Contents
Natural Sciences
Section 1
The Core Idea
For every action there is an equal and opposite reaction. Newton's third law is reciprocity in mechanics: forces come in pairs. When you push on a wall, the wall pushes back. When a rocket expels mass backward, the rocket accelerates forward. The principle generalises. In systems, interventions produce countervailing responses. In markets, one side's gain often implies another's loss — or triggers a reaction that offsets the gain. Reciprocity is the expectation that influence runs both ways; you can't act without the system acting back.
Physics gives the cleanest formulation. No force is one-sided. Momentum is conserved; every push has a counter-push. The same structure appears in feedback: you change a variable, the system responds in a way that either amplifies (positive feedback) or opposes (negative feedback) the change. Reciprocity in the negative-feedback sense is the system pushing back toward equilibrium. In strategy, it appears as competitor response, customer pushback, or regulatory reaction. Do something that hurts others or disrupts equilibrium, and you should expect a response.
The strategic use: before acting, ask what reaction you will trigger. If you cut price, competitors may cut too. If you enter a market, incumbents will respond. If you push the organisation, it will push back unless you've aligned incentives. Reciprocity doesn't mean "don't act" — it means model the reaction and either neutralise it, absorb it, or use it as part of the plan.
Section 2
How to See It
Reciprocity shows up whenever an action is met with a counter-action: competitor moves, customer churn, employee resistance, regulatory response. Look for cause and counter-cause. When you see "we did X and then Y happened," ask whether Y is the system's reciprocal response to X.
Business
You're seeing Reciprocity (Physics) when a price cut by one competitor is matched by others. The action (cut price) has a reaction (rivals cut too). Net effect may be no share gain and lower margins for all. The reciprocal response is built into competitive dynamics. Same with capacity additions: one firm expands, others follow; industry capacity rises and prices fall.
Technology
You're seeing Reciprocity (Physics) when a platform changes its API or policies and developers or partners react — by lobbying, by building workarounds, or by leaving. The platform's action has a reaction from the ecosystem. Ignoring reciprocity leads to surprise backlash and unintended consequences.
Investing
You're seeing Reciprocity (Physics) when a large order moves the market. You buy; price rises. The market "pushes back" via the price mechanism. Same with activism: when an activist takes a position and pushes for change, management and other shareholders react. The outcome depends on the strength and direction of those reciprocal forces.
Markets
You're seeing Reciprocity (Physics) when a central bank raises rates and the economy slows, or when a government imposes tariffs and trading partners retaliate. Policy actions have reactions — in behaviour, in other countries' policies. Reciprocity is the structure of action and counter-action in the system.
Section 3
How to Use It
Decision filter
"Before a major move, ask: what is the equal and opposite reaction? Who is pushed, and how will they respond? Model the counter-move. If the reaction neutralises or overwhelms your move, redesign or prepare for it."
As a founder
Anticipate how competitors, customers, and partners will react to your moves. Price changes, feature launches, and channel moves will trigger responses. Pre-position for the reaction: have the next move ready, or choose actions where the reaction works in your favour (e.g. competitors copying you into a trap). The mistake is assuming the world will stay static. The second mistake is ignoring internal reciprocity — e.g. pushing the team without addressing the forces that will push back (incentives, culture).
As an investor
When evaluating a thesis, ask what reactions the company's success would trigger. Will incumbents copy, acquire, or lobby? Will customers consolidate power? Will regulators step in? Reciprocity says the system will respond; the question is whether the response is manageable or fatal.
As a decision-maker
Use reciprocity to stress-test plans. For every material action, list the likely reactions. If the reactions are benign or aligned, proceed. If they're hostile or offsetting, either change the action (smaller, different angle) or build in capacity to absorb or counter the reaction.
Common misapplication: Assuming reciprocity means "every action is cancelled." Reciprocity means there is a reaction, not that it's equal in outcome. You can still win if your move is stronger or faster than the counter-move, or if the reaction is slow or fragmented.
Second misapplication: Confusing physics reciprocity with social reciprocity (tit-for-tat, gifts). Social reciprocity is about exchange and norms. Physics reciprocity is about forces and system response. Both matter; they're different models.
Section 4
The Mechanism
Section 5
Founders & Leaders in Action
Richard FeynmanPhysicist; Nobel Prize 1965
Feynman's lectures and popular writing made action-reaction and conservation laws accessible. His way of thinking — "what happens if I do this?" and tracing the physical response — is reciprocity in practice. Founders can borrow the habit: for any strategic action, trace the reaction through the system.
Isaac NewtonMathematician and physicist; Principia 1687
Newton's third law is the source. Equal and opposite forces are the core of reciprocity in physics. The same mindset — forces in pairs, conservation — applies when thinking about markets and organisations as systems that react to interventions.
Section 6
Visual Explanation
Reciprocity (Physics) — Action and reaction. Every force has a counter-force; every move can trigger a counter-move.
Section 7
Connected Models
Reciprocity in physics ties to forces, equilibrium, and system response. These models either formalise it or apply it in strategy.
Reinforces
Newton's Laws
Newton's third law is reciprocity: action and reaction forces are equal and opposite. The first two laws (inertia, F=ma) describe how bodies respond to forces. Together they underpin mechanical intuition.
