s impacting your savings.
22
The Peter Principle
People rise to their level of incompetence.
DEFINITION
In hierarchies, employees keep getting promoted until they reach a job they cannot perform well.
EXAMPLE
A brilliant engineer promoted to manager who struggles with leadership tasks.
23
Tragedy of the Commons
Shared resources get overused.
DEFINITION
Individuals acting in self-interest deplete shared resources, harming the group in the long run.
EXAMPLE
Overfishing in international waters leads to fish stock depletion.
24
Second-Order Thinking
Consider the consequences of consequences.
DEFINITION
Evaluate not just the immediate impact of decisions, but the chain of future outcomes.
EXAMPLE
Offering a deep discount might boost short-term sales but degrade your brand value over time.
25
The 5 Whys
Keep asking why to find root causes.
DEFINITION
Iteratively question the reason behind an issue until the underlying cause is identified.
EXAMPLE
Your car won't start → Why? Battery dead → Why? Left lights on → Why? Forgot to check before locking up, etc.
26
The Eisenhower Matrix
Prioritize by urgency and importance.
DEFINITION
A framework dividing tasks into four quadrants: urgent/important, important/not urgent, urgent/not important, neither.
EXAMPLE
Scheduling doctor’s appointments (important/not urgent) vs. responding to pings (urgent/not important).
27
Zeigarnik Effect
We remember incomplete tasks better.
DEFINITION
People recall uncompleted tasks more vividly than completed ones, affecting focus.
EXAMPLE
Leaving an unfinished puzzle keeps you thinking about it until it’s solved.
28
Spotlight Effect
We overestimate how much others notice us.
DEFINITION
People think their actions and appearance are more prominent than they actually are.
EXAMPLE
Worrying excessively that everyone noticed a small stain on your shirt.
29
Survivorship Bias
We focus on successes and ignore failures.
DEFINITION
Drawing conclusions from the few that survived, missing lessons from those that didn't.
EXAMPLE
Reading success stories of entrepreneurs might mislead you about the real chances of startup success.
30
Planning Fallacy
We underestimate time for tasks.
DEFINITION
People are overly optimistic about how quickly they can complete tasks.
EXAMPLE
Consistently missing deadlines because you assume work will go smoothly.
31
Velocity vs. Speed Problem
Direction matters more than raw speed.
DEFINITION
Moving fast in the wrong direction is worse than moving slower in the correct direction.
EXAMPLE
Quickly developing features for a product before validating the core idea might be wasteful.
32
Red Queen Effect
You must keep improving just to stay in place.
DEFINITION
In a competitive environment, standing still means falling behind.
EXAMPLE
A company that doesn't update its products constantly loses to more innovative competitors.
33
Hedonic Treadmill
We adapt to pleasure over time.
DEFINITION
After positive or negative changes, people return to a baseline level of happiness.
EXAMPLE
Buying a new car feels great initially, but the excitement fades, returning you to your usual mood.
34
Law of Large Numbers
Averages become stable with more data.
DEFINITION
As a sample size grows, its mean gets closer to the average of the whole population.
EXAMPLE
Flipping a coin many times eventually approaches a 50/50 distribution of heads and tails.
35
Black Swan
Rare events with huge impact.
DEFINITION
Unpredictable outliers that drastically change outcomes.
EXAMPLE
A global pandemic suddenly affecting all travel and industries worldwide.
36
Law of Small Numbers
Small samples can mislead.
DEFINITION
Conclusions drawn from small data sets may not represent reality.
EXAMPLE
Basing business strategy on just a few customer reviews might give a skewed picture.
37
Pygmalion Effect
High expectations enhance performance.
DEFINITION
People perform better when higher expectations are placed on them.
EXAMPLE
A teacher believing certain students are gifted unknowingly motivates them to excel.
38
Broken Window Theory
Signs of disorder invite more disorder.
DEFINITION
Visible neglect or minor crimes can lead to bigger issues if unchecked.
EXAMPLE
A single broken window left unrepaired can encourage more vandalism in a neighborhood.
39
Self-Fulfilling Prophecy
Beliefs can influence outcomes.
DEFINITION
An expectation prompts behaviors that make the expectation come true.
EXAMPLE
If you think you’ll fail a test, you might study less and actually fail.
40
Scarcity Mental Model
Limited availability increases perceived value.
DEFINITION
People place higher value on things that appear to be scarce.
EXAMPLE
Limited-time offers encourage quick purchases for fear of missing out.
41
Endowment Effect
We overvalue what we own.
DEFINITION
People assign more value to things simply because they own them.
EXAMPLE
You might price your used car higher than the market because of emotional attachment.
