Diversification means spreading your capital across assets whose performances are not perfectly correlated, in order to reduce overall risk without proportionally reducing expected return. It is often described as the only "free lunch" in finance: a reduction in risk that costs nothing in expected return. This article explains why, and how to apply it.
Why does diversification reduce risk?
When two assets do not rise and fall at exactly the same time, their variations partially offset each other. The declines of one are cushioned by the stability or rise of the other. Mathematically, a portfolio's risk is not the average of its components' risks: it is lower as soon as the correlations are imperfect. That is what makes diversification so powerful.
What is correlation and why is it central?
Correlation measures how much two assets move together, on a scale from -1 to +1. A correlation of +1 means they move identically: holding them together provides no diversification. A low or negative correlation is what you are looking for.
A common trap: holding ten different technology stocks gives an impression of diversification, while their high correlation makes them fall together.
How many positions should you hold?
Research shows that most of the diversification benefit is reached with a reasonable number of well-chosen positions; beyond that, each additional line adds less and less. Too few positions expose you to high specific risk; an excessive number dilutes monitoring and brings the portfolio closer to a simple index, without its low fees. The issue is less the number than the low correlation between the lines.
How to size a position?
A position's size should reflect the level of conviction and risk, not the enthusiasm of the moment. Approaches such as the Kelly criterion propose calibrating the size based on the probability of success and the expected gain/loss ratio. The practical rule: the stronger and the more confirmed the conviction across several signals, the larger the position can be, but never to the point of threatening the portfolio in case of error.
How to monitor risk over time?
A well-built portfolio drifts over time: a position that rises a lot eventually weighs too much, and assets that were once uncorrelated can become correlated in periods of stress. Regularly monitoring concentration, correlation between lines and sector exposure is essential. InvestIQ applies this logic at the portfolio level with an aggregated health score and correlation alerts when two positions become too linked.
What mistakes should you avoid?
The classic traps: confusing the number of lines with real diversification, neglecting correlation, letting a winning position become disproportionate, and underestimating the correlation that rises in a crash. Risk management is not a one-off act but a continuous discipline, which often matters more than the search for the perfect stock.
This is not investment advice.