Research on how people actually make financial decisions has produced genuine findings and some overstated ones.
Loss aversion
Losses weighing more heavily than equivalent gains.
Which is among the better supported findings.
Overconfidence
Investors overestimating their own judgement.
Which shows up as excessive trading.
The replication problem
Several well-known effects failing to reproduce.
Which the field has had to confront.
What survives
The general observation that decisions are systematically imperfect.
Which is enough to justify defaults and automation.
What survived the replication crisis
Psychology as a field discovered that many published findings did not reproduce, and behavioural finance was affected alongside it.
Which means some widely repeated effects should be treated with more caution than the popular books suggest.
The broad conclusion, that people make systematic rather than random errors with money, is well supported; specific numbered effects are variable.
Practical applications
Automatic enrolment and default options.
Which have measurably increased retirement saving.
Nudges and their limits
Effect sizes smaller than early enthusiasm suggested.
Which subsequent evaluation established.
Self-application
Automation removing decisions from moments of stress.
A general note
This describes research rather than prescribing behaviour.
The finding that matters most in practice
Investors reliably underperform the funds they hold, because of when they buy and sell rather than what they buy.
Which has been measured repeatedly by comparing fund returns against investor returns in the same funds.
The gap is a direct measurement of the cost of behaviour, and it is larger than most fee differences people worry about.
Recency and salience
Recent and vivid events weighing too heavily.
Which explains chasing performance.
Framing
Identical choices presented differently producing different decisions.
Which affects how products are sold.
Mental accounting
Money treated differently by source or label.
A general note
This summarises research rather than prescribing action.
Why market structure is worth understanding
Most investment writing is about what to buy. Very little of it is about how the machinery underneath actually works: how orders are matched, how funds track what they claim to track, how prices respond to information, and what regulation does and does not cover.
That machinery determines a large part of the outcome. It sets what trading costs, it explains price movements that otherwise look irrational, and it defines the boundary between a risk you accepted and a failure someone else is responsible for.
None of it tells you what to invest in. It tells you what is happening when you do, which is a different and more durable kind of knowledge than any particular recommendation.
The recurring lesson
Across almost every topic here, the same pattern appears: a mechanism that looks arbitrary from outside turns out to follow published rules, and the rules are available to anyone who wants to read them.
The gap between what is knowable and what most investors know is not caused by secrecy. It is caused by the material being dry, unpromoted and available only to people who go looking for it, while a great deal of louder content competes for the same attention with considerably less substance.
Where the reliable information is
Exchanges publish their rulebooks. Index providers publish their methodologies. Fund managers publish factsheets and annual reports. Regulators publish investor education material and firm registers. Companies publish annual reports and prospectuses. Central banks and statistical agencies publish their reasoning and their data.
All of it is free, all of it is authoritative, and almost none of it is promoted, because nobody makes money from you reading it.
A general note
This article describes how markets and financial products work and is not investment, tax or financial advice. Rules, protections, tax treatment and market structure differ substantially between countries and change over time. Where a decision is significant, advice from a qualified professional who knows your circumstances is the appropriate route.
A closing observation
Almost every mechanism described here rewards patience, low cost and understanding what you actually own, and punishes urgency, complexity and acting on incomplete information.
That is an unexciting conclusion, and it is the one that the accumulated evidence supports most consistently. The financial industry has a commercial interest in the opposite message, and the volume of content pushing it should be understood in that light.
Further reading
Regulator investor education material is written for the public, carries no product to sell, and covers this ground more thoroughly than commercial content does. It is the obvious place to start and almost nobody does.
One more thing worth stating plainly
The distinction between a risk you knowingly accepted and a failure someone else caused is the one that determines whether anyone owes you anything. It runs through every topic here: market losses are yours, firm failures may not be, and fraud is neither.
Most disappointment in personal investing comes from not having drawn that line before committing money, and from discovering afterwards which side of it a loss fell on.