42
Abilene Paradox
Group members agree to a plan none of them want.
DEFINITION
A group collectively decides on a course of action, even though individuals privately oppose it.
EXAMPLE
Family members agree on a vacation spot they don’t really like, each thinking the others want it.
43
Focusing Illusion
We overemphasize one aspect of an event or situation.
DEFINITION
Placing too much importance on a single detail, distorting overall perception.
EXAMPLE
Obsessing over job salary while ignoring company culture or work-life balance.
44
Normalcy Bias
We underestimate the possibility of disaster.
DEFINITION
People assume things will always function the way they normally do.
EXAMPLE
Ignoring evacuation warnings because “it’s never flooded here before.”
45
Status Quo Bias
We prefer things to stay the same.
DEFINITION
A tendency to resist change even when it may offer benefits.
EXAMPLE
Sticking with a subpar phone plan because switching feels like a hassle.
46
Ostrich Effect
We ignore negative information.
DEFINITION
People avoid inconvenient truths by metaphorically ‘burying their head in the sand.’
EXAMPLE
Not checking credit card bills to avoid facing the reality of debt.
47
Reciprocity Principle
We feel obligated to return favors.
DEFINITION
When someone gives us something, we often feel compelled to give something in return.
EXAMPLE
Offering free samples in a store can increase sales because customers feel indebted.
48
Authority Bias
We trust and obey authority figures too readily.
DEFINITION
Placing undue weight on opinions from perceived authority figures.
EXAMPLE
Buying a product because a celebrity endorses it, regardless of its actual quality.
49
Halo Effect
We see everything about a person as positive based on one trait.
DEFINITION
One positive impression leads us to assume overall positive qualities.
EXAMPLE
If a coworker is very punctual, you might assume they’re also more competent in all tasks.
50
Horns Effect
One negative trait taints our entire perception.
DEFINITION
A single negative impression leads us to assume overall negative qualities.
EXAMPLE
If a student is late once, a teacher might brand them lazy and irresponsible.
51
Contrast Effect
Comparisons distort perception.
DEFINITION
Judging something relative to what's around it rather than its own merits.
EXAMPLE
An average meal feels amazing after tasting something terrible right before.
52
Availability Heuristic
We judge probability by what’s most easily recalled.
DEFINITION
People assess likelihood based on how quickly examples come to mind.
EXAMPLE
Fearing plane crashes more than car accidents because plane crashes are heavily publicized.
53
Representativeness Heuristic
We judge likelihood by similarity to existing stereotypes.
DEFINITION
Assessing probability by how much something resembles our mental category or prototype.
EXAMPLE
Assuming a quiet person is a librarian rather than an athlete, even if statistically it’s unlikely.
54
Base Rate Fallacy
Ignoring general statistics in favor of specific information.
DEFINITION
Failing to consider the overall odds while focusing on details of a particular case.
EXAMPLE
Believing your lottery ticket will win due to a “lucky feeling,” ignoring the 1-in-million odds.
55
Satisficing Principle
Settle for ‘good enough’ instead of perfect.
DEFINITION
Choosing an option that meets minimum requirements rather than seeking the best possible solution.
EXAMPLE
Picking the first decent restaurant you find instead of comparing every option.
56
Pareto Efficiency
No one can be better off without making someone else worse off.
DEFINITION
A state of resource allocation where any change to help one party harms another.
EXAMPLE
In a negotiation, you’ve reached a point where any further concession benefits one side but hurts the other.
57
Tipping Point
Small changes accumulate to a big shift.
DEFINITION
The moment when a minor input causes a critical mass and leads to large-scale impact.
EXAMPLE
A social media trend that grows slowly but suddenly “goes viral.”
58
Law of Averages
Outcomes of a random event will 'even out' in the short term.
DEFINITION
People mistakenly believe outcomes of a chance process must become balanced.
EXAMPLE
Expecting the next coin toss to be heads if you’ve had several tails in a row.
59
Pareto Distribution
A few large events and many small ones.
DEFINITION
A power-law distribution where most effects come from a small cause, more extreme than the 80/20 rule.
EXAMPLE
Wealth distribution often follows a Pareto pattern, with few very rich and many with less wealth.
60
Thermodynamics (as a Model)
Energy conservation and entropy can guide processes.
DEFINITION
Thermodynamic principles can be applied metaphorically to understand resource usage and disorder.
EXAMPLE
Spreading yourself too thin across many tasks can increase ‘entropy’ (disorganization) in your workflow.
61
Gambler’s Fallacy
Belief that past random events influence future ones.
DEFINITION
Expecting a change in outcome because of a run of luck or bad luck.
