What can a written rule protect you from that willpower cannot?
Every bias in this course shares a weakness: none of them survives contact with a decision that was already made. That is the whole strategy. You cannot out-think a bias in the moment — in the moment you are the bias. You can only pre-commit.
The core instrument: a written policy
One page, written when nothing is happening, containing:
- Target weights by asset class, with tolerance bands (for example 60/40 with ±5 points).
- A rebalancing trigger — a date, a band breach, or both.
- Position-size caps, so no single idea can dominate the outcome.
- Contribution rules — how much, how often, automatically.
- Kill criteria for individual holdings, in numbers.
- A cooling-off rule — a fixed interval between having an idea and acting on it.
It is deliberately boring. Boring is the feature: a document that is dull to read in calm weather is exactly what you need in a storm.
Watch the bands do the work
Take 60/40 with ±5-point bands.
- Equities fall 30%. The equity sleeve goes from €60,000 to €42,000; the portfolio is €82,000; equities are now 51% — below the 55% band. The rule says buy, at the moment every instinct says sell.
- Equities rally 40%. The sleeve goes to €84,000 of a €124,000 portfolio — 68%, above the 65% band. The rule says trim, at the moment every instinct says add.
Rebalancing is contrarian by construction. It does not need you to be brave, or to have a view, or to call a turning point. It needs you to have written the band down before you needed it — and to treat the document as binding rather than advisory.
Three more devices worth stealing
The cooling-off period. Forty-eight hours between idea and execution. Most impulse trades do not survive two nights. The ones that do are usually the ones worth making.
The pre-mortem. Before committing, write: "It is a year from now and this decision was a disaster. What happened?" Imagining a failure that has already occurred produces far more specific risks than asking "what could go wrong" — and the specifics become your kill criteria.
The decision journal. Date, decision, reasoning, expectation, confidence. It is the only instrument that can tell you, later, whether you were right or merely lucky.
The honest limits
Rules change behaviour, not markets. A disciplined investor in a falling market still loses money; the rule is not a hedge and no allocation policy promises a return. Rules also fail in three predictable ways: written during the stress rather than before it, written so vaguely that any action complies, or quietly abandoned because nobody would notice. Fix all three the same way — write it early, make it numeric, and make breaking it visible (a note in the journal, a conversation with someone, a required written justification).
And keep one escape hatch honest: a policy should be revisable when your circumstances change — a new job, a child, an inheritance, a shorter horizon. It should never be revisable because the market moved. Learning to tell those two apart is most of the discipline.
Try it now
- Draft your policy in five lines: target weights, bands, rebalancing trigger, maximum position size, cooling-off period. Date it.
- Compare your policy weights with your actual weights today. Every holding's current price is simply the last bar of its own chart — one is below, as a reminder of how little arithmetic this needs. Any band already breached is a decision the document has already made for you, before you have had a chance to feel anything about it.
- Run a pre-mortem on your largest position and turn the two most specific failures into written kill criteria.