A balanced investment portfolio holds a mix of equities (shares), bonds, and sometimes other assets, spread across geographies and sectors. Asset allocation — how you divide between equities and bonds — drives around 90% of long-term returns. A common starting point for medium-risk, 10+ year investors: 70% global equities, 30% global bonds. Two or three low-cost index funds achieve genuine diversification for most investors. Rebalance annually to maintain your target allocation.
The portfolios with the best long-term track records tend to be the simplest. A small number of diversified, low-cost index funds, held patiently and rebalanced occasionally, consistently match or outperform more complex approaches. Complexity adds cost without reliably adding return.
The Three Pillars of Portfolio Construction
1. Asset Allocation
Research by Brinson, Hood, and Beebower found that approximately 90% of long-term return variability is explained by asset allocation — how you split money between equities, bonds, and other asset classes. Picking the "right" fund within an asset class matters far less than getting the allocation broadly right.
- Equities (shares): highest expected long-term returns; highest short-term volatility
- Bonds: lower returns; lower volatility; often rises when equities fall — a portfolio stabiliser
- Cash: safe and liquid but likely to underperform inflation over long periods
2. Time Horizon Determines Allocation
- Under 5 years: mostly bonds or cash — equity risk is too high for money needed soon
- 5-10 years: moderate — perhaps 50-60% equities, 40-50% bonds
- 10-20 years: growth-oriented — 70-80% equities
- 20+ years: predominantly equities — 80-90%. The long horizon absorbs downturns.
3. Cost Minimisation
Every percentage point of annual fees reduces long-term returns through compounding. Aim for total annual cost (platform fee + fund charge) below 0.5%. The difference between 0.2% and 1.2% over 30 years on a growing portfolio can be six figures.
A Simple Portfolio for Most Investors
Two or three index funds are sufficient for a genuinely well-diversified portfolio:
- A global equity index fund — covers thousands of companies across developed and emerging markets
- A global government bond index fund — provides stability and a different return profile
- Optional: UK equity, property, or other specific exposure
This genuinely isn't oversimplifying. Academic evidence consistently shows simple, diversified, low-cost portfolios match or outperform complex approaches largely because they avoid the high fees and behavioural mistakes complex strategies invite.
Rebalancing
Different assets grow at different rates. A portfolio starting at 70% equities might drift to 85% equities after strong equity years. Rebalancing annually — selling the over-represented asset and buying the under-represented — returns to the target allocation and enforces "buy low, sell high" without requiring any market prediction. Inside an ISA, rebalancing generates no tax.
For the evidence base supporting index fund-based portfolios, our article on index funds vs actively managed funds covers the data in detail.
Frequently Asked Questions
How often should I rebalance?
Annually is sufficient. More frequent rebalancing adds transaction costs and complexity without meaningfully improving outcomes. Set a reminder once a year, check the allocation, and adjust if any asset class has drifted more than 5-10 percentage points from its target.
Should I hold gold or commodities?
For most investors, a bond allocation provides broadly similar diversification benefits without the complexity. Gold is a reasonable small allocation (5-10%) for those with specific concerns about monetary stability or extreme market scenarios, but it's not a core requirement for a well-diversified portfolio.
For a transparent, long-running example of a simple UK passive portfolio, Monevator's Slow and Steady Portfolio provides genuine real-world data.