EXAMPLE
Thinking you’re “due” for a lottery win after losing several times.
62
Just-World Hypothesis
Assuming the world is fair and people get what they deserve.
DEFINITION
Belief that good things happen to good people and bad things happen to bad people.
EXAMPLE
Blaming victims for their misfortune because “they must have done something wrong.”
63
Hedonic Adaptation
We quickly adapt to good or bad changes.
DEFINITION
Similar to the hedonic treadmill—happiness level returns to baseline after life changes.
EXAMPLE
Getting a raise feels great at first, but soon becomes the new normal.
64
Cognitive Dissonance
Discomfort from holding conflicting beliefs.
DEFINITION
Tension arises when one’s thoughts or actions conflict, leading to rationalization.
EXAMPLE
Justifying an expensive purchase by focusing on its “long-term value.”
65
Reciprocity Fallacy
Expecting reciprocation even in mismatched relationships.
DEFINITION
Presuming that any action you do for others will be equally returned.
EXAMPLE
Doing extra work for coworkers and being upset when they don’t do the same for you, ignoring context.
66
10,000-Hour Rule
Mastery requires extensive practice.
DEFINITION
Popularized concept that achieving world-class skill demands about 10,000 hours of practice.
EXAMPLE
Spending years practicing piano daily to become a concert-level pianist.
67
Delay of Gratification
Short-term sacrifice for long-term gain.
DEFINITION
Resisting immediate temptations to achieve better outcomes later.
EXAMPLE
Skipping dessert today for better health in the long run.
68
Super Mario Effect
Focus on the goal, treat failures like stepping stones.
DEFINITION
A mindset of learning from mistakes by seeing them as part of the journey.
EXAMPLE
Treating coding errors as feedback rather than personal failures to keep motivated.
69
IKEA Effect
We overvalue things we assemble ourselves.
DEFINITION
Investing labor leads to increased value placed on the end product.
EXAMPLE
Preferring your self-assembled shelf over a higher-quality one from the store because you built it.
70
Gell-Mann Amnesia
Forgetting a source’s inaccuracy in one domain while trusting it in another.
DEFINITION
Observing an error in a news article on a topic you know well, yet trusting the outlet’s reporting on other topics.
EXAMPLE
Seeing a newspaper misreport your professional field but still believing their sports coverage is flawless.
71
Flynn Effect
IQ scores tend to increase from one generation to the next.
DEFINITION
Observed rise in standardized intelligence test scores over time, possibly due to better education and nutrition.
EXAMPLE
Modern children scoring higher on old IQ tests than children decades ago.
72
Knowledge Doubling Curve
Human knowledge grows exponentially.
DEFINITION
The total amount of human knowledge doubles faster over time, once estimated at every few years and accelerating.
EXAMPLE
Advancements in technology and science now happen at breakneck speeds, surpassing prior decades’ pace.
73
Bell Curve
Normal distribution of traits or data.
DEFINITION
Most occurrences cluster around the mean with fewer outliers at the extremes.
EXAMPLE
Height, test scores, and many natural phenomena often follow a bell curve distribution.
74
1% Rule
Small daily improvements lead to big results.
DEFINITION
Improving by 1% each day compounds significantly over time.
EXAMPLE
Learning a bit of a new language every day rather than cramming once a month.
75
Two Pizza Rule
Keep teams small enough for two pizzas to feed them.
DEFINITION
A team is most efficient if it can be fed with just two pizzas, implying fewer than ~8 people.
EXAMPLE
A startup’s product team remains nimble by limiting members to 5, ensuring quick decision-making.
76
Law of Least Effort
People naturally choose the path of least resistance.
DEFINITION
Given multiple options, humans often choose the easiest route.
EXAMPLE
Opting for the elevator instead of the stairs, even if the stairs are healthier.
77
Pareto Frontier
Optimal trade-offs where no one can improve without another losing.
DEFINITION
In multi-objective optimization, you can’t better one objective without worsening another.
EXAMPLE
Balancing cost vs. quality: you can’t improve quality without adding cost or reducing another feature.
78
Streetlight Effect
We look for answers where it’s easiest to search.
DEFINITION
People often only search for something where the searching is simpler, not necessarily where it's most likely found.
EXAMPLE
Looking under a bright lamp for your lost keys even if you dropped them in a dark alley.
79
Chicken-and-Egg Problem
Difficulty starting when each part depends on the other.
DEFINITION
A situation where two events each require the other to happen first.
EXAMPLE
Needing a credit history to get a credit card, but needing a credit card to build credit history.
80
Net Present Value
Future money is worth less than present money.
