Sector

A sector groups companies by business type — tech, energy, healthcare — so you can see where performance comes from.

Why investors care

It is the first layer of diversification beyond owning the whole market.

Where people go wrong

Sectors move on different cycles; tech and utilities rarely rhyme.

In the real market

An investor owns five of the most popular stocks of the decade, believing the portfolio is diversified because they are five different companies. In reality all five are technology platforms with correlated earnings and customers, so the portfolio is one giant sector bet wearing a diversified costume. When tech multiples compress, all five fall together. The scenario is routine: people diversify by count of holdings, while the market cares about the underlying exposure.

Using it in practice

Check your portfolio’s sector weights to avoid accidental concentration. A stock-heavy portfolio is often far more tech-weighted than investors realise, and a single sector can dominate both gains and losses.

Example in numbers

Key takeaway

Sector is the first layer of diversification, because a stock-heavy portfolio is often far more concentrated than it looks. Five different technology companies are still one sector bet, and sector leadership rotates with the cycle — energy follows oil, utilities follow rates, tech follows innovation. Check your portfolio’s true sector weights and spread across industries that respond to different forces. Diversifying by count of holdings means nothing if they all zig together. A single low-cost global index fund already spreads you across every sector, which is why it remains the simplest way to avoid accidental concentration.

Definitions reviewed by the Investing Glossary editorial team.