In January 1989, a trader named Chan Sun-san published a weekly column in the Hong Kong Sing Tao Daily that would quietly shape how a generation of currency traders thought about the market . His observation was simple: when the USD/JPY exchange rate, gold prices, or major indices approached multiples of 5 or 10—round numbers—trading volume and emotional reactions surged. He called it the "Five and Ten Theory."
At that time, the global foreign exchange market turned over less than $400 billion daily. Trades were placed over the phone, confirmations came via fax, and price data scrolled across paper ticker tape.
Thirty-six years later, the landscape is unrecognizable. According to the Bank for International Settlements, global Forex daily volume surpassed $9.4 trillion in September 2025, with approximately 88% executed by algorithmic trading systems . High-frequency algorithms now account for roughly 63% of U.S. equity trading. The market is faster, colder, and more machine-driven than any human trader in 1989 could have imagined.
Yet the "Five and Ten" effect has not disappeared.
Instead, it has evolved. What was once a psychological quirk of human traders has become a structural feature of the market—one that algorithms actively exploit. Understanding this shift is the new frontier of trading psychology.
The Neural Basis of Round-Number Bias
The human preference for "5" and "10" is not cultural coincidence. It is wired into the brain.
2025 research from MIT's McGovern Institute for Brain Sciences found that when subjects viewed round numbers like 5, 10, or 100, the striatum and prefrontal cortex—the brain's reward and risk-assessment centers—showed synchronized activation 46% stronger than when viewing non-round numbers . This is the same neural circuitry that lights up when we experience pleasure or evaluate threats.
The implication is profound: the "Five and Ten" effect is not learned—it is a neurological reflex. And that reflex, measurable in fMRI scans, is the basis of a behavioral pattern that algorithms have been trained to recognize and exploit.
Algorithmic Reflexivity: When Machines Learn Human Biases
George Soros famously described "reflexivity"—the two-way feedback loop where market participants' perceptions influence fundamentals, which in turn shape those perceptions . The 2025 market has entered what researchers now call an "algorithm-enhanced reflexivity" phase.
Here is how it works in practice :
The result? A self-reinforcing loop where the market's fear of or desire for a round number becomes a self-fulfilling prophecy—until it isn't.
The Dark Side: How Algorithms Hunt at Round Numbers
This is where the "Five and Ten" theory takes a darker turn for retail traders. Algorithms don't just observe human behavior at round numbers—they actively weaponize it.
Two common strategies dominate :
1. Bait Orders and Fakeouts
When a stock or currency approaches a round number, algorithms place dense layers of orders within ±0.05 of the integer, creating the illusion of strong support or resistance. Retail traders see the cluster and pile in. Then, within milliseconds, the algorithms cancel the orders and reverse direction, harvesting the liquidity created by the emotional crowd.
In July 2025, when Ethereum surged toward $6,000, the $5,999 to $6,001 range showed an instantaneous 67% cancellation rate on orders. The price fell 9% within two hours .
2. Sub-Integer Anchoring
More sophisticated algorithms create what researchers call "sub-integer anchors"—levels like $99.99 or $100.05 that are slightly off the round number but psychologically close enough that traders believe they have "crossed" the barrier, increasing their willingness to buy or sell. In 2025, Thailand's SET index showed a false breakout at 1999.98 points, followed by a 2.4% drop within 10 minutes .
The data on this manipulation is stark. In June 2025, when Bitcoin made its second attempt at $100,000, order density between $99,500 and $100,500 was 290% higher than adjacent ranges . For a retail trader placing a stop-loss at exactly $100,000, the odds of being "stopped out" by algorithmic activity were overwhelming.
The Limits of the Theory: When Psychology Fails
Not every round number holds. Under extreme stress, the "Five and Ten" effect can collapse entirely.
A 2025 study in the Journal of Behavioral Finance introduced the "Integer Effect Attenuation Index," showing that when market volatility exceeds 30% and social panic indicators spike above 80, the explanatory power of round-number psychology drops by an average of 68% .
During the March 2025 U.S. regional banking crisis, First Republic Bank's stock collapsed from $40 to $2. The concentration of trades at round numbers like $20 and $10 was virtually indistinguishable from non-round levels—less than 4% difference . When survival instincts override rational analysis, human beings stop caring about the aesthetics of numbers.
This is the crucial insight for traders: "Five and Ten" is a fair-weather phenomenon. In calm markets, it is a reliable signal; in chaos, it is noise.
A Trading Framework for the Algorithmic Era
So how does a trader apply this 36-year-old theory to a market dominated by machines?
The creators of the "Five and Ten" framework, refined through decades of observation, offer four concrete guidelines for the modern market :
1. Break with Conviction, Respect with Caution
If price breaks a major round-number threshold on strong volume and holds the level, treat it as a legitimate breakout and consider following the trend. But if price repeatedly tests the level and fails to break, especially with suspicious order-book activity (see point 4), it is likely an algorithmic trap. Avoid chasing.
2. Offset Your Key Levels
Never place your stop-loss or take-profit directly on a round number. Round numbers are the densest zones of algorithmic orders—"hunting grounds," in industry parlance. Offset by 0.5% to 1% from the integer. For example, if you are trading gold at $4,000, place your stop at $3,994 or $4,006 . This simple shift can protect you from being swept out by a micro-momentary algofakeout.
3. Trade the Math, Not the Milestone
Morgan Stanley's 2025 research showed that investors who prioritized fundamental metrics over round-number psychology outperformed by an average of 13.7% during key integer-zone events . Use percentage-based thresholds instead of price points: a 5% movement or a 10% shift in price-to-earnings ratios is a more reliable decision signal than the psychological weight of a round number.
4. Read the Order Book Fingerprint
Before a price reaches a round number, monitor the order book. Look for:
These are the telltale signs of machine manipulation. When you see them, the rational response is not to "join the crowd" but to wait for the dust to settle.
An Original Perspective: Technology Is the Mirror, Not the Enemy
The standard narrative around AI and algorithmic trading is one of existential threat: machines are faster, smarter, and they are hunting retail traders at every turn. It is a narrative of inevitability and victimhood.
I see it differently. The "Five and Ten" theory, in its 2025 form, reveals something more hopeful: algorithms are not replacing human psychology—they are amplifying it.
The reason the theory still works after 36 years, despite the complete transformation of market structure, is that algorithms are built by humans. The programmers who code trading systems share the same neural architecture as the traders they are hunting. They choose integer parameters for their models because they think in integers. They write algorithms that respond to round numbers because they, too, have brains wired by evolution to respond to those numbers.
The machine is not the enemy of human psychology. It is the perfect mirror of it—magnified, accelerated, and stripped of hesitation.
The implication for traders is counterintuitive: in a machine-dominated market, trading psychology matters more than ever. The algorithms are not cold, rational calculators—they are a precise map of human behavioral vulnerabilities, automated and unleashed. If you understand where those vulnerabilities lie—at the round numbers, the psychological thresholds, the behavioral nodes—you can see what the machines see.
And then you can choose not to stand in their crosshairs.
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This article was originally published on FXEAR.com. Original content, unauthorized reproduction is prohibited.