DEFINITION
Financial principle that discounts future cash flows to their present-day value.
EXAMPLE
Receiving $100 now is better than $100 in a year because you can invest it immediately.
81
Margin of Safety
Leave room for error in decisions.
DEFINITION
Invest or act with a buffer to protect against the unexpected.
EXAMPLE
If you’re building a bridge that must hold 10 tons, design it for 12 tons.
82
Circle of Influence vs. Circle of Concern
Focus energy on what you can control.
DEFINITION
Differentiate between issues you can affect and those you can’t, to use time more effectively.
EXAMPLE
You can’t control the weather, but you can plan an indoor event if rain is likely.
83
Swim Lane Theory
Assign clear, separate responsibilities.
DEFINITION
Visualizing processes in parallel lanes so that everyone knows their role and avoids confusion.
EXAMPLE
Organizing tasks by department in a project chart so each team sees only their tasks.
84
KISS Principle
Keep It Simple, Stupid.
DEFINITION
Solutions are most effective when kept as simple as possible.
EXAMPLE
A single-page checklist can often outperform a lengthy manual.
85
RAS Theory (Reticular Activating System)
We notice what we focus on.
DEFINITION
Part of the brain that filters information based on what’s important to us.
EXAMPLE
When you decide to buy a certain car model, you start seeing it everywhere.
86
Curiosity Gap
We’re driven to fill knowledge gaps.
DEFINITION
The feeling of wanting to know what we don't yet understand keeps us engaged.
EXAMPLE
Clickbait titles exploit this by hinting at interesting content without revealing it immediately.
87
Dunbar’s Number
We can maintain about 150 stable social relationships.
DEFINITION
Cognitive limit to the number of people one can meaningfully interact with.
EXAMPLE
A company with under 150 employees can often remain close-knit without formal hierarchy.
88
Gresham’s Law
Bad money drives out good money.
DEFINITION
When two currencies circulate, people hoard the more valuable one and spend the other first.
EXAMPLE
Saving real gold coins and spending paper money or coins with less precious metal content.
89
Amygdala Hijack
Emotional response overrides rational thinking.
DEFINITION
When the fear center of the brain takes control, leading to impulsive reactions.
EXAMPLE
Lashing out at someone who criticizes you before you can logically process their feedback.
90
Brain Plasticity
The brain changes with experience.
DEFINITION
Neural pathways adapt based on learning, environment, and behavior.
EXAMPLE
Consistent practice on a musical instrument reshapes areas of the brain responsible for motor skills.
91
Chunking Technique
Group information for easier recall.
DEFINITION
Splitting content into smaller, meaningful units improves memory and learning.
EXAMPLE
Remembering a phone number in segments (XXX-XXX-XXXX) instead of 10 random digits.
92
Theory of Constraints
A system’s weakest link sets its limit.
DEFINITION
Identify the primary constraint or bottleneck and fix it to improve the entire system.
EXAMPLE
A slow packaging machine in a factory limits overall production, so speeding it up increases output.
93
Iceberg Principle
Most of the problem is hidden below the surface.
DEFINITION
Visible issues are often a small part of a much larger underlying situation.
EXAMPLE
Customer complaints might hint at deeper product or process flaws not immediately obvious.
94
Babel Problem
Communication fails when everyone speaks differently.
DEFINITION
Inconsistent languages or frameworks lead to misunderstanding and errors.
EXAMPLE
A project team using different coding styles and project management approaches leads to chaos.
95
Groupthink
Harmony in a group leads to poor decisions.
DEFINITION
Desire for consensus overrides critical evaluation of alternatives.
EXAMPLE
A team ignoring obvious flaws in a proposal because they want to maintain group unity.
96
Apophenia
Seeing patterns where none exist.
DEFINITION
Humans tend to perceive meaningful connections in random data.
EXAMPLE
Spotting shapes in clouds or believing random coincidences have deep significance.
97
Capgras Delusion
Belief that a familiar person is replaced by an imposter.
DEFINITION
A psychological condition where individuals think loved ones are duplicates or imposters.
EXAMPLE
Rare, but used to illustrate how drastically perception can be skewed by mental processes.
98
Growth Mindset
Abilities can be developed with effort.
DEFINITION
The belief that skills and intelligence can be improved through dedication and hard work, rather than being fixed traits.
EXAMPLE
Learning to code is difficult at first, but believing you can improve with consistent practice leads to success.
99
Serendipity
Discoveries made by chance.
DEFINITION
The occurrence of unexpected but beneficial discoveries when you're pursuing something else.
EXAMPLE
While trying to invent synthetic rubber, scientists accidentally created Silly Putty.
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