What Trading Is — and Who Is on the Other Side

Every trade has two sides. When you buy, someone sells to you; when you sell, someone buys from you. Before costs, the money that all active traders make and lose nets out against the market's own return — trading moves money between participants, it does not create any. After costs — spreads, commissions, slippage, fees — the group of active traders as a whole must earn less than the market. This is not a moral claim or a pessimist's opinion; it is arithmetic, set out by Nobel laureate William Sharpe in a three-page 1991 paper.

Key takeaway

Active trading is a negative-sum game after costs. Any claim of trading profit is a claim of taking money from someone on the other side — usually someone with better information, faster systems, or lower costs than you.

Who is on the other side

Market makers and high-frequency firms, who quote both sides and earn the spread — they are the counterparty to most retail orders.
Institutions working large orders over hours or days with execution algorithms.
Other retail traders — the counterparty retail most often beats, and the one everyone assumes they are trading against.

The best complete-market evidence comes from Taiwan, where researchers could see every account in the market. Individual investors' aggregate losses ran at about 2.2% of Taiwan's GDP per year, and virtually all of their trading losses traced to their aggressive orders — the ones that demand immediate execution. Institutions were on the winning side. Retail losses were, in large part, a transfer.

Worked example

The arithmetic of the whole market

Imagine every investor in a market combined holds exactly the market portfolio, which returns 8% this year. Now split them into two groups: passive holders, who do nothing, and active traders, who trade with each other. The passive group earns 8% minus near-zero costs. The active group, in aggregate, also holds the market — every buy matched a sell inside the group — so before costs it also earns 8%. But the active group paid spreads, commissions, and slippage all year. Its net return is 8% minus those costs. The average active trader must underperform the average passive holder. For any active trader to beat the market, another must lose to them by the same amount, plus everyone's costs.

This is why the course keeps repeating: the burden of proof is on the claim that you, specifically, will be on the winning side of the transfer.

The default that needs no edge

Owning the whole market cheaply and doing nothing — passive index investing — earns the market return minus near-zero costs, requires no edge, and beats the aggregate of active traders by construction. This course teaches trading as a skill discipline for people who understand that and still choose to attempt it with money they can afford to lose. Choosing the default instead is not failure; it is what the arithmetic recommends for most people, and this course will say so again in its final module.

Where a retail market order goes

Step 1 of 3

Order placed: BUY 100 shares at market.

You tap Buy. Your broker routes the order — often to a wholesale market maker rather than directly to an exchange.

Common mistakes — and how to catch them in yourself

Assuming the counterparty is another amateur — 'someone has to be the loser, it won't be me.'
Self-check: When you imagine a trade, ask yourself who is selling to you and why. If your honest answer is 'no idea', you have not priced the other side.
Treating market return as the baseline you get for free while trading — it is the baseline you give up costs against.
Self-check: If you have traded before: did you ever compare your yearly result to simply holding an index fund? If you never ran that comparison, that avoidance is itself data.

Practice — Journal

Write your starting position

In your journal notes, write three sentences: why you want to trade, what you believe your edge could eventually be, and what evidence would convince you to stop. You will re-read this at graduation — the course grades your process, and this is its first entry.

Open the exercise

You are ready to move on when…

You can explain why active trading is negative-sum after costs without using the word 'probably'.
You can name three kinds of counterparties on the other side of a retail order.
You can state what the passive alternative earns and why it needs no edge.
You have written your starting-position note in the journal.

Sources & evidence status

Sharpe (1991), “The Arithmetic of Active Management,” Financial Analysts JournalReplicated evidence
Before costs, active investing in aggregate earns the market return; after costs it must earn less.
Barber, Lee, Liu & Odean (2009), “Just How Much Do Individual Investors Lose by Trading?,” Review of Financial StudiesDocumented
Complete Taiwan market data: individual investors' aggregate losses ≈ 2.2% of GDP; losses concentrated in aggressive orders; institutions on the winning side.

Educational content only — not financial advice. Nothing in this course is a recommendation to buy or sell any asset.