Rebalancing vs. Danish Taxation

Why classical portfolio theory doesn't always work in a Danish taxable account

06. mar. 2026

Rebalancing is a classic investment strategy, but Danish realisation-based taxation can significantly change the outcome. Here's why.

Rebalancing is a classic investment strategy

Almost every book on portfolio theory highlights rebalancing as a good idea.

The idea is simple:

Choose a target allocation between asset classes, e.g. 60% equities and 40% bonds.
As markets move, the weights will drift.
Rebalance the portfolio by selling some of what has risen and buying some of what has fallen.

This keeps risk stable and exploits differences in market movements.

In many countries, the strategy works well.

But in Denmark there is a detail that changes the picture:

Taxation.

An important distinction first

This article deals only with investing in taxable accounts (frie midler).

The reason is the difference in taxation.

Account type	Taxation
Taxable account (frie midler)	Realisation-based taxation
Share savings account (aktiesparekonto)	Mark-to-market taxation
Pension account	Mark-to-market taxation

With mark-to-market taxation, you pay tax every year regardless of whether you sell.

Classical rebalancing therefore works on these accounts much as described in international investment books.

The problem arises primarily in taxable accounts, where tax is only triggered when you sell.

Why rebalancing works in theory

Rebalancing exploits a simple mathematical principle.

When two assets move differently, you can generate extra return by continuously adjusting the weights.

This is often called volatility harvesting.

The intuition is:

something rises
something falls
you sell the expensive one and buy the cheap one

Over time, this process can improve the portfolio’s risk-adjusted return.

Below is a simple example.

Here you see four curves:

equities
bonds
a rebalanced portfolio before tax
a rebalanced portfolio after tax

All curves use the same random market movements, so the difference is due solely to the portfolio’s structure.

Where taxation enters the picture

In a Danish taxable account, a sale typically triggers equity tax (aktieskat).

This means:

every time you rebalance
you realise a gain
and pay tax.

The tax reduces the capital available for reinvestment.

This may seem like a small detail, but the effect can be substantial.

If the process is repeated many times over many years, ongoing tax payments can significantly reduce the portfolio’s growth.

A simple example

Assume a portfolio:

Asset	Weight
Equities	60%
Bonds	40%

After a few years, equities have risen sharply.

Asset	Value
Equities	DKK 120,000
Bonds	DKK 40,000

The portfolio is now 75% equities.

To return to 60/40, you must sell equities.

If you sell equities for DKK 20,000 and the entire amount is a gain, the tax is:

Tax rate	Tax paid
27%	DKK 5,400

After tax, you can only invest DKK 14,600 in bonds.

The portfolio has therefore shrunk, even though you were simply trying to adjust the risk.

The hidden advantage of buy-and-hold

When you don’t sell, you defer the tax.

This means the government effectively gives the investor a kind of interest-free loan.

As long as gains are not realised, the full capital can continue to compound.

This is often called tax deferral, and it can be enormously valuable over long time horizons.

Monte Carlo simulation

For a more realistic picture, we can simulate many possible market scenarios.

Below, two strategies are compared:

Buy-and-hold (tax is deferred)
Rebalancing with ongoing tax

The simulation shows both the development over time and the distribution of final outcomes.

Each time you run the simulation, new market scenarios are generated.

The left column shows buy-and-hold, while the right shows rebalancing.

You will often see a pattern like this:

the median is fairly close
but top outcomes are lower with rebalancing

The reason is simple: tax withdraws capital from the portfolio along the way.

An interesting paradox

The result can therefore be quite counterintuitive.

Even if rebalancing mathematically improves the portfolio’s properties, taxation can make the strategy worse.

In some situations, the following strategy can actually beat classical rebalancing:

Buy a portfolio and let it develop without selling.

This is because of the value of tax deferral.

How often should you actually rebalance?

In classical portfolio theory, rebalancing frequently is often recommended.

Typical advice includes:

once a year
once a quarter
or whenever the portfolio drifts from its target.

