Mental accounting

A rupee is a rupee. Fungibility is the formal name for that, and it is the assumption behind every calculation in this course: your net worth is one number, and money has no memory of where it came from.

People do not experience it that way. They sort money into accounts — this is holiday money, this is the emergency fund, this is the bonus, this is the house deposit — and apply different rules to each.

The three ways money gets sorted

By source. A bonus is spent differently from salary, and a tax refund differently again, though both are rupees. Windfalls get treated as licence; earned income gets treated as responsibility.

By destination. Jars with labels, each with its own rules about what may be taken out and when.

By position. This is the investing version and it connects directly to chapter 3. The disposition effect only makes sense if each holding is its own account with its own reference point. Someone evaluating one portfolio would ask which holding has the worst prospects. Someone running twenty mental accounts asks, of each, "am I up or down on this one?" — and gets twenty answers that do not add to a decision.

When it helps

It would be wrong to present this as a defect to be eliminated, because for most people the jars are load-bearing.

Earmarking works. Money labelled "retirement" and kept somewhere awkward to reach is less likely to be spent than the same money in a savings account. That is mental accounting plus a small barrier, and it is the single most effective savings technique there is.

It economises on decisions. Treating the monthly investment as a fixed obligation, like rent, removes a monthly deliberation you would sometimes lose.

It caps damage. A rule that speculation happens only from a separate small account is a real constraint, and better than trusting your judgement in the moment.

So the aim is not fungibility. It is to notice the specific cases where the jars cost you money, and there are only a few.

When it is expensive

Holding a low-return asset while paying high-rate debt. The worked problem below.

Treating "house money" as less real. Gains that have not been withdrawn get risked in ways the original capital would not be, because they are recorded in a different jar. The market cannot tell which of your rupees are profits.

Ignoring correlation across jars. Your equity fund, your employer's shares and your bonus all depend on the same economy. Jars hide that, because each is assessed alone.

Paying for separate products to serve separate labels, when one arrangement would have covered all of them more cheaply.

Working the problem

₹2,00,000 earning 6% against ₹1,50,000 costing 16%.

The arithmetic. The deposit earns ₹12,000 a year before tax. The loan costs ₹24,000 a year. Repaying ₹1,50,000 of the loan from the deposit saves ₹24,000 and gives up ₹9,000 of interest, leaving roughly ₹15,000 a year better off — and the saving is certain, which is more than can be said for most returns anyone will offer you. Interest earned is also generally taxable while interest paid on a personal loan is not deductible, so the gap after tax is wider still, not narrower.

The case in their favour, which is real. The deposit is not only an earning asset; it is access to cash without permission. Repay the loan and that access is gone — the lender will not give it back on the day the emergency arrives, and a new loan at that moment may be slower, dearer, or refused precisely because the emergency has impaired their circumstances. They are paying roughly ₹15,000 a year for a guaranteed, instantly available credit line. Framed that way it is not obviously foolish; it is insurance with a stated premium.

What makes it a mistake is the reasoning, not necessarily the outcome. "That money is for emergencies" never compares the ₹15,000 to the value of the access. It treats the two jars as incomparable, which is what mental accounting does. The right decision needs the comparison made out loud.

The better structure, which the jars obscure: repay ₹1,00,000, keep ₹1,00,000 liquid. Most of the interest saving is captured and a genuine buffer survives. Thinking in one portfolio makes that option visible; thinking in jars does not, because a jar is either opened or it is not.

The point

Money is fungible in arithmetic and not in experience: people sort it by where it came from, what it is for, and which position it sits in, and then apply separate rules to each. The sorting is genuinely useful — earmarking is the most reliable savings device there is — so the aim is not to abolish it but to catch the few cases where it costs real money, of which holding a low-yield asset while servicing high-rate debt is the clearest. The test is whether you have compared the jars out loud rather than treating them as incomparable.