What Is Mental Accounting
Mental accounting is a concept proposed by Nobel laureate Richard Thaler that describes the psychological tendency for people to categorize money into different mental "accounts" and apply different rules to each. For example, 100,000 yen earned from salary and 100,000 yen won in a lottery are economically identical in value, yet most people tend to spend lottery winnings more freely. The psychological label of "windfall money" changes how the money is used.
In investing, mental accounting exerts its influence in numerous ways. Spending dividends as "bonus income" while refusing to touch the principal is a classic example. Economically, dividends are simply a portion of the principal converted to cash, and the stock price drops by the dividend amount after the ex-dividend date. Yet mental accounting applies different rules - "dividends are money I can spend" versus "principal is money I must not reduce" - leading to irrational capital allocation.
Specific Examples of Mental Accounting Distorting Investment Decisions
Mental accounting distorts investment decisions in many ways. Managing "stocks with gains" and "stocks with losses" in separate mental accounts, quickly taking profits on winners while letting losers languish (the disposition effect), is a prime example. Similarly, treating a NISA account and a tokutei koza as separate mental accounts - taking aggressive risks in the NISA while being conservative in the tokutei koza - is also driven by mental accounting. Books on investment psychology and trading decisions also analyze how varying your investment decision criteria based on account type or the source of funds hinders the optimization of your overall portfolio.
Overcoming Mental Accounting Through Integrated Asset Management
To overcome the pitfalls of mental accounting, you need the perspective of managing all assets as a single integrated portfolio. Periodically aggregate assets spread across multiple accounts - NISA, tokutei koza, iDeCo (individual-type defined contribution pension), savings accounts - and review your overall asset allocation. By evaluating investment performance based on the total portfolio's gains and losses rather than individual account results, you can make decisions free from psychological labels.
That said, mental accounting is not always harmful. Purpose-based fund management such as "emergency fund," "education fund," and "retirement fund" is rational from a goal-based investing perspective. The problem arises when psychological labels degrade the quality of investment decisions. Books on integrated asset management propose practical frameworks for leveraging the beneficial aspects of mental accounting while eliminating its harmful effects.
Next Steps to Reassess Your Mental Accounting
Start by listing the balances of all your accounts - bank accounts, brokerage accounts, NISA, iDeCo, and others - and get a complete picture of your portfolio. Calculate your overall asset allocation (the ratio of stocks, bonds, and cash) across all accounts rather than per account, and compare it against your target allocation. Ask yourself whether you are applying different investment decision criteria depending on the account type, and check for any irrational "money labeling."
As a next step, review how you use dividends and distributions. If you have been spending dividends as a "bonus," consider whether this is hindering the optimization of your total assets. Use the compound interest calculator on this site to compare the 20-year portfolio value difference between reinvesting dividends versus spending them, and see in concrete numbers the long-term impact of mental accounting on wealth building.