Reinforces
Leverage (Physics)
Leverage multiplies force; the reaction (e.g. load) is still there, but the point of application and direction matter. Reciprocity says the load pushes back; leverage determines how that exchange plays out.
Reinforces
Equilibrium
Equilibrium is when opposing forces or flows balance. Reciprocity is the mechanism: when you push the system, it pushes back until a new balance is reached. Markets and organisms tend toward equilibrium through reciprocal adjustment.
Leads-to
[Feedback](/mental-models/feedback) Loops
Feedback loops are reciprocity over time: output feeds back as input. Negative feedback is the system "pushing back" to restore a set point. Positive feedback is the system amplifying the initial change.
Leads-to
Second-Order Effects
Second-order effects are the reactions to the reaction. You act; the system reacts (first order); that reaction may trigger further effects (second order). Reciprocity is the first step; second-order thinking traces the chain.
Tension
Symmetry
In physics, symmetry often implies conservation laws; reciprocity (action-reaction) is a form of symmetry. In strategy, symmetry can mean "we and our rivals have similar options" — reciprocal capability. Asymmetric moves try to break that symmetry.
Section 8
One Key Quote
— Isaac Newton, Principia Mathematica (1687)"To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts."
The law is exact in mechanics. In strategy and organisations, "equal" and "opposed" are approximate: reactions may be delayed, distributed, or unequal. The habit of looking for the counter-move is what carries over.
Section 9
Analyst's Take
Faster Than Normal — Editorial View
Always ask: what's the counter-move? Before a price change, a launch, or a reorg, list who is affected and how they might respond. Competitors will react. Customers will react. The org will react. Model it. If the reaction is worse than the gain, rethink the move.
Use reciprocity in your favour. Sometimes you want the reaction — e.g. competitors copying you into a price war that they can't sustain and you can. Sometimes you want to move in a way that makes the reaction slow or fragmented (e.g. niche entry, regulatory arbitrage). Design the move with the reaction in mind.
Internal reciprocity matters. Pushing the team without addressing what pushes back (compensation, clarity, trust) leads to resistance or exit. The "reaction" is lower morale, politics, or turnover. Align incentives and communication so the reciprocal force supports the direction.
Reactions can be delayed. The market or the competitor may not respond immediately. That doesn't mean reciprocity doesn't apply — it means the reaction may come in the next quarter or the next year. Plan for lagged responses.
Conservation of "something." In physics, momentum and energy are conserved. In strategy, value, attention, or political capital often are. When you gain somewhere, something else may give. When you ask for more from the system, the system may take from somewhere else. Look for the conservation law in your context.
Section 10
Test Yourself
Is this mental model at work here?
Scenario 1
A company cuts prices 20% to gain share. Within two months, two key competitors match the cut. Share is flat; industry margins are down.
Scenario 2
A platform unilaterally changes its API terms. Several large partners threaten to leave; the platform delays the change and negotiates.
Scenario 3
A country imposes tariffs. Trading partners announce retaliatory tariffs. Trade volumes fall; no side gains.
Scenario 4
A founder announces a big reorg without prior alignment. Key execs leave within six months; the reorg is partially rolled back.
Section 11
Top Resources
Summary: Reciprocity is action and reaction. In physics, forces come in pairs. In strategy, your moves trigger counter-moves. Model the reaction before you act; use it or neutralise it.
01
Book
The source of the third law. Newton's formulation of action and reaction and its role in mechanics and conservation of momentum.
02
Book
Feynman's treatment of Newton's laws and the habit of tracing cause and effect. Reciprocity in context of dynamics and conservation.
03
Book
Feedback loops and system response. How systems "push back" when you intervene. The systems analogue of reciprocity.
04
Concept
Game theory models how players react to each other's moves. Reciprocity is the structure of strategic interaction; game theory is the formalisation.
05
Concept
Conservation of momentum and energy as consequences of symmetry and reciprocity. Deeper physics behind "equal and opposite."
Why this matters next
mental modelsIncentives
Reciprocity (Physics) applied the Incentives mental model
mental modelsSecond-Order Thinking
Reciprocity (Physics) applied the Second-Order Thinking mental model
mental modelsLeverage
Reciprocity (Physics) applied the Leverage mental model
mental modelsMomentum
Reciprocity (Physics) applied the Momentum mental model
mental modelsInertia
Reciprocity (Physics) applied the Inertia mental model
mental modelsIntuition
Reciprocity (Physics) applied the Intuition mental model
Frequently asked questions
What is Reciprocity (Physics)?
Reciprocity (Physics) is a mental model used for better thinking and decision-making.
How do you apply Reciprocity (Physics)?
To apply Reciprocity (Physics), identify situations where this framework is relevant, then use it as a lens to evaluate your options and decisions. The model is most useful when combined with other complementary mental models.
What category does Reciprocity (Physics) fall under?
Reciprocity (Physics) falls under the Natural Sciences category of mental models. Other models in this category can be found on the Natural Sciences hub page.
Why is Reciprocity (Physics) important?
Reciprocity (Physics) is important because it provides a structured way to think about problems that would otherwise be approached with intuition alone. Understanding this model helps you avoid common reasoning errors and make better decisions.
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