What you are actually watching
Monopoly is usually explained as greed, which explains nothing, since every firm would
like more money. What actually separates a monopolist is a constraint that competitive
firms do not face.
Selling more means charging less on everything
A wheat farmer, selling into something close to perfect competition, can
sell another tonne without moving the world price, so each extra sale adds the full price
to revenue. A sole seller cannot. To shift one more unit it has to drop
the price, and that lower price applies to every unit it sells, not only the last
one.
So the true value of one more sale is the price received, minus the discount now being
given on everything else. That is marginal revenue, the purple dashed line,
and it sits below demand from the very first unit. On a straight demand curve it falls
exactly twice as fast, which is why it hits zero halfway along.
Restricting output is the whole strategy
Any seller keeps producing while one more unit adds more than it costs. For a monopolist
that means stopping where marginal revenue meets marginal cost, which happens well before
the price does.
Look at the classic preset. The monopolist charges $13.33 while the last unit cost only
$6.67 to make. It could profitably sell more at any price above cost, and it deliberately
does not, because the extra sales would drag down the price on everything already being
sold. The scarcity is the product.
Why the markup depends on escape routes
Flip between the patented medicine and the crowded market presets. Same logic, wildly
different outcomes. The drug carries an enormous markup, the crowded market barely any.
The difference is how easily buyers walk away. Where substitutes are plentiful, raising the
price loses so many customers that it is not worth doing. Where there is no alternative,
buyers absorb almost anything. The markup share works out as the reciprocal of demand
elasticity, so a firm facing elasticity of 2 keeps half the price as margin, while one
facing elasticity of 10 keeps a tenth.
This is why antitrust and competition policy pay so much attention to whether substitutes
exist, rather than to firm size on its own. Market power is the absence of
alternatives, not the presence of bigness.
Price discrimination is efficient and deeply unfair
Switch to Discriminate. This is perfect price discrimination, where the
seller charges each buyer exactly their own maximum. It no longer has to cut the price on
earlier sales to make one more, so marginal revenue becomes the demand curve itself, output
rises all the way back to the competitive level, and the red deadweight loss wedge
vanishes.
By the standard economists usually reach for, this is the best outcome available. It is
also one where consumers keep precisely nothing. Efficiency and fairness are different
questions, and this case separates them more sharply than any other on this site.
What this model leaves out
Real monopolies rarely look this tidy. This one has no fixed costs, so it says nothing
about natural monopolies where one firm serving everyone really is cheapest, which is
the usual argument for regulating utilities rather than breaking them up.
It also assumes the monopoly simply exists. Where it came from matters a great deal: a
patent is a deliberate temporary monopoly granted to reward invention, and judging it means
weighing this deadweight loss against research that would not otherwise happen. The
diagram measures the cost. It does not tell you whether the cost was worth paying.
Things to try
Each one takes seconds and lands a point the diagram alone will not.
1Compete the power away
On any preset, switch between Competition and Monopoly. Watch price, quantity, and the
red wedge move together. Same costs, same buyers, entirely different outcome.
2Give buyers an exit
On the patented medicine, raise buyer response from 2 towards 15, as though a generic
arrived. The markup collapses without anyone regulating anything.
3Remove the cost of serving
Load Software, where marginal cost is nearly zero. The markup share approaches the
entire price. This is the shape of most digital markets.
4Be efficient and ruthless
Switch to Discriminate and watch deadweight loss hit zero while consumer surplus does
too. Then decide which outcome you would rather live in.
Common questions about monopoly
Why is marginal revenue below the price?
Because a single seller has to lower the price to sell more, and the lower price applies to every unit rather than just the extra one. The gain from selling one more is the new price minus the revenue lost on everything already being sold. With a straight-line demand curve that works out to exactly twice the slope, so marginal revenue starts at the same point as demand and falls away twice as fast.
Why does a monopolist not simply charge as much as possible?
Because demand slopes downward, so a higher price means fewer buyers. Charging the maximum anyone would pay would leave it selling almost nothing. Profit is price multiplied by quantity minus costs, and pushing price too far shrinks quantity faster than it lifts the margin. The best point balances the two, which is where marginal revenue meets marginal cost.
Is monopoly always bad?
The deadweight loss is real, but the answer depends on where the monopoly came from. Patents create temporary monopolies deliberately, accepting this cost in exchange for inventions that might not otherwise be funded. Natural monopolies, where one firm serving everyone is the cheapest option, are usually regulated rather than broken up. What is hard to defend is market power that comes from blocking competitors rather than from inventing or building something.
What is the Lerner index?
The share of the price that sits above marginal cost, so a value of 0.5 means half the price is markup. It equals the reciprocal of demand elasticity at the chosen quantity, which is why it is such a useful measure: it converts an abstract question about market power into one about how easily customers can leave. Competitive firms sit near zero.