Research consistently shows that asset allocation — the split between stocks, bonds, and other asset classes — explains roughly 90% of a portfolio’s return variation over time. Not which specific funds you choose. Not when you buy or sell. The single decision of how much to put in equities versus bonds is the most consequential investment decision you’ll make.

The most important decision

Once you’ve decided to invest in low-cost index funds, the only truly important remaining question is: what percentage in equities, and what percentage in bonds?

More equities means higher expected return but more volatility. More bonds means lower expected return but a smoother ride. Your job is to find the split that maximises long-term growth while keeping volatility within a range you can tolerate.

Factors that determine allocation

Time horizon. The most important factor. Money you won’t touch for 20+ years can handle 100% equities. Money you need in 3 years should be mostly bonds or cash.

Risk capacity. Objective measures: stable income? Dependents? Emergency fund? Job security? The more resilience outside the portfolio, the more risk within it.

Risk tolerance. How would you react to a 40% drop? If the honest answer is “I’d sell everything,” you need less equity than your capacity suggests. The best portfolio is the one you’ll stick with through a crash.

Income needs. If you need income from your portfolio now, bonds and dividends matter more. In the accumulation phase, growth dominates.

Other wealth. Property ownership, pensions, and other assets affect what your investable portfolio should look like.

Simple allocation models

Aggressive (80-100% equities): Young investors, 20+ year horizon, stable income, high tolerance. Maximum growth, maximum volatility.

Balanced (60/40): The classic institutional split. Roughly 70% of equity returns with significantly less volatility. Suitable for medium horizons or moderate tolerance.

Conservative (30-40% equities): Shorter horizons, low tolerance, near retirement. Stability over growth.

Lifecycle: Start aggressive, gradually shift conservative as retirement approaches. Many providers offer this as a single all-in-one product.

None is universally “correct.” But any of them, implemented with low-cost index funds and maintained through cycles, will serve you far better than no plan at all.

Adjusting over time

Your allocation should evolve as your life changes:

  • As you age: Gradually reduce equity exposure.
  • As goals approach: Shift toward safer assets near deadlines.
  • As circumstances change: Major life events warrant revisiting.

What shouldn’t change your allocation: market movements. Your allocation is based on your needs, not market conditions. Adjusting because “stocks seem expensive” is market timing dressed as prudence.

Asset allocation is the steering wheel of your investment journey. Get it right, implement with cheap index funds, contribute regularly. That’s 95% of investing done well.