The argument is that frequent rebalancing:

keeps risk stable
exploits volatility between asset classes
and systematically sells high and buys low.

But when we introduce Danish realisation-based taxation, an interesting trade-off emerges.

Two opposing forces

There are two effects pulling in opposite directions.

1. Volatility harvesting (positive effect)

When two assets move differently, rebalancing can generate a small extra return.

A rough rule of thumb is:

\text{rebalancing premium} \approx \frac{1}{2}\sigma^2(1-\rho)

where

$\sigma$ is the volatility
$\rho$ is the correlation between the assets.

The higher the volatility and the lower the correlation, the greater the potential gain from rebalancing.

2. Taxation (negative effect)

In a Danish taxable account, something else happens:

every time you rebalance by selling an asset at a gain, you realise the tax.

This means:

capital leaves the portfolio
it can no longer compound
the effect of returns on returns is reduced.

If rebalancing happens very frequently, these small tax payments can accumulate significantly over time.

The result: a surprising optimum

The two effects create an interesting balance.

Rebalance frequency	Effect
Never	no tax, but also no volatility harvesting
Infrequently	some harvesting, almost no tax
Often	more harvesting, but also much more tax

The combined effect therefore typically resembles an arc-shaped curve:

Too little rebalancing gives no effect — too much can increase the tax burden.

Returns first rise, but then fall again as rebalancing becomes too frequent.

There is therefore often an optimal level somewhere in the middle.

A simple mathematical perspective

Assume a portfolio with expected gross return $r$ and an effective tax rate $\tau$ on realised gains.

If rebalancing triggers a taxable realisation every $k$ -th year, a rough model can be written as:

V_T(k) \approx V_0 \cdot (1+r)^T \cdot (1-\tau)^{N(k)}

where

$V_0$ is the starting capital
$T$ is the investment horizon
$N(k)$ is the number of realisations.

When $k$ becomes very small (frequent rebalancing), $N(k)$ grows rapidly — and with it the tax burden.

Why less discipline can mean higher returns

This means that classic advice such as

“Rebalance every year”

is not necessarily optimal in a Danish taxable account.

In many realistic scenarios, the following can actually produce better results:

rebalancing every 3–5 years
rebalancing only on large drifts
or in some cases no rebalancing at all.

This depends in particular on:

the tax rate
asset volatility
the correlation between asset classes
the investment horizon.

Try it yourself

The following simulation illustrates exactly this trade-off.

Here you can adjust:

tax rate
correlation between assets
rebalancing frequency
and drift threshold.

Notice how the portfolio’s median outcome changes as rebalancing happens more or less frequently.

You will often see a pattern where:

no rebalancing produces a good outcome
moderate rebalancing produces the best outcome
frequent rebalancing reduces the outcome again.

The key point:

Portfolio theory is not only about market returns and risk.

In practice, tax rules play a central role in determining which strategy is optimal.

How can the strategy be adapted?

This does not mean rebalancing is always a bad idea.

But in Danish taxable accounts it can be advantageous to use a more tax-aware strategy.

Rebalance with new contributions

If you continuously invest new money, you can buy the asset class that has fallen relatively.

This triggers no tax.

Use tolerance bands

Instead of rebalancing every year, wait until the portfolio has drifted more noticeably from its target.

Example:

Target	Rebalance if
60/40	equities > 70%
60/40	equities < 50%

This reduces the number of transactions significantly.

Rebalance primarily in pension accounts

On pension accounts, tax is paid every year regardless.

You can therefore rebalance freely without extra tax consequences.

Use mark-to-market assets

If part of the portfolio is already taxed on a rolling basis (e.g. certain ETFs), these can be used to adjust the weights.

Conclusion

Rebalancing is a solid strategy in theory.

But in Denmark, realisation-based taxation changes the rules of the game.

The key insight is therefore:

The optimal portfolio strategy depends not only on markets — but also on the tax system.

By combining:

tolerance bands
new contributions
pension accounts
and mark-to-market assets

you can still capture many of the benefits of rebalancing without paying unnecessary tax.